There are few subjects which have afforded a more profitable field to the traveling recipe-monger than iron and steel, especially as relates to welding and casehardening. The latter is a very old process, but one which still has important uses, notwithstanding the great improvements in the manufacture of iron and steel.
Casehardening is simply the rapid conversion of the surface of a piece of iron into steel, and this is usually followed by sudden hardening in cold water, which makes the casehardened portion as hard as the hardest steel, and besides gives it a beautifully mottled appearance. The special advantage possessed by an article which has been casehardened over one made entirely of steel lies in the fact that the interior or core remains soft and tough, so that the article is not liable to be broken by a fall or a blow. Owing to the irregularity of the steelifying process the surface also presents a mottled appearance, which confers upon it a beauty that can not be obtained in any other way.
The following processes have been tested by experience, and may be fully relied upon: -
1. Where it is desired that the articles should be hardened to a considerable depth: Char a quantity of bones, just enough (and no more) to enable you to powder them with a hammer. Lay a layer of this bone-dust over the bottom of an iron tray or box, which may be easily made by bending heavy sheet-iron into form. Lay the articles to be hardened on the bone-dust, taking care that they do not touch each other. Cover with bone-dust, and fill up the tray with spent dust, charcoal, or sand. Expose to a bright cherry-red heat for half an hour or an hour, and then turn the entire contents of the tray into a vessel of cold water. We have seen beautiful results obtained by this process when carried out in a common kitchen-stove.
Even raw bone-dust, such as is sold for farming purposes, may be used with good results.
Bone-black or ivory-black may also be used; and, as they may be purchased ready prepared, we may avoid the disagreeable process of roasting the raw material.
2. Moxon's recipe is as follows: -
Cow's horn or hoof is baked or thoroughly dried and pulverized. 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 by immersion in water or oil, the latter being preferred for delicate articles.
3. Take a quantity of old boots, burn these until they become charred, beat off the black and charred portion with a hammer until sufficient powdered carbon is obtained. Then place this powder with the articles to be operated upon into a sheet-iron box or a piece of wrought-iron gas-pipe sufficiently large, taking care that the articles are well covered and in the center of the mass; lute the ends of the pipe or the top of the box with clay, and place the whole into a fire made of coke, keeping them there for an hour or more, taking care that the heat shall be equal (between dark-red and red); now plunge the contents into water.
Any animal matter will answer; and, on the large scale, charcoal might be prepared from almost any refuse of the kind; and, being well powdered, might be made an article of commerce. "Charcoal for casehardening " could hardly fail to find a good market.
It would seem that in mechanical processes, as in medicines, there are those who believe that the more disgusting an article is the more effective it is. It is only on this ground that such filthy ingredients as stale urine, nightsoil, etc., can have been recommended. We have tried these abominable recipes, and they are not as efficient as the more cleanly ones. We therefore omit them.
4. As this roasting of bones, leather, etc., gives rise to most abominable odors, the editor of this work some years ago devised the following preparation, which was found to give very excellent results: -
Prepare a strong solution of prussiate of potassa; boil in it as much coarsely powdered wood-charcoal as can be mixed with it. Drain off the superfluous liquid, spread the charcoal on a board, and dry by exposure to the air. When dry, roast it at a temperature just below that of ignition, the object being to drive off all moisture, but not to discompose the prussiate, which, at a red heat, is converted into cyanide of potassium and some other compounds. The charcoal thus prepared, and afterwards reduced to a moderately fine powder, will be found to answer quiet as well as animal-charcoal; and no difficulty will be found in casehardening to a depth which will allow of a good deal of polishing before the soft metal underneath is reached.
In using the materials above described, the articles to be casehardened are always inclosed in an iron box or case while exposed to the fire. Pieces of iron tubing make capital receptacles to hold the work, the ends being stopped with loose iron plugs, which are to be cemented in air-tight with a mixture of fire-clay and sand, and the whole securely bound with wire. The entire outside of the box or tube should then be coated with loam and allowed to dry, after which it may be exposed to a fire for a period varying from half an hour to three hours, according to the size of the box and the design of the operator.
In packing the articles in the box see that the entire space is packed solidly with the powdered charcoal; and, above all, see that none of the pieces touch each other. The air must be thoroughly excluded, or mischief will ensue.
The articles are usually hardened by allowing them to drop directly from the box or tube into a tub of water, in which they are vigorously stirred until cold. For some peculiar purposes the articles are dropped into oil. They do not become so hard, but they are tougher.
When mere superficial hardening is required, heat the article to be hardened to a bright red; sprinkle it liberally with powdered prussiate of potash. The salt will fuse, and if the piece of iron is small and gets cooled, heat it again and plunge into cold water.
We have seen recipes in which various salts (sal ammoniac, nitre, and even bichromate of potassa!) are recommended to be mixed with the prussiate of potash. It is needless to say that such additions do harm instead of good, and can only serve to render the recipe more complicated and mysterious. The fact is, however, that casehardening is one of those operations which are usually surrounded by much mystery by the less intelligent class of mechanics; but to those who have given the subject a careful practical study the process is as well understood as any other operation connected with iron and steel. Even where we would least expect it, this nonsensical complication creeps out. Thus, in a "Techno-chemical Receipt-book" recently issued we find the following as the only recipe given in this department: -