Red enamel on silver

Hi

In some old enamelled silver ornaments certain red enamels get a
black then yellow edge (often quite attractive) - presumably due to
interaction between the enamel and the silver. Do any of you
enamelling folk know what the colouring oxide is that causes this. I
thought it might be selenium/cadmium, but some examples seems too
early for this? The earliest examples I’ve seen so far are about
1900 - I don’t think the selenium/cadmium reds came in for another
decade after that.

Any thoughts or comments gratefully received

Thanks
Jack Ogden

Most early reds (and blues too strangely - even a few yellows and
greens I’m told(?)) are from gold.

Tony Konrath
Key West Florida 33040