Hardening Liquids
American Watchmaker and Jeweler (1892)
If water is used for hardening, 32 deg. F. will be found about right for the sized articles hardened by watchmakers, and if the article is very small, ice may be added to the water. A solution composed of 1 quart of water, 1 1/4 lbs. of sal-ammoniac, 10 oz. of refined borax, 1 1/4 ozs. red wine, is used extensively for fine cutlery. A mixture of 1 lb. of resin, 3 ozs. of lard, 1/2 lb. train oil and 1/2 oz. of assafoetida is said to be excellent for fine steel work. Directions for Plunging when Hardening. Thin articles, as steel plates, or articles of small diameter, such as drills, should always be plunged into the hardening compound, end or edge foremost to avoid warping. If an article is thicker on one side than the other, as a knife blade, the thick side should enter the compound first. Heat the article only as far as you wish to harden it and immerse it as far as it has been made red hot. Tempering by Electricity. Watch springs have of late years been successfully tempered with the aid of electricity. The steel ribbon is passed through a bath of oil and an electric current of sufficient strength to keep it at the proper heat is passed through the ribbon. The heating is thus effected without contact with the atmosphere and the spring is not liable to blister as in ordinary methods. The temper is drawn in the same manner and the heat can be controlled to a nicety and is uniform throughout. The spring is then finished by means of rolls. Glass Polisher for Steel. French plate glass, ground on one side, makes a good polisher for flat work. A piece four inches square, nicely finished on the edge, is about the right size.
What Has Changed in the Last Century
The 1892 recipes here — sal-ammoniac and red wine for cutlery, resin, lard, train oil and asafoetida for fine steel — belong to a craft tradition that worked by trial and inherited lore rather than by any theory of what the quench was doing to the metal. That theory arrived in 1930, when Edgar Bain and Edmund Davenport published the time-temperature-transformation diagrams showing exactly how fast steel must be cooled to form martensite, and it was made practical by Walter Jominy's end-quench hardenability test of about 1940. Modern shops quench in graded oils or water-soluble polyalkylene glycol solutions whose cooling rate can be dialled in by concentration, and harden in vacuum or controlled-atmosphere furnaces that do what the writer's electric bath was groping toward: heating without contact with the air. The advice on plunging thin work edge-first, and the thick side of a knife blade first, is still exactly what a bladesmith is taught.
Written for this edition, 2026.