In hardening round sections it is necessary to use great care to have the heat perfectly uniform and not too high, because the circular form is the most rigid, offering the greatest resistance to change. For this reason a round piece will be almost certain to split if it be heated above a medium orange, or if it be heated unevenly. Many a round piece is cracked by a heat, or by a little unevenness of heat, that another section would endure safely. A roll with journals is perhaps the most difficult of all tools to harden successfully; the most expert temperers will not be surprised at losing as many as one roll in five.
Engraved dies require to be hardened without oxidizing the engraved face, so that the finest lines will be preserved clear and clean.
This is done by burying the engraved face in carbonaceous material in such a way as to prevent the flame or any hot air from coming in contact with it.
There are many ways of doing this, and many different carbonaceous mixtures are used; one simple, and known to be satisfactory, plan will be explained as sufficient to give any intending operator a good starting-point.
The carbonaceous material preferred is burnt leather powdered—and the older it is the better—until it is reduced to ash, so that the material should be saved after each operation to be used again mixed with enough new material to make up the necessary quantity.
D is the die to be heated; B is an open box about two inches deep and one inch larger each way than the die; L is the burnt leather packed in thoroughly, and as full as the box will hold. The engraved face is down, embedded in the burnt leather, and secure from contact with flame or air.
Sometimes powdered charcoal is used, with or without a mixture of tar, according to the fancy of the operator.
Some operators prefer to have the box so high as to leave only the top surface of the embedded die exposed, but the most successful workers prefer the plan sketched, because they can see more of the die, and so regulate better the even heating.
The die and box are put in the furnace, and the heating is watched, the die being turned and moved about in the furnace so as to obtain a perfectly even heat.