be seen by noting how cast-iron borings blacken the hands just
as does graphite, while steel turnings do not have the same effect.
The difference is due to the fact that the carbon in steel is not
in a graphitic form as well as because it is present in smaller
quantities.

In making steel in the old way the cast iron was melted and the
carbon and other impurities burned out of it, the melted iron being
stirred or "puddled," meanwhile. The resulting puddled iron, also
known as wrought iron, is very low in carbon; it is tough, and on
being broken appears to be made up of a bundle of long fibers.
Then the iron was heated to redness for several days in material
containing carbon (charcoal) until it absorbed the desired amount,
which made it steel, just as case-hardening iron or steel adds
carbon to the outer surface of the metal. The carbon absorbed by
the iron does not take on a graphitic form, however, as in the
case of cast iron, but enters into a chemical compound with the
iron, a hard brittle substance called "cementite" by metallurgists.
In fact, the difference between the hard, brittle cementite and
the soft, greasy graphite, accounts for many of the differences
between steel and gray cast iron. Wrought iron, which has very
little carbon of any sort in it, is fairly soft and tough. The
properties of wrought iron are the properties of pure iron. As
more and more carbon is introduced into the iron, it combines with
the iron and distributes itself throughout the metal in extremely
small crystals of cementite, and this brittle, hard substance lends
more and more hardness and strength to the steel, at the expense
of the original toughness of the iron. As more and more carbon is
contained in the alloy--for steel is a true alloy--it begins to
appear as graphite, and its properties counteract the remaining
brittle cementite. Eventually, in gray cast iron, we have properties
which would be expected of wrought iron, whose tough metallic texture
was shot through with flakes of slippery, weak graphite.

But to return to the methods of making steel tools in use 100 years
ago.

The iron bars, after heating in charcoal, were broken and the carbon
content judged by the fracture. Those which had been in the hottest
part of the furnace would have the deepest "case" and highest carbon.
So when the steel was graded, and separated into different piles,
a few bars of like kind were broken into short lengths, melted
in fire-clay cruc