In making plique a jour jewels, I prefer 18k gold as my choice of metal. But as you have seen other karats of gold can be use as well as, copper, sterling silver and fine silver. All of which except the pure metals, oxide when firing the enamels in place.
To deal with the oxidation you can deplete sterling silver by using sulfuric acid and nitric acid for gold. I use nitric acid to deplete the 18k gold in most cases before enameling. But once in a while it is necessary to do so after enameling in the technique of plique a jour. I have started a list of the enamels that are not affected by the acid. By the way if there is a B in front of the number it is Bovano.
If you are using enamels that do not take well with acids, deplete the gold first in nitric. I heat the nitric in a beaker just to a slight steam, and dip the heated jewel into it and the oxides dissolve quickly. I usually repeat this until the jewel can be heated in the kiln and produce no oxides.
It very very important to wear protection!! Nitric is dangerous. I work outside and wear acid gloves and goggles and a respirator. Also on the table is a fan blowing the fumes away from myself and the studio.
Colors in Nitric
A105B When dipped in nitric acid in a quick bath after enameling it dulls the shine and pits the surface. When fired the dullness get worse.
BOVANO #3 HOLDS UP REAL WELL IN NITRIC ACID.
B 518 This enamel was deteriorating, so I put a few drops of nitric acid in the 518, rinsed well and tested the firing of, with no effects except it harden the enamel.Which makes it a longer fire.
Ninomiya #N3, does not work well when washed with Nirtic before enameling.
B 610 Great
N G 338 Great
NG 302 Great
B 209 Great
One thing I figured out this week when using colors that are affected by the acid is to add a layer of Bovano’s # 3 over the color and all is fine.