July 18, 2009

Vitreous Enamels Coating

Vitreous enamels are essentially glasses, that is solidified mixtures of fused sodium and other silicates. Often with the addition of boro-silicates and phosphates. The chemical resistance of the glass, and hence of the enamel, its co-efficient of thermal expansion, and its softening point can be varied by additions of the silicates of calcium, magnesium, manganase, lead, cadmium and tin, the formulation of a glass for a particular use in vitreous enamelling is a complex exercise in compromise.

In the preparation of vitreous enamel, the ingredients of the glass are fused and coated together in a crucible, and are then cast into water, where the glass shatters into fragments. The glass needs to be design with a view to a suitable melting or flowing temperature, and also so that its coeficient of thermal contraction is similar to, if not identical with, that of steel. The glass fragments are then ground in pebble or ball with water to a fine powder of slurry are sieved.

Iron and steel parts must be freed from scale by pickling before enamelling, and must then be thoroughly dried, but a small amount of superficial oxide is not harmful. There are two main methods of applying vitreous enamel. The most commonly used, and that universal for sheet steel product, is a wet process. A carefully formulated charge of glass frit, opacifying oxides-usually titanium oxide (rutile), or tin, antimony or zirconium oxides (which are all white) -together with coloring oxides and suspending agents are milled with water in a pebble mill to give a thin cream like slurry, which is called the slip. The coloring materials are cadmium, iron or selenium compounds or oxides for reds, chromium compounds, for greens, cobalts compounds for blues, and antimony, vanadium and tungsten compounds for yellows. The available color are rather limited and perhaps crude.

The steel articles are then coated with the slip all over by spraying or somtimes by dipping. The suspending agents added to the slip ensure that it is a colloidal suspension which can be uniformly spread. The article is meanwhile supported on a few spikes called perrits, the marks from which will not be obtrusive. It is usually to dry off the slip, leaving the article covered will a thin film of dry, but loosely adherent mud before it is fired.

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