Ever wonder how to sharpen a file? Tiny little gnomes with even tinier little stones to sharpen all those little ridges? Nope, acid.

THE use of acids in the restoration of worn-out files is ancient; but in view of the immense number of these used up in this country, the following precise formula, translated from the Chemiaches Centralblatt, is worthy of record. A public report says upon this subject:

Some idea may be formed of the file business of the United States, when it is shown that from four to five millions of dollars worth of files are annually imported, and from five to six millions are manufactured here, and large as the supply is, it is not sufficient to supply the demand. From fifty to one hundred thousand dollars worth are used in some large iron-work shops, as well as government and railroad machine shops, yearly.

The files are first washed with a hot lye of soda, and all grease removed with brushes. They are then suspended in a mixture of concentrated nitric acid with eight parts by measure of water for 25 minutes; then well cleaned in water with brushes, and reimmersed for 25 minutes more in the acid mixture with addition of another eighth of strong acid. Brush again and reimmerse, after adding to. the bath a sixteenth part of concentrated sulphuric acid. This heats the bath, and the etching proper now commences, and is kept up for three minutes, with a vibratory motion of the bath. Wash and brush and reimmerse in a bath similar to the last with similar agitation for five minutes. Wash repeatedly with water, then with milk of lime, finally rinse again with water, dry quickly at a gentle heat and varnish, while still warm, with oil.
American Gas-Light Jour,sal.