+ They are called steel mirrors because in the past they were usually made of steel polished with emery paste. But more easily, various kinds are made of cuivre franc, which is rosette and tin, because these can be cast in a mold and made round, hollow, convex or in whatever various shapes you want to represent.
So take half rosette copper and half soft tin, i.e. fine and yet unused. Place them in a crucible, and first melt the copper. Once it has been well melted, add the tin and mix them together and cast that in a white stone mold with no hole, and grease the mold, which should be moderately warm, with oil. Then, once your mirror has been molded, you can polish it in this manner.
Set one mirror in plaster to hold it firmly, then put the other one over it with fine sand between them, and rub one against the other, whether they are hollowed or flat, and thus you will polish both at once. If you want to polish them on both sides, you only have to switch them, which means putting the one that was polishing in the plaster and [using] the one which was in the plaster to polish. Once they have been polished with you can soften them with tripoli of Venice that should not be sandy, and then with paste. You use water with arene to polish, but polishing with tripoli and paste is done dry. Once the mirror has been polished, you can set it.