The top or first layer is black and is called cupric oxide. The layer that is typically under that layer is reddish and is called cuperous oxide. It is this red layer that is called "fire scale".
Speaking of which … has anyone ever deciphered Oppi Untracht’s
descriptions of “firescale” and “firestain” ("Jewelry Concepts &
Technology, p. 416-417)? As I read it, his definition of “fire
scale” includes both cupric and cuprous oxide, with the primary
distinction being how deep the layer of scale goes.
That’s fine and good except that his definition of “firestain"
sounds exactly like what he has already defined as the deeper of the
two layers of “firescale” in the section above. And nowhere does he
give the chemical makeup of what he calls “firestain” as opposed to
"firescale.” Furthermore, everything he says about "firestain"
applies to what I’ve always considered to be “firescale,” i.e.,
Are these two sections just poorly written or are there actually two
kinds of “firescale” as distinct from “firestain”? I tend to think
it’s the former (no disrespect intended – it’s an amazing book).