Discolorations after firing

25 september 2014 // Door webmaster //
28-01-2005
Discolorations after firing.
29-01-2005
  1. Discolorations occur particularly when enamel is applied to copper or tombak. They often disappear by refiring after applying another thin coat of enamel.
    During the firing process copper oxide always forms at the copper-enamel interface; it dissolves in the enamel and aids in fusing it to the metal.
    A surplus of copper oxide often leads to dark stains in red enamels, in other enamels as green stains. One should make sure that the enamel (and the glue) is absolutely dry before firing.
    In general copper oxide stains develop if enamel has been applied too thinly, so that the copper oxide cannot be properly absorbed.
  2. Residues of acids: after cleaning with an acid solution (vinegar and salt, Vitrex, Sparex nr. 2, vitriol, sulfuric acid or nitric acid) the metal was not properly washed down with water and not subsequently neutralized (boil 5-10 minutes in 5% soda, then rinse thoroughly).
  3. The enamel was still damp. During firing water (damp enamel or glue) binds to copper and copper oxide is formed. The excess oxide is not dissolved and shows as a stain in the enamel. In general opaque enamels need not be washed prior to use.
  4. Enamel was applied irregularly, and hence too thinly in places. If it was applied so thinly that copper was exposed this gives rise to the formation of black fire scale: remove down to shining copper, clean it and reapply enamel. If enamel was applied too thinly, add a thin coat and fire again: usually the stains disappear. Keep in mind that if enamel was applied by wet packing with a lot of water, the enamel coat is often quite thin.
  5. Copper is somewhat less suited for use with dark transparent enamels. Stains due to oxides and discolorations are best prevented by applying a colourless flux first.

See also: How do I prevent white enamel from displaying greenish discoloration on copper?

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