in glas-making, a substance which serves to colour glas.

This is made by a simple calcination of copper, but it serves for several colours: there are two ways of making it. The first is this. Take thin plates of copper, and lay them on a layer of powdered brimstone, in the bottom of a crucible; over these lay more brimstone, and over that another layer of the plates, and so on alternately till the pot is full. Cover the pot, lute it well, place it in a wind furnace, and make a strong fire about it for two hours. When it is taken out and cooled, the copper will be found calcined, that it may be crumbled to pieces between the fingers like a friable earth. It will be of a reddish, and, in some parts, of a blackish colour. This must be powdered and sifted fine for use.

Another way of making ferreto is as follows. Make a number of stratifications of plates of copper and white vitriol alternately in a crucible; which place on the floor of the glas furnace near the eye; and let it stand there three days; then take it out, and make a new stratification with more fresh vitriol; calcine again as before. Repeat this operation six times, and a most valuable ferreto will be obtained.