The reverse of the cross with silver and gold reflections has an almost entirely niello surface: in the hollows of a decoration previously engraved on a metal plate, a black powder, niello, composed of a mixture of different metal sulphides (copper, silver, lead and borax) is deposited and then heated. Once the fusion is complete and the material has solidified, the whole is polished to make the surface uniform. The color of the niello varies from black to bluish gray depending on the dosage of sulphides.
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