evaporation. It has a bitter taste and acid reaction; its aqueous solution is unaffected by the ferric salts and tincture of galls.—Dose, 1 to 3 gr.; in dyspepsia, &c.
CASE-HARD′ENING. Syn. Steel sur′facing. The operation of giving a surface of steel to iron goods. Tools, fire-irons, fenders, keys, &c., are usually case-hardened.
Process. 1. The goods (finished in every respect but polishing) are put into an iron box, and covered with animal or vegetable charcoal, and “cemented” at a red heat for a period varying with the size and description of the articles operated on: these, when taken out, are hardened by plunging into water, or oil, if they are of a delicate nature.
2. (Moxon.) Cow’s horn or hoof is baked or thoroughly dried and pulverised; to this is added an equal quantity of bay salt, and the whole is made into a paste with stale chamber-lye, or white wine vinegar; the iron is covered with this mixture, and bedded in it, in loam, or inclosed in an iron box. In this form it is laid on the hearth of the forge to dry and harden, then it is put into the fire, and blown till the lump has a blood-red heat (no higher). It is hardened as before.
3. Coat the goods with a paste made of a concentrated solution of prussiate of potash and loam; then expose them to a strong red heat, and when it has fallen to a dull red, plunge the whole into cold water.
4. The goods, previously polished and finished, are heated to a bright-red, and rubbed or sprinkled over with prussiate of potash. As soon as the prussiate appears to be decomposed and dissipated the articles are plunged into cold water.
Obs. The process of case-hardening has been well conducted when the surface of the metal proves sufficiently hard to resist a file. The last two plans are a great improvement upon the common method. By the topical application of prussiate of potash (ferrocyanide of potassium) any part of a piece of iron may be case-hardened without interfering with the rest.
Case-hardening Powders. Syn. Case-hardening compositions. 1. Prussiate of potash, dried and powdered.
2. Prussiate of potash, 3 parts; sal-ammoniac, 1 part; mix.
3. Sal-ammoniac and bone-dust, of each 2 parts; prussiate of potash, 1 part. (See above.)