Description / Application
Vitreous enamel, also porcelain enamel in US English, is a material made by fusing powdered glass to a substrate by firing, usually between 750 and 850 °C. The powder melts, flows, and then hardens to a smooth, durable vitreous coating on metal, or on glass or ceramics. The term “enamel” is most often restricted to work on metal, which is the subject of this article. Enamelled glass is also called “painted”. Fired enamelware is an integrated layered composite of glass and metal. The word enamel comes from the Old High German word smelzan (to smelt) via the Old French esmail. Used as a noun, “an enamel” is a usually small decorative object, coated with enamel coating. Enameling is an old and widely-adopted technology, for most of its history mainly used in jewelry and decorative art. Since the 19th century the term applies also to industrial materials and many metal consumer objects, such as some cooking vessels, dishwashers, laundry machines, sinks and tubs, etc. “Enamelled” and “enamelling” are the preferred spellings in British English, while “enamelled” and “enameling” are preferred in American English.
Enamel may be transparent or opaque when fired; vitreous enamel can be applied to most metals. The great majority of modern industrial enamel is applied to steel in which carbon content is controlled to prevent reactions at the firing temperatures. Enamel can also be applied to copper, aluminium, stainless steel, cast iron or hot rolled steel, as well as gold and silver.
Vitreous enamel has many excellent properties: it is smooth, hard, chemically resistant, durable, scratch resistant (5-6 on the Mohs scale), long-lasting colour fastness, easy-to-clean, and cannot burn. Enamel is glass, not paint, so it does not fade with UV light. Its disadvantages are its tendency to crack or shatter when the substrate is stressed or bent, but modern enamels are relatively chip and impact resistant because of good thickness control and thermal expansions well-matched to the metal. The Buick automobile company was founded by David Dunbar Buick with wealth earned by his development of improved enameling processes, circa 1887, for sheet steel and cast iron. Such enamelled ferrous material had and have many applications: early 20th century and some modern advertising signs, interior oven walls, cooking pots, housing and interior walls of major kitchen appliances, housing and drums of clothes washers and dryers, sinks and tubs cast iron bathtubs, farm storage silos, and processing equipment such as chemical reactors and pharmaceutical process tanks. Structures such as filling stations, bus stations and Lustron Houses had walls, ceilings and structural elements made of enamelled steel. One of the most widespread modern uses of enamel is in the production of quality chalk-boards and marker-boards (typically called ‘blackboards’ or ‘whiteboards’) where the wear and chemical resistance of enamel ensure that ‘ghosting’ or unerasable marks will not occur, as with polymer boards. Since standard enameling steel is magnetically attractive, they may also be used as magnet boards. Some new developments in the last ten years include enamel/non-stick hybrid coatings, sol-gel functional top-coats for enamels, enamels with a metallic appearance, and new easy-to-clean technologies.
The key ingredient of vitreous enamel is a highly friable form of glass called frit. Frit is typically an alkali borosilicate chemistry with a thermal expansion and glass temperature suitable for coating steel. Raw materials are smelted together between 1.149 and 1.454 °C into a liquid glass that is directed out of the furnace and thermal shocked with either water or steel rollers into frit.
Colour in enamel is obtained by the addition of various minerals, often metal oxides cobalt, praseodymium, iron, or neodymium. The latter creates delicate shades ranging from pure violet through wine-red and warm grey. Enamel can be transparent, opaque or opalescent (translucent), which is a variety that gains a milky opacity with longer firing. Different enamel colours cannot be mixed to make a new colour, in the manner of paint. This produces tiny specks of both colours, although the eye can be tricked by grinding colours together to an extremely fine, flour-like powder.
There are three main types of frit, usually applied in sequence. A ground coat is applied first; it usually contains smelted-in transition metal oxides such as cobalt, nickel, copper, manganese, and iron that facilitate adhesion to steel. Next, clear and semi-opaque frits that contain little colouring material for producing colours are applied. Finally, a titanium white cover coat frit, supersaturated with titanium dioxide which creates a bright white colour during firing, is applied as the exterior coat.
After smelting, the frit needs to be processed into one of the three main forms of enamel coating material. First, wet process enamel slip (or slurry) is a high solids loading product of grinding the frit with clay and other viscosity-controlling electrolytes. Second, ready-to-use (RTU) is a cake-mix form of the wet process slurry that is ground dry and can be reconstituted by mixing with water at high shear. Finally, electrostatic powder that can be applied as a powder coating is produced by milling frit with a trace level of proprietary additives. The frit may also be ground as a powder or into a paste for jewelry or silk-screening application.
Industrial Enamel Application
On sheet steel, a ground coat layer is applied to create adhesion. The only surface preparation required for modern ground coats is degreasing of the steel with a mildly alkaline solution. White and coloured second “cover” coats of enamel are applied over the fired ground coat. For electrostatic enamels, the coloured enamel powder can be applied directly over a thin unfired ground coat “base coat” layer that is co-fired with the cover coat in a very efficient two-coat/one-fire process.
The frit in the ground coat contains smelted-in cobalt and/or nickel oxide as well as other transition metal oxides to catalyze the enamel-steel bonding reactions. During firing of the enamel at between 760 to 895 °C (1,400 to 1,643 °F), Iron Oxide scale first forms on the steel. The molten enamel dissolves the iron oxide and precipitates cobalt and nickel. The iron acts the anode in an electrogalvanic reaction in which the iron is again oxidized, dissolved by the glass, and oxidized again with the available cobalt and nickel limiting the reaction. Finally, the surface becomes roughened with the glass anchored into the holes.
Shipment / Storage / Risk factors
Care is necessary in the use of the expression ‘flaking or chipping’ as the cause of damage to enamelware. ‘Flaking’ is recognised as being due to some peculiarity in the enamel or the metal by which small portions of the enamel do not adhere to the metal, so that on cooling they flake off. Loss of value, if arising out of such a cause, would not ordinarily be recoverable under the terms of the insurance cover. Such damage is usually discovered before goods are packed and is not normally found at destination. Care should therefore be exercised in the examination of the article to determine whether the cause of damage be due to ‘flaking’ (faulty manufacture) or chipping due to shock, rough handling or other cause.
Vitreous or porcelain enameled cast iron baths – Shipped nested in crates with corrugated or mottled cardboard manufactured by the sulphite process between. If exposed to moisture, the sulphite is released and has a bleaching effect on the colour pigment in the porcelain, leaving a pattern which cannot be eradicated. The pattern disappears when the bath is wet but reappears as it dries. Scratching and chipping are sometimes found to be greater when each unit is individually cased. On occasion, improvement in the outturn condition has been observed when larger cases are used containing a greater number of units.