IRON. 
will bend or turn. Some artists ignite their 
tools and plunge them into cold water; 
after which they brighten the surface of the 
steel upon a stone : the tool being then laid 
upon charcoal, or upon the surface of melt- 
ed lead, or placed in the flame of a candle, 
gradually acquires the desired colour ; at 
which instant they plunge it into water. 
If a hard temper be desired the piece is 
dipped again, and stirred about in the cold 
water as soon as the yellow tinge appears. 
If the purple appear before the dipping, the 
temper will be fit for gravers, and tools 
used in working upon metals; if dipped 
while blue it will be proper for springs, 
and for instruments used in the cutting of 
soft substances, such as cork, leather, and, 
the like ; but if the last pale colour be 
waited for, the hardness of the steel will 
scarcely exceed that of iron. When soft 
steel is heated to any one of these colours, 
and then plunged into water, it <!oes not 
acquire nearly so great a degree of hard- 
ness as if previously made quite hard, and 
then reduced by tempering. The degree 
of ignition required to harden steel is diffe- 
rent in the different kinds. The best kinds 
require only a low red heat. The harder the 
steel thg more coarse and granulated its 
fracture will be; and as this is not com- 
pletely remedied by the subsequent tem- 
pering, it is advisable to employ the least 
heat capable of affording the requisite hard- 
ness. It is a circumstance worthy of re- 
mark, that steel has a less specific gravity 
when hardened than when soft ; but there 
are no circumstances upon which a pro- 
bable connection between these two pro- 
perties, namely, the increased hardness 
and the diminished specific gravity, can be 
made out. 
If the cementation be continued too long 
the steel becomes porous, brittle, of a 
darker fracture, more fusible, and incapa- 
ble of being forged or welded. On the con- 
trary, steel cemented with earthy infusible 
powders is gradually reduced to the state 
of forged iron again. Simple ignition pro- 
duces the same effect ; but is attended with 
oxidation of the surface. The texture of 
Steel is rendered more uniform by fusing it 
before it is made into bars ; this is called 
cast steel, and is rather more difficultly 
wrought than common steel, because it is 
more fusible, and is dispersed under the 
hammer if heated to a white heat. 
The conversion of iron into steel, either 
by fusion, viz. the direct change of crude 
iron into steel, or by cementation of bar- 
iron, present many objects of interesting 
inquiry. From various experiments of 
Bergman it appeared, that good crude iron, 
kept for a certain time in a state of fusion, 
with such additions as appeared calculated 
to produce little other effect than that 
of defending the metal from oxidation, be- 
came converted into steel with loss of 
weight. These facts are conformable to the 
general theory of Vandermonde, Monge, 
and Berthollet : for, according to their re- 
searches, it should foliqw that part of the 
carbon in the crude iron was dissipated, 
and the remainder proved to be such in pro- 
portion as constitutes steel. The same 
chemist cemented crude iron with plum- 
bago, or carbonate of iron, and found that 
the metal had lost no weight. Morveau re- 
peated the experiment with grey crude 
iron. The loss of weight was little, if any. 
The metal exhibited the black spot by the 
application of nitric acid, as steel usually 
does, but it did not harden by ignition and 
plunging in water. Hence it is concluded, 
that it was scarcely altered : for crude iron 
also exhibits the black spot, and cannot by 
common management acquire the hardness 
of steel. 
From the experiments of the three excel- 
lent chemists last mentioned, it appears, 
that the grey crude iron consists princi- 
pally of iron, with as much carbon as it can 
dissolve in the strong heat of the smelting 
furnace. They have shown also, that it de- v 
posits part of this addition when cooled in 
contact with an iron bar immersed in the 
bath. This separation must* he general in 
the ordinary or gradual way of cooling, 
whence the grey colour must arise from the 
blue white colour of the iron mixed with 
the black of the carbon. And this grey 
colour is also in a degree perceived, when 
soft close-grained steel is broken. These 
circumstances lead to an inference, that 
hard steel may in a certain respect differ 
from that which is softer by the intimate 
combination of a larger proportion of 
carburet. This accounts for the whiter 
and more metallic aspect of hardened 
pteel, than of such as is soft. For the former 
contains less of disengaged carburet. Hence 
also vv<i%iy account for the greater hard- 
ness of Iteel, which has been made quite 
hard, and then let down by tempering to a 
certain colour, than of steel merely heated 
to that colour, and plunged in water. For 
in the first method of hardening, a sufficient 
degree of heat is given to produce combi- 
nation between part of the disengaged 
