To Temper Small Steel Articles
American Watchmaker and Jeweler (1892)
Important: Safety Warning
This page describes practices involving highly flammable materials in ways that create a serious risk of fire or explosion.
In addition, this page describes trade processes that use dangerously toxic materials such as cyanide solutions, mercury amalgams, or strong acids. These processes require training, equipment, and precautions far beyond what the text describes.
This information has been left in for historical purposes but should not be acted upon.
The tempering of small drills, for drilling holes in arbors, staffs, etc., which we find are very hard and difficult to perforate, may be effected in the following manner: After having filed the drill to its proper size (being careful not to flatten the cutting face), you then warm it moderately, not allowing it to become red, and run it into borax. The drill is thus coated over with a crust of borax and secluded from the air. Now it may be hardened by heating it only cherry red; after this it is inserted into a piece of borax, or what is better still, plunged into mercury; taking care not to breathe the mercury fumes. Drills prepared in this way, without being brittle, will become exceedingly hard and the watchmaker will be enabled to drill articles which could not otherwise be perforated with a drill. Do not use broken broaches to make your drills, as the steel in them is often burned, rendering the metal unfit for use in small tools. In order to make the quality of your drill a certainty, always take a new piece of round steel for the purpose. To Harden Steel in Petroleum. According to B. Morgossy, the articles to be hardened are first heated in a charcoal fire, and, after thoroughly rubbing with ordinary washing soap, heated to a cherry red. In this condition they are plunged into petroleum; ignition of the petroleum need not be feared if no flame is near at hand. Articles hardened by this method show no crack, do not warp if plunged endwise, and after hardening remain nearly white, so they can be blued without further preparation.
What Has Changed in the Last Century
When the American Watchmaker and Jeweler printed this in 1892, judging heat by eye "cherry red" and quenching a hot drill in an open dish of mercury were ordinary bench practice; mercury vapour is now recognised as a cumulative nerve and kidney poison, and no workshop today would keep an open mercury bath, let alone plunge hot steel into it. The colour-by-eye method has also largely given way to thermocouples and pyrometers, controlled-atmosphere or vacuum furnaces, and quenching oils formulated for the job rather than whatever petroleum was on hand. Above all, the problem the piece is solving has mostly gone away: small drills for hard work are bought ready-made in high-speed steel (developed commercially in the early 1900s) or tungsten carbide, which hold an edge far better than any borax-coated carbon-steel drill filed at the bench. The borax coating trick, to keep air off the steel and limit scaling, still rests on sound chemistry and survives in brazing and forge work.
Written for this edition, 2026.