Foil, among jewellers, a thin leaf of metal placed under a precious stone, in order to increase its brilliancy, or give it an agreeable and different colour. These foils are made of either copper, gold, or gold and silver together; the copper foils are commonly known by the name of Nuremberg, or German foils. They are prepared as follows: - Procure the thinnest copper plates you can get; beat these plates gently upon a well polished anvil with a polished hammer, as thin as possible; and placing them between two iron plates as thin as writing paper, heat them in the fire; then boil the foils in a pipkin, with equal quantities of tartar and salt, constantly stirring them, till by boiling they become white; after which, taking them out and drying them, give them another hammering, till they are made fit for your purpose; however, care must be taken not to give the foils too much heat, for fear of melting, nor must they be too long boiled, for fear of attracting too much salt. The manner of polishing is as follows: - Take a plate of the best copper, one foot long and about five or six inches wide, polished to the greatest perfection; bend this to a long convex, fasten it upon a half roll, and fix it to a bench or table; then take some chalk, washed as clean as possible and filtered through a fine linen cloth, till it is as fine as you can make it; and having laid some on the roll, and wetted the copper all over, lay your foils upon it, and with a polishing stone and the chalk.
polish your foils till they are as bright as a looking glass: after which they must be dried, and laid up secure from the dust.