49 - Sapphire Colour.
You should have rocchetta frit, and to every 100 pounds of this frit add 1 pound of prepared zaffer, as described in chapter 12. Before adding the zaffer to the frit, mix it with 1 ounce of prepared Piedmont manganese [chapter 13].
Unify the two powders together, and add them to the rocchetta frit in the above dose. Put the frit into the furnace and leave it to fuse and clarify thoroughly.
Once the glass is clarified stir it well and check to see if the colour is sufficient, or needs more or less pigment. This way the work will have the color of sapphire, a most beautiful
double violet, and this from the one small dose of manganese. I have made it many times in Pisa, and always well. However, when this color is made with pure crystal frit, it is even more marvellous; more than I can say, which is shown here below.