Niello

Niello is a black mixture, usually of sulphur, copper, silver, and lead, used as an inlay on engraved or etched metal, especially silver. It is added as a powder or paste, then fired until it melts or at least softens, and flows or is pushed into engraved lines in the metal. It hardens and blackens when cool, and the niello on the flat surface is polished off to show the filled lines in black, contrasting with the polished metal around it.

It may also be used with other metalworking techniques to cover larger areas, as seen in the sky in the diptych illustrated here.
The metal where niello is to be placed is often roughened to provide a key.
In many cases, especially in objects that have been buried underground, where the niello is now lost, the roughened surface indicates that it was once there.

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Source: Wikipedia · Image: Wikimedia Commons · CC0