There are divers of these mixtures, of which one is alwaies better than the other, which by how much ’tis the harder, by so much the glass is the better; and by how much the harder the metal is, by so much the better it is polisht; nor doth the hardness of the mixture suffice, but its whiteness is also required: for red proceeds from too much copper; black from too much iron, or duskie from too much tin, and doth not make the true representations of things, but changeth the shape and colour of them: for example sake, too much copper rendereth the Species redder than they are to be, and so of the rest; let therefore the metallick mixture be very white; but if burning glasses are to be made, it is no matter what colour it be of, if so be that the mixture be hard. I will set down one of the best, ℞. of Copper plates the thinest beaten to pieces one part, of white Arsnick a quarter part; first moisten the plates with the liquor of the salt of Tartar, and make a Stratum super Stratum, with plates and Arsnick powdred, by sprinkling this on them, until the crucible be filled; to which pour the oyl of Linseed, as much as sufficeth to cover the copper and Arsnick; which done put on the cover with the best lute, then place the crucible (the lute being dryed) in sand, so that only the upper part of the cover may stick out and administer fire by degrees, at first little; secondly somewhat stronger, till at length it be hot, that all the oyl may evaporate; in the mean time, the oyl will prepare the copper, and retain the Arsnick, and will make it enter into the plates, like oyl piercing dry Leather: Or place the crucible upon a grate and put Fire to it, which administer by degrees, until the oyl evaporate in the boyling. Lastly, when it shall coole, break the crucible, and thou shalt find the copper of diverse colours, especially if thou shalt take Orpin in stead of Arsenick, and twice or thrice increased in magnitude, and brittle.