The formulas used by the Arabs and their Italian successors are partly disclosed in manuscripts in the British and South

Kensington Museums; two are given below:

Arab Italian

Copper sulphide...... 26.87       24 . 74

Silver sulphide........ 1.15         1.03

Mercury sulphide.........         24.74

Red ocher............. 71.98      49.49

These were ground with vinegar and applied with the brush to the already baked enamel. A great variety of iridescent and metallic tones can be obtained by one or the other, or a mixture of the following formulas:

I II III IV V VI Copper carbonate.. 30 .... 28 .. 95

Copper oxalate............ 5 . .

Copper sulphide.......20 .. ....

Silver carbonate..... 3 .. 2 1 5

Bismuth subnitrate. . . 12 . . . . 10

Stannous oxide........25......

Red ocher......... 70 85 55 70 84 . .

Silver chloride and yellow ocher may be respectively substituted for silver carbonate and red ocher. The ingredients, ground with a little gum tragacanth and water, are applied with a brush to enamels melting about 1814° F., and are furnaced at 1202° F. in a reducing atmosphere. After cooling the ferruginous deposit is rubbed off, and the colors thus brought out.

Sulphur, free or combined, is not necessary, cinnabar has no action, ocher may be dispensed with, and any organic gummy matter may be used instead of vinegar, and broom is not needed in the furnace. The intensity and tone of the iridescence depend on the duration of the reduction, and the nature of the enamel. Enamels containing a coloring base—copper, iron, antimony, nickel — especially in presence of tin, give the best results.