POVRCHOVÉ ÚPRAVY KOVŮ A JEJICH SLITIN
SMALT / EMAIL
|ÚVOD TIRÁŽ KONTAKT|
SMALT V TECHNICKÝCH OBORECH A PRŮMYSLU
Enamel is a glass-like coating fused to a metal base. Enamels are applied in the form of water suspensions or powder mixtures comprising pre-melted frit (e.g. silicate glass) and other additives. A coating formed like this protects the metal against corrosion, even under high temperatures, improves its chemical resistance, is smooth and glossy, and performs a decorative function.
Enamel is a type of non-metallic, inorganic coating. Its protective function comprises the formation of an insoluble, solid layer deposited on the base metal, which isolates the metal from the action of an aggressive environment. Enamels are essentially
a type of glass with modifiable properties produced from glass-forming oxides (e.g. quartz, boric acid, feldspar, kaolin, borax, sodium carbonate, etc.) and auxiliary raw materials (e.g. metal oxides and others).
Enamel coatings are applied to surfaces in the form of a suspension (slurry) by pouring, dipping, or spraying, and after drying they are fired at 800 to 950 °C. A primer and an outer layer or single-layer enamels are applied. The advantage of vitreous mineral enamels is that they are durable, have a high abrasion resistance, and possess insulating properties along with a decorative effect. Its benefit is long service life at high temperatures (typically 700 ° C, special up to 1100 ° C) and a low ecological load on the enamel coating.
For a long time, enamel was applied as a protective and decorative treatment of art and craft objects made from copper and precious metal alloys. Along with an increase in iron production and developments in enamel processing technology, enamel began to be used on iron alloy products – cast iron and steel. Until the early 19th century, enamel contained toxic impurities and heavy metals, so it was not suitable for the surface treatment of kitchen utensils. The chemist and inventor, Eduard Bartelmus, introduced important change with the production of white, non-toxic enamel for ironware. He established the first enamel factory in the Czech lands, in Brno. Some period sources point to the invention of lead-free enamelling technology as coming from Gliwic in Poland, it first being used in Lees’ enamelling plant in Frýdlant. This enamelling technology undoubtedly influenced the development of the industry and its production not only in our country, but around the world.
Enamel has found enormous application over many decades. Products with this coating can be found in every household, whether kitchen utensils, bathtubs in a wealth of shapes, colours and shades, or various devices such as washing machines, boilers, stoves, tanks, and the like. During World War I, manufacturers focused on products for the army, such as bottles, field kitchens, or sanitary ware. Around 1930, all enamel plants sold similar goods demanded by the market. The range of products made from enamelled cast iron and sheet metal was inexhaustible: jugs, mugs, pots, ladles, spoon rests, cups, lidded jars, basins, buckets, coat hangers, tea and coffee sets, candelabras, lamps, grinders, clocks, pans, griddles, baking moulds, bathroom accessories, and many more. Enamel can be found in basins, mailboxes, gas pumps etc. Enamelled plates and signs for a variety of purposes have become a special phenomenon.
Equally important is the application of enamel in industry, especially in processes that take place in corrosive environments. Thanks to its exceptional resistance to chemicals, fermentation, and high temperatures, enamelled steel is used in the production of chemical reactors, heat exchangers and columns, as well as in the food, chemical, medical and transport industries. In construction and architecture, enamel is used for building envelopes, decorative interior or exterior elements, and supporting structures, which have a lifespan of fifty or more years. The stability of glass enamel coatings under atmospheric conditions and their good sunlight absorption capability enable their use in the construction of solar collectors. The combination of mechanical strength and other advantages of metals combined with the durability of enamel ensure a high-quality and aesthetic product with superior environmental sustainability and great potential for future use.
ENAMELLING TECHNIQUES IN ARTISTIC TRADES
Cloisonné enamel / Compartment enamel
This technique employs the formation of compartments (from French cloisonné – compartment) lined with silver, gold, or copper wire or strip attached to the base material of similar metal alloy. Enamel as a powder or paste is applied to the structure of these compartments (cells). The resulting thickness of the enamel is achieved with repeated application and firing of the layers. The wires prevent diffusion of the applied enamel colours and ensure mechanical adhesion to the surface. Once the decoration has achieved a desired thickness, the wires are only visible as a fine mesh inside the enamelling. The process is completed by fine grinding and polishing to integrate the surface and enhance the enamel colours. This technique also relates to the development of filigree and the use of filigree wires to manufacture individual compartments.
The oldest surviving finds decorated with the cloisonné technique come from Mycenaean graves in Cyprus dating to the 13th to 11th centuries BC. This technique peaked during the Byzantine Empire, spreading in the Middle Ages to Western Europe and east to Russia. The enamels from the Záviš’s cross come from the 10th to 12th centuries.
Champlevé enamel / Raised field enamel
Champlevé is the second oldest enamelling technique and resembles cloisonné, but its method of execution is reversed.
The recesses are engraved or etched on the metal base, or prepared by casting into a mould. The overall result bestows a softer impression than cloisonné products. Both the metal and enamel are smoothed by honing after firing, forming a single surface. The applied metal material must be substantially thicker (about 3 to 5 mm thick) than the cloisonné. Mainly copper and its alloys (especially bronze) are used, as classic champlevé comprises mostly opaque enamels and expensive base metals would not have formed part of the decoration. It is common, however, to gild the surrounding, enamel-free areas.
Champlevé on bronze has been found at Celtic sites and was known in ancient Nubia. The greatest flourishing of this technique was seen in the Romanesque period. Reliquaries decorated with champlevé enamels represent one of the highlights in the history of enamelling as well as one of the most remarkable artistic objects of the Middle Ages.
Enamel en basse taille / Translucent enamel
Basse-taille is based on champlevé. Lines engraved in the metal are also used in this technique. This is a more advanced and exclusive version designed mainly for working with gold and silver alloys. The principle of basse-taille is the application of translucent enamel to an embossed relief on the metal surface. This procedure was used for jewellery and other, complex, three-dimensional objects. Some parts and lines in the precious metal are left enamel-free (reserves) so that the lustre of expensive material enhances the feeling of exclusivity. After application of the translucent enamel to the relief, reflections of light beautify the softness and radiance of the whole object. A flawless gloss is achieved with careful polishing of the entire enamel and metal surface. Depending on the thickness of the enamel layer, how much the base metal reflects will vary. The intensity of colour can thus be influenced by the depth of the relief.
The basse-taille technique is mainly associated with Gothic enamels, where it achieves its peak of mastery. The first reports of this technique come from Italy from the end of the 13th century. It has been used since the Renaissance mainly on jewellery, combined with pearls and precious stones. Painted miniatures came into fashion in the 17th century and the use of classic basse-taille disappeared, but its rare variation, basse-taille sur fond reserve, appeared.
En ronde bosse enamel
Ronde-bosse (meaning enamelling on high relief, or, on a rounded core) is an enamelling technique from the late Middle Ages. Its principle is in applying enamel to sophisticated, three-dimensional objects – mostly jewellery pendants, a very high relief, or stand-alone figures. Enamel is not applied to compartments but covers the surface of a whole area. These enamels resemble small, coloured sculptures. Enamel is not applied to the entire surface, and some gold or silver parts remain exposed. The surface is first roughened to allow easier application of the enamelling and for it to hold. It was necessary to add an extra adhesive, which then burned away in the furnace. The ronde-bosse technique originated at the turn of the 14th and 15th centuries in Paris and spread to many European countries in the Renaissance period.
Grisaille / Grisaille enamel
The grisaille technique is a specific variation of painted enamel applied to a copper plate without auxiliary recesses or compartments. A dark base is typical – most often black or dark blue enamel, less often green or red, to which several layers of white enamel are repeatedly applied. The aim was to achieve contrast between light and dark colours and achieve different effects of depth to the image.
The beginnings of Grisaille enamel date back into the first half of the 15th century. This new procedure is attributed to Dutch artists at the Burgundy court. The term grisaille comes from French gris (gray) and the technique is monochrome, using black and white paint and different shades of grey. The peak period when grisaille flourished was the Renaissance, in the French city of Limoges.
Émail peinte / Painted enamel
This is painting with different coloured enamels on a flat copper base, often combined with the Grisaille technique. Both processes are usually finalized with liquid gold or combined with gold or silver leaf. To achieve the best result, many layers of enamel must be applied and fired repeatedly. The development of this technique is again connected with the French city of Limoges (hence the name of Limoges painted enamel). The beginnings of painted enamels in Limoges date to the last quarter of the 15th century, but mass expansion took place in the 16th century. Workshops continued the tradition during the 17th century. Later, in the 19th century, this style returned to fashion and several imitations of historical enamels were created.
Among the most prominent artists of painted enamel were Nardon Pénicaud and his followers and family relatives, Pierre Courteys, and Pierre Reymond’s family members. Incredibly realistic portraits and mastery was achieved by Léonard Limousin, one of the most famous Renaissance artists of Limoges.
Émail des peintres / Enamel miniatures, painting on enamel
Although the technique of enamel miniatures is one of the classic, historical enamelling techniques, it does not work actively with enamel itself, but fine, coloured pigments (metal oxides) called colours. Colours are fired at lower temperatures for a shorter time than conventional enamels. The paint becomes firmly attached to the enamel base in the furnace. The same colours are used to paint porcelain. Thus, painting on enamel is very similar to contemporary painting on porcelain. Enamel here serves only as a lightly-coloured base and final, translucent, colourless layer that gives the painting gloss and durability. This is not classic painting with enamel, as with the Limoges techniques, but in fact is painting on enamel. The miniatures were very small, their maximum size usually not exceeding 15 cm.
Enamel miniatures are typical for the Baroque period, but the technique was later used in the 19th century. The most significant centres of painted enamel were Geneva and Vienna, the technique developing in Western European countries, England, France, and Germany.
Émail guilloché / Guilloché
From the point of view of enamelwork, it is a technique close to the basse-taille technique. A patterned surface also reflects through the translucent enamel. The main difference, however, is in the preparatory work with metal. In the case of basse-taille, the pattern is engraved or hammered into the surface of the base metal. In a guilloché pattern, everything is engraved by machine and the repeated pattern is symmetrically accurate. Abstract patterns such as lines, waves, spirals, chess patterns, rosettes, or similar are typical. The entire surface is finally polished to a high gloss after enamelling and firing.
The beginnings of guilloché date back to the 18th century. The masters of this technique were mainly French goldsmiths, although the centres of guilloché products were also Geneva, Birmingham, and London. Guilloché enamelling is also typical for the Russian family of Fabergé and their famous Easter eggs.
Émail plique-á-jour / Window enamel
This is a demanding enamelling technique typical on Art Nouveau jewellery. The basic principle involves the enamel being attached to a metal grid rather than a metal base, so light may shine through it and produce an effect like stained glass. The base metal is finally removed, or alternatively, the work is done completely without it. Either a thin metal sheet or mica is used as a temporary substrate when applying enamel to the grids. The temporary support is removed from the enamelled object after final firing.
The discovery of the technique is usually associated with the Middle Ages, but surprisingly, the ancient Nubians were familiar with it. It re-emerges in the second half of the 19th century and flourishes with the Art Nouveau period. The Japanese were also masters of plique-a-jour.
CONSERVATION AMD RESTORATION OF ENAMEL AND ENAMEL PAINT
Enamels and enamel paints are made from differently coloured glass-like material which is fused onto a metal plate. Although this surface treatment is generally resistant to environmental and mechanical damage, the surface is affected by various conditions and fatigue over time. Especially objects that form parts of museum collections or other memory institutions, the degree of damage to the enamel layer varies from object to object. The stability of enamels depends, in particular, on their composition, application technology, the nature of the metal plate and environmental conditions, including the ways of handling them. Whether we consider enamel as artistic decoration or functional surface treatment of consumer goods or technical equipment, both processes of glass and metal degradation occur. From the conservation-restoration point of view, it is therefore necessary to address the issue of stabilization and protection of these distinctly different materials.
Determining the variety of enamels requires knowledge of historical recipes and craft techniques that are relevant to this application. The enamel decoration is generally associated with gold and silver crafts - beating, engraving, chiselling, soldering, grinding, polishing and other procedures. Characteristic features of historical craftsmanship are often found during the restoration interventions only, after the object is disassembled, during the microscopic examination or when applying other display techniques. Conservation-restoration investigation therefore plays an important role in identifying objects and determining the extent and causes of their damage.
Although enamels seem to be an everlasting type of decoration, they are subject to degradation due to the reaction of unstable enamel components with the surrounding environment, metal corrosion, mechanical wear, and surface deformation.
In the case of technical enamels, where iron and its alloys prevail as the base material, the metal bases are more intensely degraded due to corrosion processes if the enamel layer is damaged. The treatment procedures are aimed at stabilizing the mechanism of corrosion of both glass and metal material, fixation of damaged parts and suitable surface treatment. The basic challenge of the restoration work is maximum preservation of the authenticity of the object, minimum intervention, and reversibility of the means used. An important aspect is also the provision of optimum climatic conditions for the long-term storage of enamelled items in museum repositories or exhibitions.
Audiovizuální dokument vznikl jako výstup výzkumného cíle podpořeného z prostředků Institucionální podpory DKRVO Ministerstva kultury ČR.
© Technické muzeum v Brně, 2017.