This injury fortunately can be cured either by reheating the steel to Ac 3 when it " refines," i.e.
A variety of animal charcoal is sometimes prepared by calcining fresh blood with potassium carbonate in large cylinders, the mass being purified by boiling out with dilute hydrochloric acid and subsequent reheating.
But the invaluable and rather delicate art of tempering the hardened steel by a very careful and gentle reheating, which removes its extreme brittleness though leaving most of ifs precious hardness, needs such skilful handling that it can hardly have become known until very long after the art of hot-forging.
This brittleness has therefore in general to be mitigated or " tempered," unfortunately at the cost of losing part of the hardness proper, by reheating the hardened steel slightly,
Hence steel which has been heated very highly, whether for welding, or for greatly softening it so that it can be rolled to the desired shape with but little expenditure of power, ought later to be refined, either by reheating it from below Are to slightly above Ac 3 or by rolling it after it has cooled to a relatively low temperature, i.e.
