This instrument is composed either of iron or forged steel, cut by means of a chisel and mallet, in small furrows of various depths, and in different directions, according to the grain or touch required. After being thus cut, it is tempered with a very hard and dry soot, which is diluted and worked up with urine, vinegar, and salt, to the consistence of mustard. The process of tempering consists in rubbing the files over with this preparation, covering them with loam, and then placing them in a charcoal fire, whence they are removed as soon as they become red-hot. Immediately after being taken out, they are immersed into cold spring water ; and, when cold, cleaned with charcoal and a rag; after which operation, they are laid up in bran, to prevent them from becoming rusty.
Files are of different forms, sizes, cuts, and degrees of fineness, in proportion to the various uses and occasions for which they are designed ; such are the common square, flat, triangular, or round files; the rough-tootlicdfiles, which are intended to cut more speedily than any other; and thefine-tooth ed file, which cuts more slowly, and is appropriated to finer work-manship.—The best and most durable instruments of this description are manufactured at Sheffield.