Saws are hardened in oil, or in a mixture of oil with suet, wax, etc.
They are then heated over a fire till the grease inflames. This is called being Mazed.
After blazing the saw is flattened while warm, and then ground.
Springs are treated in somewhat the same manner, and small tools after being hardened in water are coated with tallow, heated till the tallow begins to smoke, and then quenched in cold tallow.2
This is effected by placing the article to be case-hardened in an iron box full of bone dust or some other animal matter, and subjecting it to a red heat for a period varying from half an hour to eight hours, according to the depth of steel required.
The iron at the surface combines with a proportion of carbon, and is turned into steel to the depth of from 1/16 to 3/8 of an inch.
If the surface of the article is to be hardened all over, it is quenched in cold water upon removal from the furnace. If parts are to remain malleable it is allowed to cool down, the steeled surface of those parts removed, and the whole is then reheated and quenched, by which the portions on which the steel remains are hardened.
Gun-locks, keys, and other articles which require a hard surface, combined with toughness, are generally case-hardened.
A more rapid method of case-hardening is conducted as follows : - The article to be case-hardened is polished, raised to a red heat, sprinkled with finely powdered prussiate of potash. When this has become decomposed and disappeared, the metal is plunged into cold water and quenched.
The case-hardening in this case may be made local by a partial application of the salt.
Malleable castings (see p. 266) are sometimes case-hardened in order that they may take a polish.
Steel may be distinguished from wrought iron by placing a drop of dilute nitric acid (about 1 acid to 4 water) upon the surface. If the metal be steel a dark grey stain will be produced, owing to the separation of the carbon.3