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Case hardening is a process that converts the surface of wrought iron into steel, which is accomplished by placing them in a box filled with bone dust, animal charcoal, or leather hoofs, etc.

The bones are either used for manure, or are converted into animal charcoal, valuable for various purposes in the arts.

Second—Another grade of these natural oils which are filtered again at high temperatures and under pressure through beds of animal charcoal to improve their color.

One turned out to be a piece of animal charcoal, another was chalk, and a third a vegetable substance like a bezoar.

It is a form of bone oil, a product obtained in the manufacture of bone-black, or animal charcoal, by the distillation of bones.