FOIL, a thin bar of elastic steel, mounted as a rapier (q.v.), but without a point, and additionally blunted at the end by the presence of a button covered with leather. It is used in fencing (q.v.).
FOIL (from folium, a leaf), a general name for thin metal intermediate in thickness between leaf-metal, such as gold, silver, and copper leaf, and sheet-metal.
There are two distinct kinds of F. in common use—the tin-foil used for silvering looking-glasses, lining tea-caddies, and other similar purposes, and for the conducting coatings of electrical apparatus; and time bright foils employed by the jewelers for back ing real or artificial gems, and thereby increasing their luster or modifying their color.
The former is made by rolling out tin, or, more recently, by the method of Mr. Winishurst. who casts a eylindef of the metal, and then, by means of a knife or cutter, shaves it into a sheet as the cylinder rolls to the knife, which is gradually moved inwards towards the axis of the cylinder at a rate proportionate to the required thickness of the sheet.
The bright F. used by jewelers 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 jewelers. The metal is rolled in a flatting mill, and the requisite brilliancy of surface is produced by finishing between burnished rollers and polishing. The various colors are produced by coating the white metal with transparent colors mixed in isinglass size. A similar varnish without color is laid over the white F., to prevent tarnishing. The socket or setting in which the stone or paste is mounted is lined with the F., and by reflecting from the internal facets the light which passes through the stone, adds considerably to its brilliancy. The natural colors of real stones are sometimes heightened or modified by colored F., and factitious colors are thus given to the glass or "paste," as it is called, of which spurious gems are made.
There are two other methods of foiling gems, distinct from the above: one of them is to line the socket of the setting with tin-foil, then fill it whilst warm with mercury; after a few minutes, the fluid mercury is poured out,, and there remains an amalgam of tin, precisely the same as is used for backing common mirrors; the gem is fitted into this, and thus its back has a mirror surface. The other method is to precipitate a film
of pure metallic silver upon the back of the stone, by submitting a solution of the animonia nitrate of silver in contact with the stone to the reducing action of the oils of cassia and cloves. The silvering of looking-glasses being the chief use to which the ordi nary is applied, its purity is a matter of great consideration; its employment'also by chemists, as a ready means of forming some of the tin compounds, renders this absolutely necessary.
Nevertheless, the spirit of adulteration has extended to the tin-foil makers, • and lead has been extensively alloyed with the tin. In some analyses recently made, it has been shown that as much as 85 per cent of the adulterant metal has been used, the effect of which in the process of silvering mirrors is most injurious to the brilliancy of the amal gam, which should consist of perfectly pure tin and quicksilver. For chemical pur poses, it is now absolutely necessary to test for lead before using tin foil.
The foils used by jewelers for backing gems, consisting of small sheets of silvered copper rolled very thin, are colored with the following preparations, to suit the different gems under which they are to be placed, or for use as tinsel in the manufacture of the atrical ornaments, toys, etc. Lake and Prussian blue, and pale drying-oil finely ground with a slab and mullar—for amethyst color., Prussian blue, similarly prepared—for sapphire color. Dragons' blood dissolved in pure alcohol—for garnet color. Sesquifer rocyanide of iron and bicromate of potash, equal parts very finely ground and sifted, then ground with a quantity of gum-mastic equal to the other two ingredients, until the whole forms an impalpable powder; gradually form this into a thin paste with pure wood-spirit (pyroxylic)and preserve in stoppered bottle; when used, a portion is diluted with wood-spirit to the necessary thinness—for emerald color. Various shades of yel lowish or bluish green can be produced by varying the proportions of the two coloring mate rials. Lake or carmine ground iu solution of isinglass—for ruby color. A weak solu tion of orange shellac, sometimes tinted with saffron, turmeric, or aloes—for topaz color. Several other color-varnishes are made by similar methods for various shades of tinsel and gem foils. See SILITERLNG.