This is undoubtedly the most important metal used in the arts. Directions for working it, such at least as would be valuable to professional blacksmiths, would occupy more space than we can afford, and we therefore content ourselves with a few hints for amateurs.

Forging

As a general rule, those who are not practical blacksmiths had better take their work to a smith's shop. Cases may, however, arise where it is necessary to forge some little job, and the following hints may prove of use.

In working iron a great deal depends upon the degree of heat to which it is raised. Blacksmiths distinguish five degrees, which they name as follows:

1. The black-red heat, just visible by daylight.

2. The low-red heat.

3. The bright red heat, when the black scales may be seen.

4. The white heat, when the scales are scarcely visible.

5. The welding heat, when the iron begins to burn with vivid sparks.

Of these temperatures the 1st, 2nd and 3rd are easily attained in a common stove or grate. It requires good management to secure the 4th in a common stove, and the 5th can hardly be obtained without a blast. The higher the temperature the softer and more easily worked the metal becomes, and the less liable to crack or split; and as good iron is not easily spoilt, like steel, by a high heat, it is always best to get the metal pretty soft.

Welding

This operation requires considerable skill. The two great points to be attended to in making a perfect weld are that the metal shall be brought to a proper temperature, and that the surfaces to be united shall be perfectly clean. The latter point can only be secured by protecting the iron from the action of the air by means of some flux. Sand is generally used by blacksmiths and answers very well. When sand is brought into contact with oxide of iron at a high temperature, it combines with it and forms a fusible glass which flows over the surface of the iron and is easily driven out of the joint by pressure. Borax makes a still more fusible flux and may be successfully used by amateurs, but is too expensive for common use.

When two surfaces of iron, which have been cleansed by means of sand or borax, are brought together at a high heat and forcibly pressed into contact by hammering or pressure, they unite to form a solid mass. Bearing these principles in mind, a little practice will soon enable any one to make a respectable joint by welding.

Case-Hardening

This process is simply the conversion of the surface of a piece of iron into steel. Case-hardened articles, when plunged into cold water while highly heated, become as hard as the hardest steel, but they may be annealed and softened so as to be easily worked with files and turning tools, and afterwards hardened again so as to be as durable as ever. There are several processes for performing this operation. The following have been tested by experience:

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. Pieces of gas pipe make good receptacles to hold the work, the ends being stopped with iron plugs. When packing the articles in the tubes or trays, see that they do not touch each other.

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.

As this roasting of bones, leather, etc., gives rise to most abominable odors, the author of this manual 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 decompose 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 quite as well as animal charcoal, and no difficulty will be found in case-hardening to a depth which will allow of a good deal of polishing before the soft metal underneath is reached.

2. Where 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.