What Is Vitreous Enamel? Glass Fused Onto Metal

Enamel is genuine glass, chemically similar to window glass and modified to do a specific job. The base is silica with fluxes, typically sodium, potassium, boron and lead in historical work, lowering the melting point so the enamel fuses well below the melting point of the metal beneath. Opacifiers such as tin oxide, titanium or zirconium compounds make it opaque rather than transparent by scattering light within the layer. Colour comes from metal oxides dissolved or suspended in the glass, with cobalt giving blue, copper green or turquoise, iron browns, manganese purple, chromium green, and gold producing a deep red through suspended colloidal particles. Adhesion oxides, historically cobalt and nickel, are included specifically to make the glass grip steel. The mixture is melted, quenched into water so it shatters, and milled to a fine powder called frit, which is applied wet as a slurry or dry by dusting or by electrostatic spraying in industry.

The join between glass and metal is chemical rather than mechanical: during firing the molten enamel reacts with the metal's oxide surface, forming a transition layer, which is why surface preparation and the presence of adhesion promoters matter so much. The vulnerability is that glass and metal expand by different amounts when heated. A coating designed so that it ends up slightly in compression at room temperature is durable, because glass is very strong in compression, while one left in tension will craze and flake. The same mismatch means enamel chips when the metal beneath is dented or flexed, since the coating cannot deform, and it fails at edges where the layer is thin. This is also why enamelled cast iron cookware should not be subjected to sudden temperature changes and why a chipped enamel pan exposes bare metal that then rusts. Counter-enamelling, coating the reverse side as well, balances the stresses and prevents warping on thin work.

Artists developed several methods of controlling where each colour sits, and each has a distinctive appearance:

• •Cloisonne, in which thin metal strips are soldered to the surface to form compartments that are filled with different colours, so the wires remain visible as bright lines in the finished piece
• •Champleve, in which recesses are carved or cast into a thick metal base and filled, leaving broader areas of exposed metal between them
• •Basse-taille, where a low relief is carved into the metal and covered with transparent enamel, so the depth of colour varies with the depth of the carving and the design glows from beneath
• •Plique-a-jour, in which the compartments have no backing at all, producing a miniature stained glass window that light passes through, which is extremely difficult because the unsupported enamel must survive firing
• •Painted or Limoges enamel, where the piece is coated and then painted with fine detail in enamel and fired repeatedly, allowing pictorial work approaching the freedom of painting
• •Grisaille, a painted technique building an image in white over a dark ground

Enamel moved from the jeweller's bench to the factory in the nineteenth century once enamelling steel rather than precious metal became practical, and its properties made it the obvious surface for anything needing to be cleanable, weatherproof and durable. It coated cooking vessels, baths, sinks, cookers and washing machine drums, since the glass surface resists food acids and cleans easily; it made outdoor signage that holds its colour for decades in sunlight where paint fades, which is why old enamel advertising signs survive in legible condition and are collected; it lines chemical reaction vessels and storage tanks because the glass resists almost every acid; and it appears on architectural panels and on transport systems where the maintenance interval matters more than the installation cost. The recurring limitation is the same one that governs the art: it is brittle, chips on impact, and once chipped it cannot be repaired without refiring the whole piece.

Vitreous enamel is powdered glass fused onto metal at around eight hundred degrees, bonding chemically through a reaction with the metal's oxide surface. Colour comes from metal oxides and opacity from scattering particles such as tin oxide. The coating must be designed so it ends in compression, since a mismatch in expansion causes crazing, and it chips wherever the metal beneath deforms. Cloisonne, champleve, basse-taille and plique-a-jour control where the colours sit, and industry uses the same material for cookers, signage and chemical vessels.