We shall begin the experiment in hardening with the simplest tool,—a chisel. We suppose you must dress the chisel first; then be careful, for the trouble begins with tool steel the moment you place it in the fire. Now place the chisel in the fire, far in, so the extreme edge will reach over the hottest place in the fire,—that is, the center,— otherwise you are liable to burn the edge, as it is thin, and is liable to be too hot before the rest of the chisel is hot enough; and if the edge is heated to a white heat, you better cut it off, for it matters little how you temper it, if it is overheated. Steel for edge tools should not be hammered much on the edge, as this hammering is liable to crush the fiber. When the chisel is dressed, and you are ready to harden, proceed as follows: Place it in the fire and heat slowly. When it is of a dark red heat on a bright day, it is hot enough. In the dark this heat will look many degrees hotter. This you should always remember in hardening of all kinds. The temperature of the water should be taken into consideration. If the water is ice cold, the heat should be lower, and vice versa. For hardening of all edge tools, water is the best. When your chisel is of the right heat, stick it into the water an inch, or in proportion to the range of the heat and of the tool. When cool enough, take it out, and rub it with an emery cloth or a piece of a grindstone, or anything that will brighten it so that you can easily discern the color as the temper is drawing.
For a cold chisel, let the temper go back until it turns blue; but here be quick, for as soon as it is blue, stick it in the water. A moment’s delay will make it too soft.
For a “hot cutting” chisel, the temper may be a little harder. This blue temper is right for all cutting tools. If you dip the tool in the water just when the blue color reaches the edge, and then pull it out while it is yet sizzling, you will notice, while it is yet wet, a copper color under the water at the extreme edge. Then you will know it is plenty hard for even wood-cutting tools, if you have the right kind of steel.
In brazing anything, the joints should be filed clean, and fitted snugly together; then have the brass, in thin pieces, placed over the joint; then a little borax on top. The article should then be placed in a clean fire, the heat striking directly over the joint. When the brass is melted, take it out of the fire and cool off.