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Originally Posted by Battara
Be very careful with the niello - a type of oxidation easy to take off and not to place back on. In fact, I'm not sure if anyone does niello anymore since the chemical fumes are very toxic.
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Niello is a horrible mix of lead, and a little copper. When it's thoroughly mixed and still completely molten, a handful of powdered sulfur is mixed in, stirred, and poured into long thin (1/4") strips and allowed to cool.
Once a decorative design is chiseled into a piece of metal, the metal is heated and the niello strip is "mushed" into the design leaving excess. Or, it can be ground into a fine powder and applied like vitreous enamel and then fired. Allow to cool, and file the excess away, using a coarse single cut file. It's like filing graphite, soft, with a tendency to chip if you rush it. Polish any file marks polish with emery.
I did some of this while a student forty years ago, under a laboratory vent hood, and, believe me, it's a smoky, sulphurous, stinking mess. Glad I did it once or twice, but I don't think I ever want to do it again.
There are many formulae recorded, each with slightly different compositions and working attributes coming from early sources from around the world, but, I post this here for reference and not as a recipe, so unless you have laboratory ventilation, don't do it.
If repairs are absolutely necessary I would look into using an epoxy based mixture instead.