Small tools, fish-hooks, and
keys, etc., are usually made of iron; they are fashioned first, and
case-hardened afterwards. There are good reasons for this: first, because
it is the cheapest way of making them; and secondly, because while steel
is hard, iron is tough; and anything made of iron and coated with steel,
combines some of the advantages of both metals. The civilised method of
case-hardening, is to brighten up the iron and to cover it with prussiate
of potash, either powdered or made into a paste. The iron is then heated,
until the prussiate of potash has burned away: this operation is repeated
three or four times. Finally, the iron, now covered with a thin layer of
steel, is hardened by quenching it in water. In default of prussiate of
potash, animal or even vegetable charcoal may be used, but the latter is
a very imperfect substitute. To make animal charcoal, take a scrap of
leather, hide, hoof, horn, flesh, blood--anything, in fact, that has
animal matter in it; dry it into hard chips like charcoal, before a fire,
and powder it. Put the iron that is to be case-hardened, with some of
this charcoal round it, into the midst of a lump of loam. This is first
placed near the fire to harden, and then quite into it, where it should
be allowed to slowly attain a blood-red heat, but no higher. Then, break
open the lump, take out the iron, and drop it into water to harden.
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