Cast Steel. 
393 
manner. A cementation or converting furnace, consists 
of two parallel troughs, constructed of fire-brick, suffi- 
ciently long to admit with convenience a common bar of 
iron ; these troughs rest upon a long grate from which 
Hues proceed so as to distribute the heat as evenly as pos- 
sible to every part : an arched vault is thrown over the 
top, and the whole is inclosed within a cone of masonry 
as the glass house furnaces are. The bars of iron intend- 
ed for cementation are of the very best quality, (in Eng- 
land none but the Swedish Oregrund iron is employed 
for this purpose) and are carefully examined to ascertain 
that they are quite free from cracks, flaws, and every ap- 
pearance indicative of their not being completely mallea- 
ble. The requisite selection being made, a stratum of 
coarsely bruised charcoal is laid at the bottom of the ce- 
menting trough, upon which is arranged a layer of iron 
bars : to this succeeds another of charcoal, and so on till 
the trough is nearly filled, observing that the upper as well 
as the lowest layer is charcoal : it is then covered with a 
mixture of hard rammed clay and sand in order to exclude 
the air. A trough thus charged will contain from seven 
to ten tons of iron. The fire being lighted, the heat pass- 
es into the flues and raises the temperature of the troughs 
to a glowing red which is maintained for the space of from 
seven to eleven days according to the quantity of 
iron. At the extremity of each trough is a small hole, 
through which two or three bars project a few inches in 
order that they may be occasionally withdrawn to ascer- 
tain the progress of cementation : when by the trial bars, 
it appears to be compleat, the fire is put out, and after 
the troughs are sufficiently cool they are emptied of their 
contents. The form of the bars thus converted, remains 
unaltered, but their surface is covered over with bubbles 
or blisters, whence the steel in this state is called blister 
steel : it is heavier than the iron from which it was made 
