Firescale

Firestain can be largely prevented by heating the object in an atmosphere in which the oxygen has been replaced with another combustive gas such as hydrogen or ammonia.

Chemical agents called fluxes can reduce firestain when applied to metals before heating.

[2] Techniques such as polishing, sanding, grinding, filing, and wire brushing can be used to cure existing firestain.

[3] Other approaches include electroplating the object with a layer of the principal metal of the alloy, as well as, for sterling and similar grades of silver, depletion silvering the piece, and for gold-copper alloys, a sodium dichromate pickle solution with a low percentage of sulfuric acid has been occasionally found effective.

The only long-term techniques to prevent the appearance of firestain are through its time-consuming physical removal or preventing its formation in the first place, by using an oxygen-free environment (which is often expensive and requires specialized equipment), or by using an alloy which is resistant to firestain such as argentium.