412 
Cast Steel. 
surface of the metal in the form of a dense slag ; hence the 
residual iron differs from the crude pig- metal in contain- 
ing less earth and oxyd. By the subsequent operations 
the carburet and oxyd of iron mutually decompose each 
other, forming carbonic acid and carbonous oxyd, by which 
the metal is freed both from its oxygen and carbon. In this 
state it forms bar iron, which may or may not retain a 
small proportion of vitrified earth, but which seems even 
when purest to hold a little both of carbon and oxyd. If 
this bar iron is exposed in a close vessel to a high heat, 
the carbon and oxygen that it contains (if they are in due 
proportion to each other) will be entirely got rid of, the me- 
tal will become very soft, and will be at the same time 
malleable and fusible. If the bar iron instead of being 
heated by itself has access to carbon either in the state of 
charcoal or plumbago, its oxygen will be expelled and 
carbon will at the same time be absorbed : if the portion 
of this latter be small, the mixture will partake of the pro- 
perties both of iron and steel, hence it will be very malle- 
able and capable of being welded, but also will be harder 
than pure iron, somewhat more fusible and susceptible of 
being tempered. By being united with a fresh portion of 
carbon it will become still more fusible and will lose its 
welding property ; it will become harder, more compact, 
and will form the fine cast steel. A further portion of 
carbon increases the brittleness and hardness, so as to ren- 
der it incapable of being wrought, and its colour and tex- 
ture will approach to that of white cast iron : in this state 
it may be regarded as saturated with carbon. It is how 7 - 
ever capable of uniting to this substance even to supersa- 
turation, by which its colour and texture resembles that of 
grey or black cast iron ; its fusibility is somewhat increas- 
ed but its hardness is so much lowered by this excess of 
carbon as to allow it to be wrought with ease by a com- 
mon file, nor can it be materially hardened by sudden 
