116 Observations on Iron and Steel. 
nion I can by no means accede, as it is well 
known. that the Swedish* ores contain much less 
heterogeneous matter than ours, and are generally 
much richer, as they usually yicld about seventy 
per quintal of pure iron, whereas the average of 
ours is not more than thirty or forty :+ add to this, 
that the Swedish ores are smelted in wood fires, 
which gives the iron an additional superiority. 
Iron instruments are case-hardened by heating 
them in a cinder or charcoal fire; but if the first be 
used, a quantity of old leather, br bones, must be 
burnt in the fire to supply the metal with carbon. 
The fire must be urged by a pair of bellows to a 
sufficient degree of heat; and the whole operation 
is usually,completed in an hour. 
The process for case-hardening iron, is in fact 
the same as for converting iron into steel, but not 
continued so long, as the surface only of the article 
is to be impregnated with carbon. 
Some attempts have been made to give cast iron, by 
case hardening, the texture and ductility of steel, but 
they have not been very successful. Table and pen- 
knife blades have been made of it, and, when ground, 
* Steel is commonly made of Swedish iron. 
_ + The iron made from the ore found in the neighbour- 
hood of Sheffield, contains a great deal of phosphate of 
iron, or siderite, which renders the metal brittle when 
cold. 
