20 - To Calcine an Orpiment Called Tinsel, Which in Glass Makes the Celestial Color of the Blue Magpie
As is very well know, the orpiment known as tinsel is composed of copper, which from calamine [zinc silicate] takes a tint in colour similar to gold. The calamine not only tints the copper, but incorporating it adds quite a bit of weight. This augmentation, when it is well calcined, gives glass a colour that is quite delightful to see, holding the middle between aquamarine and the colour of the sky when it is very clear and serene; a thing of beauty. You must be very diligent in calcining it, so here is the way to make it step by step.
Take orpiment, also known as tinsel, and to save money purchase some that has already been used for decorative wreaths and garland. Cut this into small pieces with scissors, and then put it into covered chamber pots coated with clay, amongst the coals in a strong fire. I put it in the burning coals of the furnace on the side where it is stirred up, and I leave it be for 4 days in a large fire, but not hot enough to fuse, because if it melts, all the work will be lost. After the prescribed time it will be calcined quite well.
Next, I grind it into powder and pass it through a fine sieve, then I mill it over a very fine porphyry stone, at which point it becomes a black powder. I then spread it in oven pans and hold it in the annealing chamber close to the 'eye' for 4 days. Remove the ash that accumulates on top, pulverize, sift, and store it for use.
The test for good calcination is that when sprinkled into the glass melt it causes [the glass] to swell quite a bit. If it does not make the glass swell and boil vigorously, this is a sign of poor calcination or too much burning, in either of these cases, if it does not make the glass boil it will not colour well. Therefore, this is a warning for all to heed in the practice.