This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
Rough files are oftener made of inferior steel than smooth ones, and if the metal is not capable of properly hardening in ordinary water, salt water is used; and if an extraordinary hardness is requisite, the file may be hardened in mercury. Rough files are often softer than they should be, to prevent their teeth breaking off during use; this should be remedied by forming the teeth so that they shall be inclined at a proper angle to the file's broad sides, and by properly polishing the sides previous to forming the teeth; smooth teeth are more durable than rugged ones, and teeth having smooth extremities cannot be produced if the blank sides are not smooth. The cutting sides of a file must be convex, and to obtain this form the middle of the file is made thickest. The convexity of one side of a flat file is destroyed if the tool bends much in hardening, and if found to be thus bent, it is heated to dull red and hammered with a wood hammer while lying across a wood block having a concave face; this hammering is equally administered along the entire length to avoid forming crankles, after which it is heated to redness and hardened. Half-round files are always preferable if the half-round sides are convex and the point very much tapered.
A rough file which is made of soft steel that cannot be properly hardened, is improved by heating it to a bright red and rolling it in a long narrow box containing powdered prussiate of potash; the file is then held in the fire a few seconds until the powder attached is melted, when the work is cooled in water. The tangs of files are not hardened, or, if hardened, are always made quite 6oft afterwards, to prevent them breaking while in use.
In order to crank the tang of a file without softening its teeth, it is necessary to bind a couple of thick pieces of iron to that portion which adjoins the tang, and to heat the tang as quickly as possible by putting it through the hole of a thick iron ring which is at near welding heat; this ring is narrow enough to allow the greater part of the tang's length to extend beyond the hole, by which means the thick portion in the hole is heated to redness while the thin end remains black. When the proper heat is thus obtained, the first bend to commence the cranking is made by bending the work while in the hole, if the hole is small enough; if not, the bending is performed on the anvil edge. The situation of the first bend is near the file's teeth, and the second bend nearer the tang's point is afterwards easily made, because it is not necessary to heat the tang in its thick part.
File handles frequently slip off through the tangs being too taper: this is remedied by grinding and filing the tang at its thickest end, without heating it and thinning it on an anvil, especially if the file is a good one. Handles also slip off through their holes being of a wrong shape, resulting from using one handle for several files. The proper mode of fitting a handle to a tang consists in making a small round hole which is nearly as deep as the length of the tang, and next shaping the hole to the desired form by burning out the wood with the tang; for this purpose it is heated to a bright red at the point, and a dull red at the thick part; it is then pushed into the handle, and allowed to remain in a few seconds, when it is pulled out and the dust shaken from the hole; the tang is then again heated and put the same way into the hole, to obtain the proper shape. One heating of the tang is sufficient, except it happens that the round hole were too small or too shallow, when two or three burnings may be necessary.
In order to avoid the danger of softening a good file, it is proper to use the tang of an old file, observing that its shape is similar to that of the tang to be fitted.