COLOURED HOT ENAMELS AND ARTISTIC ENAMEL COATING TECHNOLOGY

Enamels are glassy coatings applied to a metal or a glass surface. There are cold (polymer), and hot enamels. The first type of enamels either does not require a thermal exposure, or is polymerized at a relatively low temperature (up to 200 °C). Actually enamels we will presented here by the second type of enamels, which requires a high-temperature treatment (600 - 900 °C). Most of these enamels is produced on the basis of silicon glasses. Enamel differs from usual (container) glass by significantly lower melting point (container glass melting point makes 1350 - 1500 °C), a considerable color palette, an enhanced fluidity, strictly defined parameters of linear expansion, and surface tension.

Enamel history

Nowadays, it is assumed that the artistic enameling art was originated more than three thousand years ago. The oldest known metal object with glass cover for artistic purposes was found in Mycenae and on Cyprus (15-14 cent. B.C.). The art of glass covering for metal objects has been developing since the 7-th century B.C. on the territory of Azerbaijan, Greece, northern Italy. A more recent technique of cloisonne enamel appeared, apparently, as a variant of the metal inlay with precious stones and colored smalt. This technique was widely developed in ancient Egypt since 2000 B.C. Golden walls were installed on a golden plate, the spaces between them were filled with color inserts. The pieces of bluestone, malachite, coral, turquoise, glazes, colored glass were processed according to the form of cells, and were fixed with a special glue.

Despite this, a real enamel appeared in Egypt only during the Greco-Roman period. However, the Egyptians created the artistic foundations of metal colored finishing by enamel and stones that are still valuable. During the 5-th cent. B.C. Celtic tribes of Britain and France developed a notched enamel on bronze. The special cells on the surface of molded articles were fused by opaque glass of saturated colors. This technique was transferred from Greeks to Romans, but it was not widely spread among them.

At the same time the enamel art has been developing in Asia: Persia, India and China. Chinese cloisonne enamel became a unique and an original technique, the traditions of Chinese enamel master are kept to this day.

Egyptian and Middle Eastern art greatly influenced the culture of the Byzantine Empire, the next center of enamel art. Byzantine cloisonne enamels are considered to be classic ones. Enamel was used for the first time not as an imitation of stones, but as an independent artistic tool. The heyday of the Byzantine enamel continued till the 12-th cen. A.D.

The experience of Byzantium had a radical impact on the development of medieval European enamel art. From Byzantium the enamel art spread to neighboring countries: Georgia, Armenia, Serbia, Kiev Rus, where the original enamel art schools were created.

In medieval Europe, the most famous enameling centers were situated in Limoges (France), at the monasteries on the Rhine and in Lorraine. In the middle of the 12th century Limoges became a leading center of church utensils production using different types of cloisonne and champleve enamel. The peculiar technique of transparent colored enamel on stamped terrain appears where different thickness of enamel coating creates a semblance of light and shadow play due to terrain roughness. Later (15-17 cent.) Limoges became a leading center for the development of a new original technology - a beautiful enamel in grisaille style. Learn more about this technique in "Picturesque enamel" chapter.

Since the mid-17th century due to the great development of chemistry metal oxides appeared, the application of which to a white enamel base and the subsequent firing make it possible to achieve the presentation of subtle color shades and the creation of miniatures on enamel, where a metal, even if it was gold, served only as a substrate and was covered by a continuous layer.

In the 19th century the technical consumer enamel on steel and cast iron was spread. Household items (dishes, parts of fireplaces and stoves) are covered with enamel in order to improve their performance properties.

At the turn of 19-20 centuries during the wake of Art Nouveau the art enamel experienced a new rise. Enamel is becoming a popular way of jewelry and decorative item design. Due to the advances in the field of silicate chemistry enamels a large palette of enamels of different colors and shades was developed.

Russian enamel. The earliest extant metal products with a decorative enamel decoration are referred to the 10-th - the beginning of the 13th century. In ancient Rus enamel was called finift' (from the Greek "fingitis" - bright, shining stone). As we mentioned above, the technique of cloisonne and notched enamel was taken over by Russian masters from the Byzantines. The largest centers of Russian enameling were Kiev, Chernihiv, Galich, Vladimir, Ryazan and Novgorod. From the extant specimens of Russian enamel art of that time turn-down collars are met most frequently - the kind of paired adornment for a female headdress in a lunar form made of two connected convex plates. The diadems made of several interconnected gold plates Ryasna (paired chains made of plaques), shoulder-mantle (pectorals, consisting of individual medallions), plates that were sewn onto the front clothes, tabernacles, body crosses and icons, Gospel linings, icon sets. Gold and sometimes silver was used for cloisonne and champleve enamel and the enamel on cast items (borrowed from Western Europe - the legacy of the Celts) was performed, as a rule, on bronze or copper. The enamel production of that period was not a widespread one. The main customers who ordered enamel items were the dukes and their families, the highest hierarch of the Church, the boyars.

After the Mongol invasion in 1237, many enameling centers ceased to exist, knowledge transfer tradition was interrupted. Many artists were deported to the Golden Horde. The cultural relations with Byzantium and Western Europe were broken. Since the mid-13th to the mid-15th century a general decline of Russian enamel art is observed. The recipes of many colors were lost, cloisonne was forgotten. Only casting enamel continues its development. Religious things were made mainly using the technique of cast enamel, the production of which was concentrated in the monastery workshops and Novgorod. Monasteries, as the whole Russian Orthodox Church, as opposed to a secular society, had a protection document from the khans of the Golden Horde, which provided some security for the local artisans from robbery and captivity.

Since the second half of the 15th century the revival of Russian enamel tradition begins. The resumption of cultural relations with Byzantium allows Russian masters to be acquainted with the experience of the Italian Renaissance art schools. The opportunities of working with enamel are expanding due to imported Western European enamel raw materials. Russian enamellers receive large orders from the royal court. Greek and Western European masters were invited in Moscow, which, along with Novgorod became the largest center of Russian enamel. At the end of the 15th century a new technology appeared - the enamel on filigree. The enamel on filigree became a favorite method for enamel decoration creation. Over time it became a recognizable characteristic symbol of Russian enamel. Russian enamellers also widely used the relief enamel technique, which was well known in Western Europe. The 17th century is considered the enamel prosperity period. During this period, a lot of household things, knife handles, windings, inkwells, cases, etc. were decorated by it. Enamel becomes an integral part of ecclesiastical items and secular things decoration.

During the period of Peter the Great the Russian enamel experienced a tremendous influence of Western culture. The miniature painting on enamel which appeared in the 18th century overshadows the traditional techniques of past centuries. By the end of the 18th century Russian miniature enamel painting goes beyond the element of arts and crafts, and becomes an independent kind of the easel art.

By the 19th century Moscow, Rostov and St Petersburg become the leading Russian enamel centers. Since the mid-19th century Russian enamel art is experiences a new rise due to the increasing public interest in ancient Russian traditions. A unique "Russian style" appears, as the variant of pan-European "historicism". The traditional enamel on filigree became widespread again as a mandatory attribute of the "Russian style". At that time the art of enamel becomes widespread, it becomes accessible to various strata of Russian society. The leading role in the enamel democratization was played by Russian jewelry firms. Using the achievements of chemistry and new enamel technologies, which allowed to manufacture the enameled products with an industrial basis, Russian jewelry firms could generate a wide range of things in accordance with the demands of a Russian buyer.

The most famous jewelry companies Russia of the late 19th - early 20th century, which used actively the art enamel in their products is Ovchinnikov, Faberge, Sazikov, Khlebnikov, Grachev, Nemirov-Kolodkin, Postnikov, Olovyanishnikov and other factories. Besides, there were also small jewelry workshops and cooperatives.

The appearance of such a wide range of enamellers, working in a variety of techniques, led to the fact that the art of Russian enamel of that period developed two ways of creative problem solution: as a decorative element of an item decoration, and as an independent easel painting, executed in enamel technique.

After the revolution of 1917, Russian enamel art was experiencing a decline. On the one hand this was due to the fact that the enamel art was perceived as the art of the ruling classes and on the other hand by the devastation in the country brought by the civil war. Of all the traditional Russian centers of enamel art, only the school of Rostov the Great remained. However, the subject was changed in the Rostov enamel. The ecclesiastical subjects were replaced by floral painting on a white or a colored background, close to porcelain painting and also Soviet symbols and miniature painting on enamel within the spirit of socialist realism. Later, on the basis of "Rostov enamel" cooperatives "Rostov finift'" factory was established.

Only in the seventies and the eighties of the 20th century a new wave of interest to the rich possibilities of artistic enamel begins among Soviet artists. Soviet enamellers have the opportunity to take part in the international creative workshops first of all in Hungary and Baltic States, and later in other Western European countries.

The possibilities of enamel technique are far from being exhausted. Despite thousands of years concerning this technology development, many aspects of enamelling are not studied completely.

Technical part

General technological information

Enamels are referred to the group of glasses and have their characteristic features: light transmission, water and acid resistance, brittleness, they do not burn. Glass has no crystalline grid, it is fully isotropic, amorphous and can be perceived as a frozen liquid.

Enamel - is formed by the partial or complete melting of the inorganic is a glassy frozen mass of an inorganic matter formed by the partial or complete melting, mostly of oxidic composition, sometimes with the addition of metals applied on a metal substrate.

Enamel is a glassy alloy containing a number of components that form glass. The enamel is applied onto the surface of metal products in a finely grinded state and it is fixed by firing at high temperatures as a durable and a thin coating. Enamels are produced by fusion at high temperatures (1250 - 1400 °C), specially selected charge materials: rocks (quartz sand, clay, chalk, feldspar) with fluxes (drill, soda, potash) and adjuvants: 1) oxides to improve enamel adhesion to a metal surface (NiO, CoO); 2) silencers for opaque state obtaining (TiO2, ZrO2, SnO2, fluorides, etc.); 3) colorants to provide a desired color for enamel.

According to its optical properties enamels are divided into transparent, non-transparent (opaque or opacified) and semi-transparent.

Metals for enameling.

Gold and silver are considered as the best metals for enameling according to their technological properties. However, due to the extremely high cost of gold and silver, as the basis for the enamel covering these metals are used only in jewelry. Pure copper and enameling tombac are used in decorative - applied art.

Due to its special chemical and physical properties copper has one of the best adhesion indicators with enamel. A relatively high melting point (1084 °C) ensures metal stability when a metal melts. The cost of copper is significantly less than the cost of noble metals. However, copper and tombac as the basis are ideal ones only for opaque enamels. Copper color makes a significant influence on transparent enamels: they darken and attain a brown hue. To preserve the natural color of transparent enamels copper is pre-coated with colorless transparent enamel (fondón), which preserves the natural reddish-golden copper color and prevents the contamination of enamel colors. The second method is the use of a silver foil substrate.

Enameling tombac is a copper zinc alloy with zinc content no more than 3 - 5%. The greater content of zinc is highly undesirable due to the enamel-metal adhesion decrease. For this reason, all other decorative alloys with higher zinc content, such as brass, bronze, nickel silver are not suitable for the enameling with conventional enamels.

The melting point of tombac makes 1055 - 1065 °C. The particular advantage of enamelled tombac is in its light yellow color, and therefore the enamel colors on tombac are clean and bright. This alloy is usually used for the manufacture of small production line items, such as icons. There is a great danger of enamel chipping during the enamelling of large surface products.

Cast iron, sheet steel, titanium and aluminum are also used in industrial enameling. Steel and aluminum may be used as the basis for artistic plaques. However, aluminum and its alloys have a sufficiently low melting temperature (659 °C for pure aluminum), which leads to the use of special low-melting enamels with the melting point no more than 620 °C. The aluminum alloys containing zinc are not suitable for enamel application.

Enamel production

Glass structure.

Enamels, like any glass are characterized by so-called glassy state which may be defined by chaotic arrangement of substance atoms in a space that do not form a geometrically correct ordered spatial structure, that is by the absence of a crystal lattice. The vitreous state is not unique to the glass. For example, amber, plexiglass (polymethyl methacrylate) and other substances are characterized by this state. This allows you to call some polymeric coatings as enamel which have similar decorative features with a classic hot enamel on metal or glass.

A liquid or a molten material passes into a crystalline or a glassy state by cooling. The properties of an anisotropic crystalline material depend on the configuration of crystals and differ in different directions. On the contrary, a glassy substance is an isotropic one, i.e. its properties are the same in all directions.

Crystals are characterized by a strictly fixed melting point, above which the crystalline material is not heated during the melting process, all the additional heat is not consumed for heating, but for the destruction of a crystal structure. At a rapid cooling some substances, such as silica SiO2 and silicate metal salts (silicates) and others are transferred into a glassy state. The melts of these materials have a high viscosity and thicken so fast that the atoms do not have time to line up in a correct crystal structure. This chaotic arrangement of atoms and molecules is the main feature of liquid or gas. Consequently, glass may be called a solid (more precisely an overchilled) liquid.

During the re-heating a glassy substance may be transferred into a crystal one due to its instability. Such crystallization is undesirable in the production of enamels. This is one of enamel defects.

Enamel production and composition.

The basis of most inorganic glasses and enamels is silica SiO2, introduced into a charge in the form of quartz sand (silicate glasses). Glass formation substances are presented by boron trioxide B2O3, phosphorus anhydride P2O5, lead oxide PbO and others. Accordingly, such glasses are called boric, lead ones, etc.. Besides, the composition of enamel includes modifiers (oxides of alkali and alkaline earth metals, which influence the properties of enamels) as well as dyes and pigments - coloring metal oxides, aluminum, lead oxides, fluorine compounds, etc...

Pure quartz sand (silica), feldspar (potassium, calcium or sodium aluminum silicate), magnesite (magnesium carbonate) are used as a refractory raw material for the manufacture of enamel. Silica (SiO2) makes 30 - 55% of the most artistic enamels. Such mechanical properties of glass as the compressive strength, elasticity and chemical resistance depend on quartz content in enamels. However, the increase of SiO2 amount in the charge composition increases considerably the refractoriness of enamel (pure quartz melting point makes 1800 - 2000 °C). Conventional glasses, including art ones contain 60 - 75% of silicon oxide.

In order to reduce the melting point fusible components (fluxes) are injected into a charge. The following substances are used most frequently: boric acid (H3BO3), borax (Na2B4O7), soda (Na2CO3), calcspar (CaCO3), red lead (Pb3O4).

The melting of charge and the boiling of enamels is carried out at the temperature of 1000 - 1400 °C. The cooking time makes from 20-30 minutes up to several hours. Such a long time is necessary in order to achieve a uniform (homogeneous) matter structure. During cooking, complex chemical reactions take place in a charge melt accompanied by the release of gases. Industrially enamels are cooked in special melting or crucible furnaces or in vitro - in small crucibles.

All charge components are ground and mixed thoroughly before melting. Usually, the enamel cooking is carried out in two stages. First a transparent glass is cooked - frit. Then frit is ground and used as the basis for the production of color enamels.

Quartz is used as a particularly clean sand, but the melt is contaminated still by a number of impurities, particularly by iron oxides. Some impurities penetrate into the resulting frit with other natural charge materials. These materials react with each other in a melt as oxides.

The resulting frit composed of considered components is a transparent one and serves as the basis for transparent enamels. When silencers are added into a glassy melt its transparency decreases, thus the raw material is obtained for opaque enamels.

Until now enamels are composed on the basis of experimental data. Many factors are impossible to foresee, as the component interaction in a melting process leads to various abnormalities. The enamel composition depends on predetermined process parameters. Table 1 provides an approximate composition of enamels produced by industry. 

Table 1. Original recipes of jewelry enamels

Quartz

34 — 55

Borax (boric acid)

0 — 12,5

Soda

3 — 8

Potash

1,5 — 11

Red lead

25 — 40

Fluorspar

0 — 2,5

Cryolite

1 — 4

Potassium nitrate

0 — 2

Arsenic

0 — 4

Coloring oxides

0,1 — 0,5

Careful preparation of original materials is the prerequisite for a complete dissolution and a uniform distribution of all components in a melt. Precisely weighed amounts of charge materials are ground thoroughly and mixed in such a way to obtain a homogeneous mixture of solid, fine component granules. Enamel charge is melted in a furnace to obtain a glassy mass which is the basis for future enamel.

The melting temperature for various enamels ranges from 1000 to 1400 °C. The minimum charge melting temperature is determined by the melting point of component melting. This implies that the course of complex reactions in a charge requires a certain time and may be accelerated by a sharp temperature rise.

Here the reaction rate increases with temperature increase as with any chemical process, but to a certain limit, the violation of which leads to undesirable phenomena: considerable changes in the enamel composition due to component volatility.

During the manufacture of glass a charge is melted, the melt is seasoned at the melting temperature as long as gas bubbles are removed and the mixture becomes a homogenous one. Similar procedure takes place during enamel cooking: a charge is heated up to the melting point, the melt is stirred, and is cooled quickly after some seasoning. Consequently, a solidified melt is obtained in the form of glass particles with the inclusions of gas bubbles. Chemical reactions between the components in a raw enamel are not finished yet, and physical and chemical processes go on as long as a glassy mixture becomes fully homogeneous one during the subsequent melting on a metal substrate. Cooking process is rather complicated, as chemical and physical processes take place simultaneously influencing each other.

Opacification.

 If non-transparent (opaque) enamels are boiled, some silencers are added in a glassy melt, special silencing additives with other refractive indices than a glass base. Light when passing through an enamel mass is deviated irregularly, scattered and reflected. The greater the difference of a basic glass and a silencer refractive index the greater the silencing effect is. Some silencers, dissolving in a liquid enamel mass during cooling, are extracted as solids or gases. Small gas bubbles or crystalline particles reflect light.

The following substances can be used as a silencer: bone ash (widely used since ancient times, now superseded by other materials), tin dioxide (SnO2); titanium dioxide (TiO2); fluor spar or calcium fluoride (CaF2); cryolite (Na3AlF6).

Coloring of glasses and enamels.

Color (coloring). It is known that the bodies with a selective absorption of light in one or more regions of a visible spectrum are represented as colored ones. A body has the color which it transmits or reflects.

A standard man's eye perceives the fluctuations with wavelengths from about 380 to 760 nm, receiving the impression of various colors.

If a body has the greatest uptake in the violet, blue and green areas and a minimum one in a long wavelength portion of a spectrum, then its color changes from yellow to red one. On the contrary, if the maximum absorption occurs in a long wavelength, and the minimum absorption in a short wavelength region of the spectrum, a body color will change from blue to purple one. The coating of enamels is also based on selective absorption phenomenon in the visible part of the spectrum.

To impart a certain coloring for enamel some specific coloring components - pigments and dyes - are introduced into a charge for repeated cooking (up to a few percent of the total mass). There are two types of coloring - ionic and colloid one.

Ionic coloring is conditioned by the presence of positively charged ions, certain transition or rare earth metals. Various ions of the same metal are characterized by different attitudes to glass coloring. You may draw an analogy between the glass coloring and the aqueous solutions with ionic dyes. For example, an aqueous solution of copper sulfate is blue, potassium permanganate solution is purple one. Such colors are also obtained at the introduction of these substances in the glass charge. In this case an original colorless glass (frit) may be regarded as a solvent, and metal oxides may be considered as dissolved pigments. When coloring oxides are mixed numerous color shades are achieved used in jewelry enamels.

The degree of selective absorption, and hence the transmittance of color rays depends on the concentration of ions in enamel and an enamel layer thickness (for transparent enamels). At the repeated heating of the solidified enamel mass with ionic dyes coloring hardly changes. These dyes color the glasses and enamels of any compositions.

Enamels colored by colloidal colorants have different properties. In this case, coloring is caused by the selective scattering of color rays: purple, dark-blue and blue rays are scattered (shortwave radiation), glass transmits only yellow, orange and red rays. These enamels contain the finest (i.e. colloidal ones) particles of such metals as gold, silver, copper, or some sulfides. The sizes of the colloidal particles make 10 ... 50 nm. The coloring occurs when these particles grow in a glass up to a specified size. However, the process of an excessive enlargement of particles may cause enamel blurring and opacification. With a sharp cooling colloid - painted enamels become colorless; coloring occurs only at a secondary heating of solidified enamel (striking). The result of striking is the extraction processes of coloring particles in enamel. A color intensity depends on the number of extracted colloidal particles and on their size. The sizes of colloidal particles and the distances between them are comparable to the color radiation wavelengths.

An example of such a coloring may be some red transparent jewelry enamel, in which the finest particles of gold are present as a colloidal dye.

Dyes and pigments.

Dark-blue and blue enamels are obtained by the introduction of 0.02% - 1% of CoO (cobalt oxide) into a charge. In order to obtain blue-green shades 1 - 2% (weight fraction) of copper oxide (CuO) is added.

A violet shade is obtained by manganese oxide Mn2O3 additives. Mn3+ ions give a purple-violet color to transparent enamel. Nickel oxide NiO administered in the amount of up to 3%, paints only glass containing K2O in reddish-purple color.

If copper oxide CuO is introduced into a charge in the amount of 2 ... 4%, the enamel color becomes an emerald green one. Warmer shades of green (without blue shades) are conditioned by the presence chromium oxide Cr2O3 in enamel composition. In order to obtain different shades of blue-green chromium oxides are used in the combination with copper oxide and iron oxides FeO and Fe2O3. At that FeO colors glass in blue, and Fe2O3 colors it in yellow. Different shades of green (bottle) color are obtained at the mixture of these colors. The use of iron oxides as the colorants of art glass and enamel is limited because they are conventional colorings of bottle and other container glass.

In order to obtain yellow enamels the sulphides of certain metals are used (cadmium sulphide CdS, copper sulfide CuS, lead sulphide PbS) and also iron sulfide of FeS, the high concentration of which (up to a few percent of the total charge weight) provides an intense amber-brown color. Enamels containing sulfides, are the typical examples of the molecular colloidal glass coloring. Various shades of yellow may be obtained using antimony and lead compound Pb2Sb4O7 with the addition of ZnO and Al2O3. In the lead fusible enamels (the glass former - PbO) chromate potassium (potassium bichromate) KCr2O is used to obtain the shades of yellow, orange and red colors. Depending on the concentration of potassium bichromate particles a corresponding shade is obtained.

Red enamels are obtained using the additives as cadmium sulfide CdS, and cadmium selenide CdSe in various proportions. Orange color is obtained at the ratio of CdS:CdSe = 3:1. Red transparent enamels of different shades, from crimson to purple ones are called ruby shades. Jewelry ruby enamels contain colloidal particulate gold (up to 0.03%) - the result of gold chloride AuCl3 decomposition into elemental gold.

Brown enamels are colored with the mixture of iron, zinc and chromium oxides.

Black color is obtained by mixing various metal oxides (the oxides of chromium, cobalt, copper, nickel with the additions of iron and manganese oxides).

Table 2 below demonstrates the data concerning enamel coloring with some dyes.

Table  2.  Pigments and colorings for enamels.

Coloring

Enamels

Coloring

Yellow

Cadmium sulfide CdS,

lead and antimony compound Pb2Sb4O7 with the addition of ZnO and Al2O3

Antimony oxide Sb2O5 (with concentration increase it becomes ocher with a brown shade)

Brown

The mixture of iron, zinc and chromium oxides

Red and orange

Cadmium sulfide CdS and cadmium selenide CdSe mixture,

Chromate potassium (potassium bichromate) KCr2O

Main lead chromate Pb[CrO4]•Pb[OH]2,

Colloid-dispersed gold (up to 0.03%)

Dark-blue

Cobalt oxide CoO

Manganese oxide, tin dioxide, aluminum oxide, chromium oxide are added to obtain shades

Green and blue-green

Copper oxides CuO  Cu2O

Chromium oxide Cr2O3, aluminum, cobalt, iron oxide additives soften the shades

Red-purple

Manganese oxides Mn2O3 MnO

Black

Mixtures of chromium, cobalt, copper oxides with the additives of iron, nickel, manganese oxides. In most cases, it is impossible to obtain a clean deep black color, and usually a brown or bluish shade is obtained

The color intensity at color silencing depends on the amount of pigment in enamel, on the size of its particles, and also on enamel silencing degree.

If a pigment is added to an opacified enamel, the coloration becomes weaker. The stronger enamel is opacified, the weaker the coloration caused by a pigment. Therefore, it is necessary to apply opacified enamels in order to obtain an intense coloring with a small addition of pigment.

Some physical and chemical properties of enamel

Thermal characteristics intended for the enameling of special glasses should conform to the thermal characteristics of the base metal. The cohesion between these materials should take place as the result of firing without a binder use.

The following properties have the highest value for the artistic enamels:

- Thermomechanical ones - enamel melt viscosity (thermoplasticity), surface tension, thermal expansion.

- Mechanical ones - adhesion strength, hardness, elasticity.

- Chemical ones - chemical resistance.

Viscosity.

It is one of the main properties for glasses and enamels. In contrast to crystalline materials, amorphous materials (to which enamels are referred) do not have fixed thermal points. And the transition from a solid brittle state into a plastic one, and then into liquid one takes place smoothly. At that it is impossible to define clearly the condition boundaries.

Viscosity is an internal friction between molecules conditioned by the flow of liquids and gases. The viscosity index at a given temperature determines enamel fluidity (the spreadability along a metal surface). The viscosity of a fluid state makes 102 - 122 Pa • s, it should be achieved (for artistic enamels) at the temperatures from 800 to 900 °C.

Thermoplasticity is referred to the basic properties of glasses and enamels. During heating, a hard brittle material softens, gradually transforms into a plastic state, becomes viscous liquid and then liquid at temperature increase. At that it is impossible to determine state borders.

While the changes of an aggregate state among crystalline materials, for example among metals, may be fixed by temperature points (the melting point of pure crystalline substances, the melting range of alloys), amorphous substances do not have fixed thermal points.

The degree of heated glass thinning is characterized by viscosity, and this physical property has a special importance for the glass characteristics.

The viscosity of an enamel frit should provide a sufficient fluidity, full metal coating, the dissolving of scale and technically optimal degassing.

The surface tension and wettability.

This property is determined by the forces of molecular interaction on the surface of a molten or a softened enamel. The wettability of a metal surface by enamel melt is determined by surface tension value. Surface tension σ is equal to the work (energy) that must be expended to increase the liquid surface by 1 cm2.

A fluid is influenced by the force under the impact of which the liquid tends to take the form of a ball - a body with a a minimum surface. It will be sufficient to recall the behavior of mercury or water beads on a greasy surface.

The value of surface tension depends on a fluid temperature and composition. During the fusing of enamel on metal a reduced value of surface tension is required, and during the application of one enamel layer on the other one, or on a glass substrate or a primer one should use compositions with a higher surface tension (more refractory ones) in order to avoid the mixing with a substrate (when it is not provided specifically) or perform firing at a lower temperature than the temperature of a substrate firing. Lead oxide and boric acid additives as well as K2O, Na2O, Li2O, CaF2, V2O5, MoO3, WO3 additives reduce surface tension significantly and hence increase the spreadability of molten enamel.

The surface tension and hence the strength of a metallic base wetting and bonding with an enamel melt are important for enamel. For example, when enamel is applied on a high relief the surface tension is reduced so that enamel is spread over the entire area and wets the substrate splendidly. As can be seen from Fig. 1 a drop of liquid applied on the surface of a solid body either spreads out, forming a thin layer of liquid (complete wetting) or remains more or less flattened (partial wetting).

Figure 1. Surface and adhesion tension


а – general scheme; б – partial wettability; в – full wettability; г – full nonwettabillity.

Thermal expansion.

It is known that a body expands during heating, and it shrinks to its original size and shape during cooling. The thermal expansion of enamels and their agreement with the base expansion is essential for the adhesion of enamels to metals and therefore it is one of the main factors affecting the quality of products. Varying the combinations of charge components, we may attain the enamel thermal expansion increase to make it higher than the household glass thermal expansion, close to the thermal expansion of metals. At that the thermal expansion of enamel should not be higher than the thermal expansion of a metal or equal to it. Durable enamel adhesion to a metal requires that the linear expansion coefficient of enamel was slightly less than the corresponding coefficient of a metal. At that enamel is under a slight compressive stress, which positively influences its mechanical properties, due to a relatively high compressive strength of glass.

The factor characterizing linear expansion for enamels is the most important one due to the fact that the thickness of an enamel layer on a metal surface is generally far less than its area.

The comparative values of linear expansion for enamels, glasses and other materials are presented on Fig. 2.

Figure 2. The temperature of enamel, glass and other materials linear expansion coefficient


Strength.

Mechanical strength is the material resistance to irreversible deformation and crack spread under an external mechanical load. The break of ties between body particles is caused by the action of tensile forces. Since glasses and enamels are very sensitive to tensile stresses, tensile strength is an important parameter of their properties.

The tensile strength of glass under compression is about ten times greater than the tensile strength under tension, and the abovementioned rule is conditioned by it. Glass and enamel are highly sensitive to impacts and have a low impact strength. However, unlike glass, enamel has a high strength under tension, bending, impact strength, due to the combination of enamel with a metal substrate. In spite of this, it is necessary to avoid anything that may increase quite low ultimate strength values when enameled products are used.

Bond strength.

Enamel bond strength with a metal is one of the main characteristics within metal-enamel system; it determines the stability of the system before a product comes into use. After product firing covered by enamel a metal gets a strongly bonded cover with it. The force that must be applied for the enamel layer separation from the metal surface, is called the enamel-metal adhesive strength. The practice of enameling, and a large number of research papers determined the dependence of adhesion on a number of factors. Enamel layer stresses, the elasticity of enamel and metal, the thickness of an enamel layer and other factors are important factors.

A good wetting of a metal surface with an enamel melt is the necessary condition for the adhesion. The metal surface shall be free of gross irregularities and impurities. The surface tension of molten enamel should not be too large. It is known that completely clean, not oxidized surfaces are not wetted by glass. In order to spread the enamel on metal the surface of the latter should have a thin oxide film. The metal surface structure plays certain role. Enamel is kept stronger on a loosened, rough surface than on a smooth one.

The cohesion of enamel with platinum, gold and silver carried out by mechanical means. A thin oxide film on a metal surface provides its wetting by enamel melt and the contact of enamel with metal. In order to obtain a durable adhesion the surface of precious metals before enameling is loosened by mechanical or chemical means. The enamel melt fills the surface notches and is retained therein after enamel hardening.

During copper enameling the adhesion is observed also on a smooth surface. It is carried due to copper oxide layer formed at on copper - enamel boundary. This layer can be seen by a naked eye. Copper oxide, on the one hand, is highly soluble in enamel. On the other hand, it diffuses into a crystal lattice of copper forming a solid adhesive layer.

The enamel composition is very important for cohesion development. It determines the surface tension and thermal expansion coefficient. Of particular importance is The presence of substances in enamel composition which enhance the adhesion strength (the oxides of cobalt, nickel, arsenic sulfides, antimony, molybdenum compounds, and some others) is of particular importance. The introduction of these substances in small amounts strongly increases the adhesion strength.

Besides the metal quality and enamel composition the adhesion strength is influenced by enamel firing mode. If the firing time or temperature are not sufficient in order to go through all the processes leading to adhesion, enamel will be easily separated from metal. There are no reliable quantitative determinations of enamel adhesion strength to metal. In order to measure directly the adhesion of enamel to metal it is necessary to apply the force that would tear off enamel in the direction perpendicular to the enameled metal surface.

Theoretically, the adhesive strength (adhesion) is realized as an absolute rupture resistance along a plane between a metal and enamel under tension with complete exposure of metal surface. However, at a thick enamel layer the rupture takes place not on metal - enamel border but on enamel layer. Hence one may conclude that the value of enamel adhesion to metal exceeds the tensile strength of enamel.

Elasticity.

The enamel coating elasticity determines the durability of enamel adhesion to a metal since the tensions that occur due to differences in the coefficients of linear thermal expansion of metal and enamel become level ones due to enamel elasticity. The gas bubbles in enamel increase enamel elasticity and solids reduce it. A prolonged firing (within reasonable limits) and a small coating thickness increase enamel elasticity.

Hardness.

Enamel hardness is an enamel coating resistance to point loads (abrasion, scratch). The indicators of enamel hardness are below quartz glass, as enamel contains the components, reducing its hardness, however, the hardness of enamel is much higher than many other art materials (oil paints, tempera, varnish, wood), that allows to compare it by durability with mosaic and incrustation with stones and metals.

Chemical resistance.

Enamel coatings under the influence of various chemicals - water, acid, weather impact - are decomposed gradually. It is manifested first of all in the loss of gloss, then the coating becomes dull and rough. Such reagents as strong acids, within a few minutes of boiling completely destroy some enamel coatings.

The ability of enamel to resist the action of reagents is determined by its chemical resistance.

According to the nature of impact on enamels and glasses, the following four main reagents are distinguished: water, acids, caustic alkali solutions and the solutions of alkali carbonates. The enamels which are resistant to one or more of these reagents can be unstable to other ones.

For art enamel chemical resistance does not have such a paramount importance as for industrial and dish enamel, but when enamels are applied on jewelry that can be exposed to direct contact with an open body, one should take into account the possibility of enamel gloss loss because of reaction with skin secretions (perspiration). In this case, the enamels in use shall be more stable than the enamels applied for interior and applied art and easel enamel painting. You should also take into account the degree of enamel chemical resistance held for a long time in an open atmosphere (for example - exterior enamel panels).

In general, enamel according to its properties is one of the most durable polychrome art materials. Only mosaic properties are better than enamel ones.

Technological part

Enameler tools

Electric muffle furnaces with the temperatures up to 900 °C are used for enamel firing.

Enamel preparation starts with the crushing of large pieces. A steel mortar fits this purpose (Fig. 3). In order to divide the milled enamel into fractions a set of sieves is used with the meshes of various sizes.

Fig. 3. Mortar for enamel milling


In order to work in cloisonne enamel technique it is advisable to use the enamel powder with the grain size of 0,1-0,5 mm. Finer grinding can cause enamel oxidation and hydrolysis, and a coarser setting increases the shrinkage during firing and makes enamel laying in small cells more difficult.

It should be remembered that the coarse milling of enamels requires a magnetic alloy mortar, as enamel powder is mixed with metal filings during grinding. These filings are inevitably formed during the grinding of enamel pieces. The filings should be removed from the enamel powder with a magnet, otherwise they can form black spots on the surface of the fired enamel.

The enamel of finest grinding is used for enamel painting. After rough grinding in a metallic mortar enamel is ground additionally in an agate or a porcelain mortar.

The bulk layers of enamel (for example, counterenamel or the substrate of a product front side) are applied with trowels. They can be made using a piece of thick copper wire. It's enough to flatten and bend its ends (Fig. 4).

Fig. 4. Trowels for enamel application


In order to apply a thin picturesque layer and provide details the brushes number 1 - 3 are used made of soft hair (weasel and synthetic).

A set of pliers is necessary in order to bend partitions: the pliers with long, pointed grips, round nose pliers, side cutters. Of course, they must be small (13 - 15 cm.). Besides, it is necessary to have several tweezers.

Enamel grinding is performed by grinding bars of different grits.

Glass or plastic jars with tight-fitting lids are suitable for the storage of finished dipped enamel.

Enameling process sequence

Metal base preparation

In training practice, copper plates are suitable to use as a metal base for enamel. The optimum thickness of such plates makes:

0.8 - 1.0 mm for large objects (up to 150x250 mm)

0.5 mm for small objects and samples (up to 50x50 mm)

When a plate thickness is selected for notched enamel the notch depth is taken into account.

Enameling plates should be free from internal stresses. For this it is necessary to anneal a workpiece after the provision of a desired shape.

Enamel coating has the best grip with a rough basis. Therefore it is desirable to apply frequent but shallow furrow on a plate surface. This can be done using a graver or an abrasive wheel. Long etching of copper in a citric acid solution also gives an easy roughness to a workpiece surface. The plate is washed in running water after etching to remove residual acid from its surface.

Enamel preparation

First of all enamel pieces must be crushed in a steel mortar. The pieces are divided into several stages and the grinding is sieved through a sieve from time to time. The remaining large fractions into the sieve are milled again. Relatively large pieces may be used for an enamel layer embedding in the basic color.

Once obtained the necessary mixture of fractions is obtained, enamel is poured into a jar and filled with water. It is stirred then and left for some time. When enamel settles it is necessary to drain muddy water in a separate tank and replace the muddy water with fresh one. The process is repeated for as long as the water over enamel is completely transparent. This degree of cleaning purity is particularly necessary for transparent enamels; the cleaning of opaque enamels may be stopped stop in some muddy water, it has no significant effect on enamel surface quality.

The muddy water should not be poured out - after its complete settling and the draining of clear water an ideal counterenamel is obtained. Of course, it should be used on unseen surfaces. It has a good adhesion and levels voltages resulting from metal firing due to its elasticity.

A desirable requirement confirmed by many years of experience, is the washing of transparent enamels with distilled water in order to avoid the enamel contamination by impurities which are present in tap water, such as Fe, Ca, and others. The opaque enamels can be washed with tap water without a risk.

Counterenamel

Thick metal sheets, which are commonly used for notched enamels have such a high strength that the resulting voltages during enameling can not lead to the changes of a product shape. Thin sheets coated with enamel on the one side, warp and sag because of the difference in the stresses which appear in metal and enamel. Therefore, thin sheets should be enameled on both sides to ensure that they have the same tensile stress on both sides and remained resistant to deformations.

Taking into account the abovementioned facts, it is necessary to stick to the following rule: a thick sheet of metal - a thin layer of enamel on one side; a thin sheet - a thick layer of enamel on both sides.

The thinner a sheet the thicker both enamel layers, the less is the danger of inflection. In order to achieve a complete equality of voltages, ideally a plate must be coated with the same enamel on both sides, made according to the same technique and should have the same thickness. However, it is almost impossible, and in most cases it is hardly justified, from an artistic point of view.

Waste counterenamel is mostly used for the products, the reverse side of which remains an invisible one. These wastes are obtained after enamel wetting. It comprises finely divided fractions of many types of enamels and therefore combines their properties. Due to a prolonged contact with water it becomes particularly resilient and resistant to wear. Furthermore, it is profitable, as it allows to use waste.

Typically, when bilateral enameling takes place counterenamel should be applied first of all to a reverse side and then to a front surface. A product, coated with counterenamel on one side, is calcined, and then it is etched, cleaned and a front side is prepared for enamel application.

If after the counterenamel firing a plate deformed slightly, you should not pay attention to it, since this deformation is usually eliminated during the enameling of the front side.

Enamel application

The enameling of a plate face is performed usually in multiple layers.

The lower (primer) layer of enamel is applied directly to a metal substrate. During priming metal should be covered with a solid layer of enamel. Bare places form a solid black copper oxide, which shall be eliminated prior to the next application of enamel. Copper oxides may be removed by etching in acid, or mechanically using an abrasive bar. If enamel is unstable to the etching solutions, burnt black edges should be polished.

By applying several layers of enamel one may adjust the results of previous firings and achieve specific color effects due to the mutual penetration of these layers.

In order to apply some moistened enamel a brush or a spatula are used as a rule. The spatula applies the bulk of the material, it fills recesses and cells. Brush is more suitable for some fine work, for example, glazing and detailed painting.

Applied enamel drying

An applied enamel shall be thoroughly dried before firing, as the water boils at the firing temperature and evaporates, at that enamel does not adhere to a substrate. It is recommended to dry a work directly over a hot furnace. An optimum temperature for enamel drying makes 60 - 80 °C to avoid enamel boiling.

If an enamel coating is not dried within the time required for a complete evaporation of moisture, you may experience the following problems:

1) Water steam evaporates explosively and captures enamel particles, therefore, spaces, voids and pores are developed;

2) During water boiling enamel particles are displaced into neighboring fields of different coloring, forming foreign color spots;

3) Enamel of fine grinding develops wrinkles and cracks;

4) Enamel is extended along the edges of recesses and cells;

5) Enamel is peeled from a lower side and vertical planes;

6) Bubbles may appear in enamel during firing.

Enamel firing

During firing in a muffle, the temperature is set at 800 - 850 °C, although enamels are transferred into liquid state at the temperature of 800 °C. This reduces the firing time, as the products are heated rapidly to a desired temperature absorbing the excess temperature in a muffle. Thus, the furnace temperature does not correspond to enamel firing temperature.

Cooling and finishing after firing. When a product is taken out of a furnace, it is allowed to cool slightly. Further a work is removed from a stand and put on an asbestos plate for further cooling. Due to the low thermal conductivity of an asbestos plate the cooling process is slowed down.

Sometimes during the process of firing an enameled object is deformed and warped. In most cases, this defect may be fully corrected during cooling while a product is in a plastic state. This should be done at the moment when red heat color disappears on enamel. Otherwise, a tool may leave marks in a soft enamel coating during finishing. If the enamel cooled too much, it becomes brittle and further finishing of a sample is not possible.

In order to finish flat surfaces wooden punches are used, covered with asbestos. For these purposes one may also use a correct steel plate, since steel does not provide any harmful effects on an enamel surface due to its relatively high thermal conductivity. During finishing a product is mounted on an asbestos stand and the necessary form is given by a load plane.

The etching of enamel products. During firing a bare metal surface is oxidized. The easiest way to remove scale is chemical etching. In this case it is necessary to check the acid resistance of used enamels. Red, light yellow, green and black enamels are particularly sensitive to acids.

The solutions of sulfuric, nitric or hydrochloric acid are usually used for etching.

A citric acid solution is used as a soft reagent requiring a prolonged exposure (from few hours to a day), but causing less damage to enamel.

If chemical etching is risky, because of a low chemical resistance of enamels, it is possible to remove an oxide layer mechanically.

Enamel grinding

The abrasive bars of different grits are used for grinding. Grinding should always be carried out with water. Liquid removes the broken abrasive grains from a bar. These grains leave scratches on a product. Besides, the residues of abrasive materials may be introduced into enamel pores during dry grinding.

After enamel firing on copper, the metal, projecting along product edges is covered with a black layer of scale. These places are not etched, since the easier and safer to grind this scale. The product is held in hands or is pressed against a a plate of wood or rubber, mounted on a table edge. A metal is treated along or across by a carborundum square bar of medium grain from an edge until the complete removal of an oxide layer (if grinding is carried out across the edge, enamel may be scratched or chipped).

The irregularities of the enamel surface are also aligned by a bar, carefully grinding the parts that protrude too much on the edges.

The surface of classical notched and cloisonne enamels must be aligned to form a single plane with recess edges and partitions. To do this, one has to fill the cells or recesses completely. If the enamel after firing is very low, it must be ground off to the deepest place, if you can not add some enamel and perform a repeated firing. This operation is labor intensive and time consuming. Besides, a deep filing weakens partitions which - especially along the edges of a plate - can be separated from a substrate.

Pre-grinding may be performed on a rotating carborundum round of a grinder, and a mill gun with a flexible sleeve is used for more delicate operations. But grinding process should take place with a frequent washing by running water. At a mechanized processing enamel is removed quickly, sometimes too quickly, but a surface remains rough. Once partitions appear, the surface is polished by hand with a carborundum bar.

Grinding bars and a product are washed with water from time to time. Prior to grinding with a more fine-grained bar, a plate should be rinsed well under running water.

Glossy firing

In order to obtain a shining surface of enamel, polished, thoroughly washed products are subjected to a final firing. At a temperature of about 800 °C a prepared product is being fired as long as enamel starts to shine. However, one should not keep a product too long during last firing to avoid undesirable effects caused by excessive melting of enamels.

Scale shall be removed from partitions by etching or mechanically after firing.

Different techniques of enameling

Picturesque enamel

In this enameling technique the colored enamel powder is applied directly to a metal substrate on foil cover or onto prepared layer of ground enamel layer without metal partitions. Then a plate is fired to melt enamel and form a dense coating, resembling stained glass. Enamel may be applied by a wet method with a spatula or a brush and by a dry method and by stenciling. In the latter case, one may use special stencils to create clear contours of colored patches.

Substrate preparation. First of all counterenamel is fused on a metal substrate from the back side. The front side is degreased and cleaned to a metallic gloss. The substrate treated in this way is ready for an enamel base application. In order to apply a preliminary drawing on a copper plate it is necessary to do the following. The contours of colored planes of a colored sketch drawing are transferred on tracing paper and then on a substrate by impaling with a steel needle or using transfer paper. If necessary, a copper plate may be pre-coated with a white enamel layer. In this case, lines will stand out more sharply. The obtained lines are drawn by a scriber. To remove the traces of carbon paper, the surface is washed with a stiff brush with soap, rinsed under running water, the surface is rubbed with the paste for cleaning and then it is washed again. The plate can be slightly fired after drying in order to remove the traces of grease. One may apply marking lines on a metal surface using an alcoholic marker. But it is prohibited to make a drawing with a pencil! During a line firing the lines made by a marker burn out without any damage to enamel, while the lines made by a pencil leave unrecoverable defects in the form of bubbles and burnouts along the lines.

Picturesque enamel on copper

Using a test firing one should be ensured that the selected enamels are suitable for these colored compositions and their firing ranges coincide. It should also be borne in mind that transparent enamels are applied better not on a copper substrate, but on a silver foil.

Copper and enameling tombac react with enamel, which particularly influence the tone of green and blue enamels. Besides, opaque red enamel is covered with black spots and white enamel becomes green on the edges.

For this the copper surface prior to the application of colored enamel is coated with white or colored ground enamel. Ground enamel should be somewhat more refractory than the enamels for a picturesque layer in order to avoid the mixing of a surface layer with a ground enamel layer during subsequent firings, if it is not intended originally as a special decorative effect.

In order to avoid a direct contact of enamel with metal, you can also use the following method: an intermediate enamel layer is fused on a metal before colored enamel application. This layer is called fondón. Fondón is colorless transparent enamel. It serves as an insulating layer between a metal and a coating enamel, the background for colored enamel application. Properly covered by a correctly chosen fondón a metal surface must retain the reddish-golden color of copper.

After firing the pattern drawn on copper is clearly visible under the layer of fondón. Moreover, the advantage of this method is that the colored planes do not necessarily fit tightly to each other. You may be restricted with a contour image. Colors may have various intensity depending on an enamel layer thickness which may be applied in the form of spots or streaks. In those places where colored enamel was not applied a warm copper tone covered by fondón stands out after the firing between color planes.

Enamel properties are manifested most completely artistically in the combination of the opaque and transparent enamels, when transparent enamels are applied on opaque ones. Opaque enamels, overlapped with transparent ones, are becoming softer and more vivid. Transparent enamels acquire a special brightness and liveliness on a light non-transparent basis, on a substrate fully or partially covered with foil they shine especially brightly. However, this method of enamel application requires a vast experience and high enameller skills.

Limoges enamel

This method of art enameling was widely used in Limoges during 15-18 centuries, and along with the Byzantine cloisonne enamel it is one of the most distinctive and highly artistic techniques, where enamel has an independent significance. In this technique plaques with images, vessels and dishes were produced. They were fully decorated by enamel with decorative ornaments, images of animals, people and even by whole scenes.

Unlike the later picturesque miniatures on enamel only colored enamels but not coloring oxides (overglaze paints) were used for painting. Besides counterenamel was also used by all means.

The feature of Limoges enamel is that depicted motifs are located on a dark background, and a relief is applied by white enamel. First a slightly convex copper substrate is coated with a black, dark blue or dark brown ground enamel, then an image is applied on this basis with white enamel and fired.

When a thin layer of white enamel is applied a dark background shines through it. In this case gray tones prevail, and a pure white color is formed at sufficiently thick layer. By varying the thickness of a layer, you can get a wide range of shades between black and white. However, this effect can not be achieved by a one-time application. The application of white enamel and firing alternate layer by layer, until the dark base is covered completely in locations corresponding to the brightest details.

If the work is done properly a relief image is created on a surface. For this purpose, the refractory enamel with high surface tension is used. Sometimes color shades were given to monochrome images by coating a white layer with transparent colored enamel: clothes, foreground details, images of flowers and animals were set off with colored enamel. Sometimes, to in order to provide a special decorative effect gold or silver foil was fused under the layer of transparent colored enamel.

Cloisonne enamel (filigree enamel)

Previously we described the techniques of product creation, the artistic value of which was determined primarily by the beauty of enamel coating. The metal of these products plays a supporting role - it serves as a substrate for a fragile enamel coating.

Let us consider the traditional methods of enamelling - cloisonne enamel technique. A characteristic feature of cloisonne enamel is the presence of metal partitions that prevent the mixing of enamels during firing.

Before we proceed to the description of techniques, let's present the definitions of basic concepts that will be used then.

Wire (in enamelling) is a rod of a circular or a square cross section.

Filigree wire is a braid twisted of two round wires. Filigree wire may be a rolled one (rolled in rolls).

Partition is a flat wire with a rectangular cross-section or a thin narrow sheet strip. Partitions are mounted on a substrate edge.

Cell is a space on a substrate surrounded by a partition, which is filled with enamel.

Groove is a depression on a substrate which is filled with enamel, flush with the recess edges. Fig. 5. demonstrates different types of jewelry enamels

Fig. 5. Types of jewelry enamel


a - a fragment of a cloisonne enamel ornament; б - enamel on filigree ornament: в - filigree enamel; д - medieval cloisonne enamel; е - stained glass enamel; ж - combination of notched and cloisonne enamel techniques; з - notched enamel

Enamel on filigree ornaments and on filigree

A metal sheet is used as a substrate, and in the simplest case a flat plate is used. However, for this purpose one may use a plastic sheet part or a complex product made of a material that may withstand the enamel firing temperature.

You can use a ready-made wire or make it out of a cast rod. A rod is forged, annealed and then rolled into rolls. When a wire reaches a desired length, it is pulled through draw plates to obtain a certain profile (usually a round or a rectangular one). During the drawing process a wire is subjected to intermediate annealing, and it is annealed in the end in order to impart a greater plasticity to it. Then it is bleached and brushed.

Filigree enamel is a special type of enamel. Cord wire is twisted of two thin round wires according to one of the following methods:

Fig. 6. Filigree wire production:


а — manual twisting; б — mechanical twisting; в— cord wire compaction

The ends of short wire sections are brought together and clamped in a vise. A wire loop is slipped through a tube, a thin rod is inserted into the loop. One hand presses a tube to a rod in order to pull the wire, and the other hand rotates the shaft until the wires are intertwined (Fig. 6a).

If it is necessary to obtain a long cord, then both ends of the wire are clamped in a three-jaw chuck of a small machine or a drill and a rod is inserted into a wire loop on the other end. In this case, wires are twisted by a chuck rotation and an arm tension (Fig. 6, б).

Wires may be twisted to certain extents by specified methods, conditioned by the bending stress and tensile stress: at an overload a stranded wire breaks during twisting. In order to twist wires more tightly, free torsion is applied. A stranded wire is annealed and cut into pieces of about 30 cm long. The end of a workpiece is clamped in a hand vice (Fig. 6, в). The wire is placed on a table and rolled tighter holding a fixed end firmly, drawing a flat piece of wood along it with a push until a desired density of turns is reached. Ready filigree wire should be like a homogeneous wire, i.e. it should not be separated during bending. At the end the wire is annealed once again. After etching and brushing the wire is ready for further use.

In accordance with the sketch the wire is bent and soldered to a metal substrate.

A wire must always be soldered for filigree enamel and filigree relief enamels. It is not necessary to cover a metal surface completely by wire cells: beauty often lies in the alternation enamelled surfaces and a clean metal substrate.

Since enamel is applied only in some places, it is not possible, and there is no need to apply counterenamel. Cells from a wire are filled with enamel to the brim, i.e. to a full height of a cell. The volume of enamel weight during firing decreases markedly as interstitial voids disappear during the melting of enamel grains.

Since the softened enamel particles are firmly held on metal edges, the wire is covered almost to the filling height; volume reduction is particularly noticeable in the middle of a cell. This leads to the fact that the surface of the melt and then solidified enamel becomes a concave in a cell. In order to correct such a defect cells are refilled with enamel and a product is fired. This operation is repeated until an enamel surface in a cell becomes flat. It should be noted that at each enamel application and firing, a cell becomes less deep.

During the manufacture of filigree enamel it is almost impossible to grind an enamel surface, as the rim waviness will be ground off for sure. A round wire changes in a similar way during grinding. When a rectangular wire is used one may perform grinding as a final operation.

The beauty of enamel on a filigree ornament lies in the fact that you can put tender, delicately drawn patterns on a metal, which are formed by the combination of a bent wire and color spot included therein. Russian and Hungarian filigree enamel with stylized flowers, climbing plants are a prime example of it.

When enameling on a filigree relief takes place it is necessary to consider some limitations conditioned by technological features.

1. As the beauty of an ornament is determined mainly by the elegance of filigree contours and, as a result, a relatively small height of a wire, an enamel layer can be thin only.

2. Stresses appear on thin layer of enamel because a product is coated with enamel on one side only, which allows to use certain types of enamel. As a result, the color palette of enamels is limited.

Decorative cloisonne enamel (cloisonne)

This technique has the following features. The cells, limited by flat metal partitions, are soldered with enamel. The cells are filled with enamel up the upper edge of the partitions. A product surface is polished so that the partitions and enamel are in one plane. If these conditions are satisfied we can talk about cloisonne enamel.

Let's consider the manufacturing technology for enamelled products in the form of flat plates.

Fabrication for partitions.

Partitions are cut from narrow sheet strips, but this method is rather laborious one. More often, partitions are made of wire. The wire is rolled in rolls up to the required thickness, and then it is pulled through a square draw plate. If a four edged hole of a draw plate is absent the wire can be pulled through a round element and then flattened by rolling. A slight rounding of partition narrow sides has no practical significance. The height of a partition depends on the thickness of an enamel layer, and the width of it depends on a drawing finesse. A ready dressed (straightened) flat wire is annealed, etched and brushed.

Partition bending.

Forceps are used for partition bending. Besides, a set of tools includes pliers, pointed pliers, round nose pliers, wire cutters, the tongs for small operations and tweezers.

A wire is bent in accordance with a pattern; each time partition is applied to pattern for comparison. Two identical or symmetrical motifs may be bent simultaneously with partition pre-folding. Cutting pliers straighten bent parts. Finished ornaments made of wire are straightened and aligned on an anvil.

When you create a sketch for cloisonne enamel it should be borne in mind that small parts are fixed badly on a substrate; too long, straight partitions are deformed and curved at a high temperature. One should connect two straight at an angle or bend a straight line in an arc, or to give it a U-shaped form. If two pattern lines intersect or overlap, up to a half of a partition should be cut and inserted into each other, or an intersection place should be depicted using the angles established against each other.

In classical cloisonne enamels partitions were installed to develop a closed cell, which was often filled with one color enamel. This principle was observed in the Byzantine and European works of the Middle Ages. Byzantine enamellers had a fine taste and used the expressive possibilities of this technique skillfully.

Although one may solder opaque enamels now close without partitions, maintaining clear boundaries, but, as in the old days, clear outlines of the curved partitions are used as a specific means of enamelling technique. The novelty is the following one: a single cell may be filled with enamels of several colors or multiple color shades. An image will be softer if you use the transitions and intermediate tones. At that partition is used in a drawing as a certain iconic contour. We may proceed further and use partitions only for the most important parts of an image, and make the other parts in a picturesque enamel technique. The contours of partitions on a monochrome enamel create the effect of a purely graphic image.

Mounting of partitions.

In accordance with a pattern partitions are applied on a substrate and soldered. During the manufacture of simple motifs partitions should not be soldered: their adhesions with soldered enamel are quite enough to fix them firmly and securely. Partitions shall be installed on an enamel substrate and the cells shall be filled with enamel. Of course, partitions are shifted easily, and enamel shall be applied with a special caution.

Sometimes, if they want to do without a soldering paste the partitions are glued to a substrate with tragacanth. BF-6 glue suits well for this purpose. A few drops of glue are applied on a glass plate, slightly moistening a partition with it and holding it with tweezers. Then, the partition is mounted on a substrate. If a substrate is covered with silver foil, it is the only way to secure the partitions.

During fondon coating with a metal surface, as it is often practiced on copper, the following method proved to be good: a thin continuous coating layer of fondón is applied on a plate and then it is fired. A pattern may be scratched on a metal in advance. It will shine through a transparent enamel. Partitions soaked in a small amount of tragacanth, are set on fondón according to the pattern. Once the glue dries well, the plate is placed in an oven and the partitions are immersed in the softened fondón. Then the plate is removed from the oven. Tweezers slightly straighten the partitions after they are pressed in a still hot soft ground enamel. This rational and reliable method is used most often for large copper plates. Partitions are fixed quickly and properly.

Notched enamel (shampleve).

If cloisonne enamel is characterized by the application of equal width partitions, the notched enamel technique provides the obtaining of partitions with various thickness and free metal surfaces.

Notched enamel involves the combinations of various methods. For example, an image is performed in such a way that colored enamel covers an entire surface, and only metallic contour lines remain or on the contrary only an engraved line drawing is covered with black enamel. You can enamel a base with colored enamel, and revive planar metal parts with an engraved line drawing, filled with black enamel.

Notches may be formed on a molded object. Furthermore, they may be engraved, stamped, cut with a graver and finally etched in a metal surface. Iron chloride solution is used for the etching of copper and iron tombac: 400 g of Fe (III)-Cl per 1 liter of water.

Enamel application into notches and cells. Cells and notches are filled from the middle and enamel is shifted gently into corners. In order to prevent the formation of voids steel with a steel spatula enamel is repeatedly and vertically pierced to a metal substrate to remove air bubbles and compact enamel. Voids are formed most often in narrow cells and notches and at a base, where enamel wets metal partially. In such cases, they take a dry enamel powder and fill cells with it by a thin pin for enamel application in order to condense enamel to the ground.

First small, then large cells or notches are filled with enamel. It is also reasonable not to fill neighboring cells immediately one by one, but miss one cell as the case may be. Then you remove excess water and fill in the remaining cells. Thus, it is possible to avoid the penetration of one enamel into another one.

One should not fill cells immediately to a full depth, as thick enamel layers become porous and dull after firing. This is most clearly manifested among transparent enamels. First, enamels are applied so that the base of cells and notches is covered with enamel slightly after firing.

During the first firing the metallic edges and partitions are oxidized since they are not covered with enamel. In such cases, edges and partitions may be lubricated with boric acid solution prior to application.

Usually, one should always enamel a reverse side of a product, and then the front side of it. During the enameling of small products one may burn both sides simultaneously. At that a front side is covered with enamel first of all. Then the product with enamel, which is in a wet state, is turned gently and, if possible, the product is held between fingers and covered by counterenamel. Counterenamel shall be applied evenly, but not with a too thick layer, since thick layers split off a metal during firing.

In some cases enamel is mixed with a small amount of tragacanth to increase the adhesion of enamel powder with a substrate.

When you work in notched enamel technique it is enough to apply and fire enamel twice, and sometimes one may apply enamel in one step. As we described previously, a cell is filled with enamel up to the upper edge of a partition. As the enamel volume decreases at melting, it settles in the middle of a cell. Its surface takes the form of a concave meniscus. Thus the play of light and shadow appears at the towering partitions and there is an effect of light reflection in the concave surfaces. Transparent enamels are added by light refraction effect depending on a layer thickness in a cell.

During the manufacture of cloisonne enamel one should apply and fire enamel until all the cells are not filled completely with enamel.

Some enamels (especially opaque red ones) change color during multiple firings, so it is recommended to apply them completely in two stages.

Processing of jewelry enamels after firing

Grinding.

Cloisonne, notched and filigree enamel are ground by carborundum bars of different grits (preferably square ones) after a complete firing.

Since artistic enamel should be in contact with acids as less as possible, it is not etched prior to grinding; metal oxides are removed mechanically during grinding.

An enamelled product is placed on a soft surface (paper or cloth), the enamel surface and metal partitions are treated by a grit stone with water. A grinding bar shall be washed with water from time to time. Otherwise removable enamel particles are retained in the pores of a grinding bar, therefore scratches appear on enamel surface and there is no continuous removal of enamel. Furthermore, the abrasive particles can penetrate into the finest pores of enamel.

The processing is started by the roughest abrasive and a workpiece is processed until metal partitions appear. Then the product is washed, a workplace is cleaned and the product is processed by medium grit abrasive bars until the shining of partitions and other metal parts. After that, the product is thoroughly washed and is subjected to polishing until the elimination of all scratches and visible grinding marks.

As during the metal processing by a file, in this case, it is necessary to change a grinding direction at each subsequent finer grinding. It must be remembered that not only superfluous enamel is removed and a metal is exposed, but low enamel layers of other color and brightness may become bare. Such a grinding conceived in advance may create an interesting effect, but if it is not provided by artistic conception, the whole previous work will be reduced to zero.

Thus, the grinding operation deserves more attention than it usually receives. One should proceed to a finer grinding in time. A fine, enamel-filled engraving, where the notches are very shallow shall be ground with a special care. A special care should also be observed during the grinding of plate edges with cloisonné enamel and partitions, which are located close to an outer edge, so as not to break the enamel with a grinding bar. In these cases, it is recommended to carry out grinding from the edge to the middle.

Enamels may be treated by a grinding machine with a rotating carborundum, but very carefully, because the grinding process in this case is even more difficult to control.

Flushing.

This work is often underestimated. If after washing the abrasive particles remain on enamel surface, then at the subsequent firing they are fired into the enamel as gray dots, and it is almost impossible to remove them. Abrasive residues are removed from enamel surface, after grinding they are removed by a glass brush (or a tooth brush) under running water, keeping a product in an upright position. If necessary, it is boiled in water with detergent with a grinding surface down and then it is washed. After washing the product is wiped by coarse calico or filter paper. Sometimes gray inclusions i.e. the abrasive stuck in the pores expanding to the bottom can not be completely removed by washing. Then pores are cleaned carefully by a needle and the product is washed again. Flushing requires patience. Final inspection should be carried out under a magnifying glass. The things which are not clearly evident after grinding, appear after a brilliant firing.

Fire polishing.

Before a final firing after the grinding of enamel large pores are filled with a few grains of enamel. The product with a clearly purified opaque polished enamel surface is placed in a hot muffle furnace for the last time. At high temperatures, due to an unequal thermal expansion of enamel and metal cracks are formed in an enamel layer which are melt again with temperature increase. When an enamel surface starts to shine, the product is immediately removed from the furnace. The firing temperature is of great importance: the higher the temperature is, the greater the enamel gloss. The copper parts not covered by enamel are oxidized slightly, and the partitions and the edges may be finished by a scraper or polished, without exposing the product to etching. If you use rather acid-resistant enamel, the etching and brushing is performed as well as for annealed metal objects. Fusible opaque enamels after the firing at high temperatures become opalescent sometimes and opalescent enamels become completely transparent. In such cases, in order to restore the enamel color it is necessary to cool down the product and to reduce the temperature of the furnace. Then, the product should be remained with enamel in a moderately heated furnace until the enamel melts and a color change occurs. At that it is necessary to control the temperature strictly.

Glossy grinding.

Opaque enamels, which experienced fire polished often look too coarse and rough. In order to get a shiny enamel surface with a silky shimmer, the enamels pre-polished by carborundum bars shall be finished by sandpaper with water: first coarse and then fine grain paper. If you change a skin you should be changed and the grinding direction. This process should be carried out with the abundant water addition to remove the grinding waste.

Of course, after a glossy grinding the product is thoroughly washed and dried. Enamel surface quality depends on grainy paper by which the final grinding is performed.

Dry enamel surface can be heated slightly and some paraffin may be rubbed into with a soft cloth. This will close the remaining finest pores, the surface will acquires a silky shine, and metal parts will be protected against oxidation.

Glossy polishing.

Fire polishing is dangerous for some products or is not desirable for artistic reasons. In such cases, a product is subjected only to glossy grinding.

Modern enamel processing technology is not as laborious as in the Middle Ages. In particular, the use of corundum powder greatly facilitates polishing. A slurry is mixed from this powder and water. This slurry is spread on a pigskin, and then under a slight pressure an enameled plate is polished until the desired degree of polishing.

If, light irregularities remain during fire polishing, then a smooth, mirror-like surface is formed after gloss polishing. For all kinds of enamel surface treatment one should not used grinding and polishing pastes for metals, as their residues due to a high content of fat is firmly retained in pores, leaving black spots on enamel surface. These spots are quite difficult to remove.

Pictorial enamel, overglaze painting

Substrate preparation for pictorial enamel:

- A metal substrate preparation

- Enamel preparation

- Enamel application

- Enamel drying

- Enamel firing

Enamel painting process:

- Paint preparation for painting

- Application of colors on an enamel substrate

- Firing of enamel painting.

Metal substrate preparation.

A thin copper plate of M1 brand is taken usually as the basis for enamel application, which will be painted. The last is processed manually or by a stamping machine. The preparation of a metal workpiece is obligatory here. After that it takes a slightly curved shape. The deformation of products occurs during firing. Deformation is minimal one among slightly curved surfaces.

The thickness of a plate depends on a product size: up to 50 mm - 0.3 - 0.5 mm plates are taken usually; a sheet metal with the thickness of about 1.0 mm is used at a greater diameter.

Metal preparation consists of the following operations:

1- Brush cleaning;

2- Whitening;

3 - Burning.

Bleaching.

The aim of bleaching is to remove scale from a plate. It is carried out with a hydrochloric acid solution, prepared at the rate of 2 parts of hydrochloric acid per 3 parts of water. The product is boiled in this solution until you all scale comes down. A concentrated solution of citric acid is also suitable for bleaching.

Burn.

In order to make enamel stuck to a workpiece surface better and perform a good distribution along it, a workpiece is kept about 30 seconds at the temperature of 600 - 700 °C prior to enamel application. A thin oxide film on a metal appears during the burning process.

Enamel preparation. Typically, enamel comes to a consumer in lumps of various size. A required amount of enamel pieces is taken for work and they are ground in a mortar until desired fractions without adding any water. At intervals the contents are put down on a sieve and sieved into a cup, and the remaining large parts on the sieve are ground again. Stainless steel mortars suit best of all.

Then it is necessary to grind enamel. An agate mortar is used for this. Enamel is rubbed by calm circular motions, starting with small portions with the gradual addition of enamel. Rubbing takes place until a desired grain size of grinding is achieved.

After the grinding process the enamel elutriation is performed. Fully pounded enamel is placed into a porcelain cup and fill which is filled with distilled water. Then it is stirred with a spatula to cover the whole enamel weight with water. After that the enamel is left to stand, a settled muddy water is poured out and it is replaced with fresh one, until the water over the enamel becomes completely transparent.

Enamel application.

First, a counterenamel layer is applied on the opposite side, excess moisture is removed. Then the plate is turned and enamel is applied gently from the center to the edges on the front, convex side. Excess moisture is removed.

An applied enamel must be thoroughly dried before firing. A sample with an applied enamel is put on a firing rack carefully and then it is placed in an oven. The optimum temperature for enamel drying makes 60 - 80 °C. The sample becomes dry after about 30 minutes.

Enamel firing. The 13-th and the 16th enamel of Dulevo paint factory suit best of all for the painting with overglaze colors. Once a uniform layer is obtained from the 13-th enamel (generally this requires two firings at the temperature of 820 °C and the holding time of 3 min.) a thin layer of 16-th enamel is applied. It is dried in a muffle furnace for 1 min. 30 sec. A prerequisite of the 16th enamel first firing is the curing in an oven without a complete enamel spreading. Then the second layer of the 15-th enamel is applied carefully with the filling of all cracks and unevenness. It is fired then for 2 min 30 sec. at the temperature of 820 °C. It is necessary to achieve a full spreading of enamel and a perfectly smooth glossy surface during the last firing.

Preparation of paints for painting.

Enamel painting is carried out, usually with the help of porcelain colors. Overglaze paints are presented by a mixture of pigments with fluxes. Paints are fixed on enamel at the temperature of 720 - 860 °C. The oxides of coloring metals are used as pigments.

It is necessary to prepare paint in advance, a day or two before painting. Glass is used as a palette. Glass is pre-washed and dried.

Powdered paints are poured onto glass with a spatula, then some turpentine oil is added in a paint and rubbed with a spatula. Paints are rubbed very carefully. A paint should not be spread along glass.

Dust causes irreparable damage to paints. Therefore, paints must be kept closed.

When a paint is applied to a surface the following aspects should be considered:

1 - the paint should be put quickly, do not put a brush on the same place several times;

2 - the subsequent strokes on already applied ones, do not correct them, but finally spoil the original strokes;

3 - lines shall be drawn without interruption, with one stroke if it's possible;

4 - when strokes are applied a brush movements along an enamel surface must always be directed to a painter.

In all cases of paint application it is necessary to monitor the thickness of a paint layer, as some paints may change its color in thick layer, be underfired, become dull, may blacken, etc. Therefore, you need to determine empirically the change of colorful shades after firing at different ways of their application on enamel.

Firing of painting on enamel.

The firing of paintings takes place at the temperature of 740 - 820 °C. Temperature and curing time depend on paint properties and an enamel substrate. Applied paints are dried until all turpentine evaporates before each firing. When a paint dries and ceases to shine, the product is put in a muffle furnace.

The duration of firing usually makes no more than 3 minutes. Shine is the sign that a product may be put out of a muffle furnace. Keep in mind that at properly conducted firing the brightness of colors is amplified a little, and their tone and color changes more rarely. During the process of painting the product is fired more than once. There are patterns that require up to 5 - 7 firings. The defects which appear after firing, are usually the result of poor manufacturing processes, an improper firing mode, the use of low-quality paints, old materials, the errors committed during the execution of painting.

The use of silicate compositions based on smalt and colored beads as colored enamels

Currently enamel artists have a wide range of colored art enamels of domestic and foreign production different works of art, both in the field of jewelry, as well as in the framework of enamel painting and other kinds of arts and crafts.

However, commercially available hot enamels of art have several disadvantages: there are significant restrictions concerning the possibility of color solution use and physico-chemical properties.

It should be noted that art enamels, produced by industrial method have standard colors and usually do not allow mixing with each other. A significant obstacle for the creative work and training is a relatively high cost and the shortage of hot enamels in retail. If the creation of jewelry, especially precious metals makes the use of expensive often import enamels reasonable due to the high cost of a product as a whole, large enamel painting demands that the cost of consumables (enamel, substrate) was comparable to the cost of materials for other types of painting and drawing.

On this basis, it is reasonable to propose the use in artistic practice, along with the classical industrial enamels some colored silicate compounds with similar physico-chemical properties. These compounds are more affordable and allow to expand the artistic possibilities of hot enameling.

An important advantage of these materials is also the absence of harmful impurities of lead and arsenic which are present in many art enamels of industrial production, as well as improved chemical stability of experimental enamels, compared with most known color art enamels, which allow to clean uncoated enamel copper surfaces from scale in organic acid solutions.

Experimental enamels based on smalt and beads can be used directly as hot enamel art, and as a solid and refractory substrate for enamel painting. This provides new consumer properties of already known materials, and also greatly enriches the decorative possibilities of enamel painting and cloisonne enamel. In particular, the use of enamels based on colored smalt and beads allows to obtain complex transitions and color effects as a mosaic structure of enamel coating. Experimental enamels may contain colored smalt insertions which allow to provide contrasting color transitions, not delimited by partitions. Besides, experimental enamels based on smalt and beads are combined well with hot enamels which are commercially available. Their addition with industrial enamels allows to obtain an artistic effect which can not be performed when only enamels are used produced by industry.

The use of colored smalt and beads as enamel has historical analogues. For example, in medieval France the manufacture of cloisonne enamel on copper and tombac required the colored smalt of Roman mosaics. And Rostov enamel was made on a substrate from molten white beads before the beginning of the 20th century [6].

However, the literature in fact does not provide any specific details concerning the use of these materials as enamel art. On the basis of the Department of Materials Science at SPbSMTU the studies were carried out on the use of colored and artistic smalt and beads as hot enamel on copper. The technological advice on the use of experimental art enamels for cloisonne enamel and painting creation by overglaze paints.

The influence of process parameters on physicochemical properties of enamel coating on the basis of smalt and beads

During the manufacture of cloisonne enamels at various stages of production: from the preparation of surface and enamel compositions to a finished product obtaining, the main process parameters that influence the quality of enamel coating will be the following ones:

- The size of enamel particles grinding;

- The material and the quality of a metal plate used for enameling;

- Counterenamel availability;

- Complete removal of moisture from enamel powder applied on a metal surface;

- Temperature and firing time of an enamel product;

- The thickness of enamel coating layer and a product shape.

Now let's consider the impact of these factors individually.

The size of ground enamel particles.

 The compounds with the grain size of 0.1 - 0.5 mm in diameter proved to be the best ones for the manufacture of cloisonne enamels. This grain size is most suitable for enamel laying in small cells between the partitions, as during the work in cloisonne enamel technique larger grain size of ground enamel complicates the laying of compounds in small cells, and a finer grinding (less than 0.25 mm) makes enamel drying more difficult and may cause the oxidation of silicate enamels during firing (typical for smalt), which adversely affects their color characteristics. However, in order to save material and simplify the manufacturing process, most cases do not require the sieving of enamel fractions with a grain size less than 0.1 mm in order to obtain some qualified enamel coating in terms of physical and chemical properties of enamel coatings, although this can be done for the convenience of work with enamel. The size of particle grade is also important for the decorative properties of a finished product, but this will be discussed in detail in the next chapter.

Material and the quality of a metal plate preparation designed for enameling. The best adhesion of experimental enamels with metal demands the cleaning of a metal plate surface from all possible impurities: grease, dirt, thick oxide films, paint, etc. Purification may be performed mechanically using abrasives and also with an acid solution. Despite the fact that the complete absence of an oxide film decreases the wettability of a metal surface with enamel melt, the case of copper enameling demands a metal surface treatment in a weak acid solution followed by rinsing in running water. This allows to identify the crystalline macrostructure of copper, which creates a microrelief on a metal surface and promotes the adhesion of enamel with copper. A thin oxide film, which improves the adhesion of enamel with metal on a surface free of copper impurities is formed rather quickly at the contact with oxygen during enamel application, a product drying and firing.

As in the case of conventional hot opacified (non-transparent) enamels use [9], the adhesion of experimental enamels with a metal substrate is obtained more durable by their applying on a rough surface of metal than on a smooth or a polished one. In this case the adhesion is enhanced due to metal coating area increase with enamel and an uneven terrain of a metal surface.

Availability of counterenamel.

Usually cloisonne enamels are carried out on metal surfaces which have considerably larger area relative to the metal thickness. Consequently, the enamel coating on a metal surface only on one side may lead to the warping of a product due to the difference of metal and enamel linear thermal expansion coefficients. In order to avoid this unwanted effect, cloisonne enamel is performed as bilateral one or the reverse side of a product is covered by a counterenamel layer.

In most cases counterenamel is applied on the reverse side of a product before the making of a pattern from partitions on a front side. The thickness of counterenamel should not exceed the thickness of a surface layer in order to prevent a product deformation. As we noted above, counterenamel is comprised from the mixture of various enamels in order to provide more versatile properties to it, and moreover it is desirable that counterenamel contains fine fractions to give a greater plasticity to it. When counterenamel is fired on the reverse side, a metal plate becomes slightly curved and convex on the side of counterenamel. This defect may be reduced after a plate cooling, when it is pressed along an entire plane. In order to avoid the splitting off of enamel during this operation, it is recommended to apply a thin layer of counterenamel, no more than 0.5 - 1 mm in fired condition. One may achieve a final alignment of a plate after enamel firing on the front side.

Complete removal of moisture from the enamel powder applied on a metal surface.

In order to obtain a qualitative enamel coating during the application of powder enamel to a metal surface by a wet method, it is necessary to evaporate all moisture in enamel powder before a product loading in a kiln. Otherwise there is a danger of a sudden boiling of moisture remained in enamel powder. At simultaneous production of large quantities of products it is advisable to carry out enamel drying before firing in drying cabinets at the temperature of about 80 °C. However, during the manufacture of single products with a simple configuration it is enough to dry the enamel applied on a hot substrate or place a workpiece in a heated muffle gently for a short time (no more than 5 sec).

The temperature and the firing time of an enameled product.

In order to obtain high-quality products using hot enamel technique the ratio of temperature and firing period shall be observed, depending on a product size and weight. An average temperature of pilot enamel firing makes 850 °C, which was found out during experiments. However, as we mentioned above, it is appropriate to use different values of operating temperatures at various stages of cloisonne enamel production. Thus, during the first firing of enamel powder applied on a product, firing should be carried out at maximum possible temperature in the case of experimental enamels for the copper of M1 grade, this range makes 850 - 890 °C. This is necessary to ensure that the molten enamel properly fills all of the cells formed by partition walls. A firing period must be sufficient enough to make an enamel melt spread on a product surface completely and to remove gas bubbles from enamel melt (degassing). But we must not unduly increase the period of product firing, as the burning out of thin layers may occur because of this reaction with the copper oxide, and undesired chemical reactions in enamel melt, which could lead to enamel color change. For experimental enamels the first firing time of a product makes 3 - 5 minutes for small products with a surface area up to 4000 mm2. Firing time may be extended up to 10 minutes for larger products.

Subsequent firings for a product finishing may be produced at lower temperatures and in a shorter time interval. The final firing to obtain the effect of a gloss surface of mechanically ground enamel (so called hot grinding) may be carried out in the temperature range of 780 - 820 °C for 1 - 3 minutes, depending on a product size.

The thickness of an enamel coating and a product shape.

This factor is important for the proper operation of finished products, not including the decorative coating properties, as discussed below. As we noted earlier, the ideal strength characteristics of an enameled product are shown at the equal thickness enamel coating of a copper surface on both sides with the same properties. However, it is poorly applicable in practice. Most often, counterenamel composed from different enamels is used to compensate stresses. Usually counterenamel is applied to the reverse side of a product, and then a front side is decorated, if a product is one-sided one. A counterenamel layer should not be too thick (no more than 1 mm), in order to prevent a product deformation after cooling. Due to some difference of metal and enamel thermal expansion coefficients, the copper plates with the thicknesses up to 1 mm, are coated by counterenamel, slightly bent towards an enamel surface. This can be avoided by using initially slightly convex plates the coating of which by counterenamel from a concave side does not provide any deformation. When flat workpieces for cloisonne enamel are used, as was described above, the deformation may be reduced by pressing a plate with counterenamel after the unloading from a muffle. The final alignment may be obtained after the coating of a front side with enamel. However, too uneven proportions should be avoided when flat products are enameled. If the length of a flat plate under the enamel is greater than its width twice or more, the strain can not be aligned during cooling. In this case, you may increase a metal thickness, reducing an enamel layer thickness as far as possible.

The influence of technological parameters on the decorative properties of an enamel coating on the basis of smalt and beads

The main process parameters that influence the optical and decorative properties of enamel coatings are the following ones:

- Temperature and firing time of enamel compositions;

- Enamel powder cleaning degree from various contaminants (metal particles entering enamel during the grinding in a metal mortar, the impurities contained in the water for enamel powder washing and wetting, etc.);

- The size of enamel grains formed during glass material grinding;

- The degree of enamel powder purification from dust fractions (this factor is most important for transparent enamels);

- At repeated firing after the grinding of an upper enamel layer during covering surface alignment - the extent of surface roughness before re-firing and its purification degree from abrasive particles used for polishing.

Now one may consider these factors separately.

Temperature and firing time of enamel compositions.

From the viewpoint of decorative property dependence concerning the studied silicate compositions on firing modes, the main feature of experimental enamels based on color smalt and beads is that a portion of cool colors lose the effect of opacification during primary firing at a temperature over 800 °C and require a secondary firing (striking) with a temperature no more than 820 °C. The mechanism and the reasons of this process are described above. In this case opacification is restored completely or partially, depending on the temperature and curing period. Opacification loss effect can be used as an artistic means to obtain darker shades. The restoration of opacification occurs usually during final firing (hot polishing) of cloisonne enamels, occurring in the range of 780 - 820 °C, so this effect can not be considered as a disadvantage. It should be noted that the effect of striking is inherent to a series of jewelry enamels, commercially available and is widely described in literature.

As we mentioned above, during the manufacture of cloisonne enamels an average experimental firing temperature for enamels makes 850 °C. An optimal spreading of enamel on a metal surface, and the proper of cells requires to perform the first firing of a product in the range of 860 - 900 °C. Despite the fact that this temperature range is insufficient for a complete spreadability of some experimental enamel compositions (but it is quite sufficient to obtain the desired product quality). The firing at the temperatures above 900 °C is not recommended, as this may lead to the change (browning) of enamel colors and burnout appearance - dark spots a thin layer of enamel surface due to the interaction of enamel pigments with oxygen, metal and other components of enamel compositions.

As for the firing time of experimental enamels, from the viewpoint of decorative property optimization, the firing time should be as minimal as possible to prevent the decomposition of pigments and color loss. Typically, an average burning time of small products makes 2 - 3 minutes, depending on a product size, temperature and a product availability. However, an original firing of opaque cloisonne enamels may be carried out for a longer period (up to 10 minutes), since all colors will be added by the second layer and fired at a lower temperature because of a strong shrinkage. This is necessary for a high-quality enamel melt filling of all voids and an enamel layer degassing. However, at a final firing of a product, firing time should be sufficient to prevent the above defects. Final firing temperature makes 780 - 820 °C for cloisonne enamels on the copper with the thickness up to 1 mm, using experimental enamel compositions for small products with a surface area up to 8000 mm2. Curing time for the products with a given surface area makes 1 - 2 minutes. Final firing of harder products shall be performed at a temperature of at least 820 °C due to a high heat capacity of a product (the data are valid at the use of a muffle furnace with the specified parameters).

The purity degree of enamel powder from various contaminants. In order to obtain some high-quality enamel coating it is necessary to observe some purity requirements for materials:

- The absence of foreign impurities in enamel powder (enamel particles of other colors, not compatible with the basic, mechanical pollutants - dust, dirt, metal particles);

- Chemical purity of water to wet enamel powder.

When large pieces of glass materials are ground an inevitable chipping of fine metal particles occurs in a steel mortar during grinding. This leads to the appearance of black spots on a fired enamel surface. In order to prevent this defect enamel powder must be processed immediately after firing with a magnet as long as a magnet surface stops to collect steel dust, located in enamel powder.

It is also important to observe that enamel powder was not contaminated with various impurities: dust, organic particles, the enamel particles of other color or chemical composition.

Enamel grain size.

Unlike commercially available enamels during the firing of ground experimental enamel powder, there is no complete fusion of ground enamel grains. This effect can not be attributed to the shortcomings of experimental enamels, as it does not affect the strength of enamel coating in general. One may state on the basis of experimental data, that the strength of pilot enamels on a copper substrate exceeds commercially available art enamel strength. Moreover, it gives a particular decorative effect to artistic products. This effect is difficult to achieve using "conventional" enamels. Depending on the use of experimental enamels different with various degrees of grinding and various color shades mixing concerning the compositions based on smalt and beads one may obtain complex discrete experimental colors of experimental enamels.

From a technical point of view, in order to manufacture the cloisonne enamels of small size, it is not recommended to use too small or too large fractions of ground experimental enamels, which was described in detail above. But during the manufacture of larger products (panels, etc.) one may use coarse ground glass materials, up to the inclusion of separate glass beads and pieces of smalt in an enamel layer.

The cleaning degree of enamel powder from dust fractions and foreign inclusions. This factor is the most important one during the manufacture of small jewelry with evident defects caused by foreign matter in enamel composition of organic and inorganic origin. In order to avoid dust and foreign particles in enamel compositions, ground enamel should be stored in tightly closed containers, while the operation with enamel requires to monitor the cleanliness of a workplace and used tools. You should also carefully monitor the cleanliness of glass materials before enamel grinding.

The degree of surface roughness before re-firing and the degree of its purification from abrasive particles is used for polishing. During the manufacture of cloisonne enamel using the "medieval" technique, when an enamel layer is gradually ground off with partitions one should observe that during the grinding of products at intermediate stage, before a next firing, an enamel surface is as smooth as possible, without any noticeable scratches and irregularities. This is especially important when at a glossy ("fire") polishing. For this grinding must be carried out by progressive decrease of abrasive grain size and a thorough washing of a surface with running water after grinding. Otherwise (at insufficiently smooth surface, especially in the presence of noticeable scratches and roughness) after an enamel product firing defects may be observed on an enamel surface in the form of whitish "abrasions" where scratches and roughness are situated. Insufficient washing of a ground enamel surface from abrasive material microparticles, small abrasive particles may clog the pores of enamel and also cause the color defects of an enamel coating.

The use of experimental glass-forming materials as the basis for painted and picturesque enamels

Here and further the compositions of some art glass and beads in different proportions or in a pure form are meant as the glass-forming materials. The melting point of such compositions, sufficient for a full coverage of a copper surface and subsequent firing of a pattern applied by ceramic overglaze paints is within 740 - 850 ºC.

The technological process of basis manufacturing for picturesque enamel with the use of glass-forming materials:

- The manufacture of a blank from a copper sheet of M1 grade, with predetermined shape and size;

- Mechanical preparation of a metal surface (a rough surface creation);

- The laying of counterenamel made of glass-forming composition. Firing in a muffle at 850 - 900 °C;

- A repeated mechanical preparation of a metal surface face part after cooling;

- The laying of glass-forming composition (slurry). Firing. Number of firings for a front side depends on surface requirements (Perfectly smooth one or with a slight roughness).

Once a surface is prepared, you may start painting. The method of a ceramic pattern application by overglaze paints on the surface made of glass-forming composition is not different from the method of a pattern application on an enamel base. This method requires the same work sharpness and clarity. The basic requirement at the use of non-traditional compositions is the production of a palette with ceramic overglaze colors. Each new foundation requires a new palette preparation, temperature, time and the number of firings should coincide with the terms of finishing product firing.

The possibilities of unconventional materials use for the art enameling are studied insufficiently and open broad prospects for future studies. But we may safely state that the use of glass materials as art enamels which were not originally designed for this purpose is justified and enables the obtaining of not only new decorative effects, but also provides some interest from a technological point of view.


REFERENCES

1. Brepol E. Artistic enameling. / Erhard Brepol; - L. M.: "Engineering" 1986 - 127 p.

2. V.V. Vargin. Enamel technology and the enameling of metals. / V.V. Vargin - M. "Stroyizdat" 1965. - 316 p.

3. Gilodo A.A. Russian enamel of XIX-XX centuries. / A.A. Gilodo - M.: 1996. - 196 p.

4. Latsetti A.G. The production of art glass. / Latsetti A.G. Nesternko M.L. - M.: 1986.

5. Nekrasova E.N. Azure and gold of Limoges. The exhibition catalog. / E.N. Nekrasova - SPb.: "State Hermitage Publishing House" 2009 - 182 p.

6. Essay on the origin and the development of enamel craftsmanship in respect of glassmaking and ceramics. Proceedings of the Research Scientific Institute of Ceramics. Vol. 3. - L.: 1926.

7. A.A. Pouparev. Art enamel. / A.A. Puparev - M.: 1948. - 57 p.

8. Russian enamel of the XII-th - the beginning of XX-th century from the State Hermitage collection. L. "The artist of the RSFSR" 1987 - 258 p.

9. Jewelry enamels. Specifications. - M.: The Ministry of Light Industry (RSFSR), 1980

10. http://hotemal.ru

© A.U. Emelyanov, E.V. Emelyanova, D.V. SMOLNYAKOV, T.N. Fedyaeva 01.10.2014