This compound is very efficacious for casehardeniug iron. It consists of 16 parts of lampblack, 18 of sal soda, 4 of muriate of soda, and 1 of black oxide of manganese."
This recipe is almost worthless. The lampblack is the only efficient agent present, and it is not half as good as charred leather or bone-black.
Should the articles require to be blue, such as the barrels or chambers of pistols, repolish them on an emery-wheel; put them into a sand-bath or powdered charcoal, and heat until the blue color is attained, taking them out the instant that this change takes place. It should be borne in mind that articles treated in this way are comparatively soft.
Owing to the extreme hardness of their surface, articles that have been casehardened are capable of taking a very high polish. The ordinary processes of polishing and buffing are sufficient to produce beautiful results.
Articles which have been casehardeneu. may be annealed and made so soft as to be readily worked with files and turning tools, and they may be again hardened so that those parts from which the steely surface has not been removed will be as durable as ever. Of this principle advantage has been taken to cause the casehardening to terminate at any desired point. The article is left with a band or projection at the place which is desired to be soft; the work is allowed to cool without being immersed in water; the band or projection is now removed by turning or filing, and the work when hardened in the open fire is only affected so far as the original cemented surface remains. This ingenious method was introduced by Mr. Roberts, of Manchester, who considers the success of the casehardening process to depend on the gentle application of the heat. Mr. Roberts thinks that by proper management, so as not to overheat the work, the cementation may be made to penetrate three-eighths of an inch in four or five hours. " In the general way, the conversion of the iron into steel by casehardening is quite superficial, and does not exceed the one-sixteenth of an inch. If made to extend to one-quarter or three-eighths of an inch in depth, to say the least it would be generally useless, as the object is to obtain durability of surface with strength of interior, and this would disproportionately encroach on the strong iron within. The steel obtained in this adventitious manner is not equal in strength to that converted and hammered in the usual way; and if sent in so deeply, the provision for wear would far exceed that which is required. "Holtzappfel.
By combining a hard steely surface with a soft interior, the article is enabled to resist sharp blows as well as wear. If left soft, it is easily worn down by friction; if hardened throughout, it will break like glass by a mere fall.