Page 254 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

but essentially calls for two-thirds silver, one-third copper, and a sixth part of lead. The partial
recipe states:
[ W]hen you have melted the silver with the copper, stir it together with a piece of charcoal,
and instantly pour onto it the lead and the sulphur from the copper crucible, and again stir
vigorously with the charcoal. Rapidly pour this into the other cast crucible over the sulphur
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which you have put in t. 1 0
Theophilus then goes on to describe how the niello is to be used in practice. He advises that
the powdered niello be carefully applied to the moistened areas to be covered and that the object
be carefully heated until the compound has melted into position. 11
Many niello preparations were made using this mixture of copper and silver sulfides, which
has great practical merit. Because pure silver sulfide decomposes below its melting point, it can
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be applied only as a solid, but f a mixture of metallic sulfides is used, a eutectic-type system is
formed, and the melting point of the mixture is greatly reduced, allowing the niello to be applied
in the molten state.
The medieval Mappae clavicula (Smith and Hawthorne 1974) contains six recipes (variously
numbered) for the preparation of niello. Three of the recipes are for a niello that can be painted
on the surface by grinding up the compound and mixing it with vinegar; the thin paste is
then applied and the vessel heated. For gold surfaces, a mixture of silver, copper, and lead sul­
fides is recommended (recipe 56) ; for gilded surfaces, a mixture of copper and lead sulfides (rec­
ipe 206); and for silver surfaces, lead sulfide alone (recipe 89B). Recipe 56 provides the following
instructions:

[M]elt together equal parts of silver, red copper, and lead, and sprinkle native sulphur over
it. When you have cast it, leave it to cool, put it in a mortar, grind it, add vinegar and make
it the consistency of the ink with which writing is done. Write whatever you wish on gold
and silver [vessels]. And when it has hardened, heat it and it will be [as f] inlaid. Melt it
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like this: carve charcoal and so put the silver and the copper in it and melt them (and while
you are heating them add in the lead, then the sulphur). When you have mixed it, pour it
out and do as was said above. (Smith and Hawthorne 1974:10)

The remaining three recipes are for making niello to be used for inlaying. The instructions
state that for application to gold and silver, the niello must contain both copper and silver sul­
fides (recipes 195 and 196), but there should be a greater proportion of silver sulfide in the niello
if it is used on gold. A pure silver sulfide is recommended for general use (recipe 58). These reci­
pes suggest that niello prepared from mixtures of copper and silver sulfides was in use long
before the eleventh century and that the use of the ternary mixture of lead, copper, and silver
sulfides can be traced back to at least the eighth century and probably to the third century.

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