Foil (Lat. folium, 'a leaf') consists of metal reduced to very thin sheets, intermediate between the extremely thin leaf metal, such as gold and silver leaf, and sheet metal. There are two distinct classes of foil in common use—the foil which in tenuity approaches leaf metal, and the much stouter tinsel foil used by jewellers, and for theatrical ornaments, &c.
Tinfoil is made by rolling bars of tin down to fine sheets, which are further attenuated by being laid in piles and beaten with a wooden mallet. Formerly tinfoil was very largely used for the 'silvering' of mirrors, by amalgamation with mercury; but that process is now little employed (see MIRROR). The chief consumption of tinfoil is now in connection with the wrapping up of chocolate and other confectionery, and of tobacco and other products which must be kept from drying in the air. It is also largely used for lining small boxes and cases, and in the preparation of Leyden jars and other electrical apparatus.
The bright foil used by jewellers and for theatrical and other ornaments, under the name of 'tinsel,' is made of copper, tin, tinned copper, or silvered copper. The last is now chiefly used by jewellers. The metal is rolled in a flattening mill, and the requisite brilliancy of surface is produced by finishing between burnished rollers and polishing. The various colours are produced by coating the white metal with transparent colours mixed in isinglass size. A similar varnish without colour is laid over the white foil, to prevent tarnishing. The socket or setting in which a stone or paste is mounted is lined with the foil, which, by reflecting from the internal facets the light that passes through the stone, adds considerably to its brilliancy. The natural colours of real stones are sometimes heightened or modified by coloured foil, and factitious colours are thus given to the glass or 'paste,' as it is called, of which spurious precious stones are made.