IRON. 
Jy, that the steel which is naturally the 
hardest is that which requires the least 
heat, and that the best temper for each 
kind of steel is that produced by the lowest 
degree of heat suitable to that peculiar 
kind. Hence, various methods have been 
proposed to ascertain the degree of heat 
most advantageous to any particular sample 
of steel. They are all reducible to that of 
igniting one end of a bar to a white heat, 
and plunging it into water. The hardness 
of file several parts may then be ascertained 
by examination. 
It appears that the considerations on 
which the first-mentioned general rule is 
founded, are rather of a complicated na- 
ture. If it be true, as conjectured above, 
that the hardness of steel depends princi- 
pally on the intimate combination of its 
carbon ; it will follow, that the utmost 
hardness any steel is capable of, will be 
produced by a degree of heat sufficient to 
effect this purpose, and that, any superior 
degree will only degrade the quality of the 
steel ; and consequently, that the general 
rule will apply only to heats inferior to 
this maximum. It may also happen, when 
the mass of water is not considerable, though 
the fact has been doubted, when the quantity 
is large, that the heating of the water, 
and the production of hydrogen, may so 
far modify the rate of cooling, as even 
to render it less sudden than might have 
happened with an inferior degree of igni- 
tion. Lavoisier has remarked, that the 
conversion of iron into the hard brittle 
oxide, when water is decomposed, may 
perhaps be of the same nature as the 
hardening of steel. There is another fact, 
which is of great consequence, and may per- 
haps afford the principal ground for Rin- 
man’s apparent exception to the general 
rule concerning the hardening of steel. 
Hardness is that property of bodies by 
which they resist indentation, and rather 
break than bend or suffer contusion. It is 
the opposite to softness. Tenacity, which 
is very frequently confounded with hard- 
ness, is not the property by which they re- 
sist indentation, but separation of their 
parts. It is the opposite to friability or 
brittleness. Now that steel, in common 
language, is said to be the hardest, which 
unites most eminently the properties of 
hardness and tenacity. But the effect of 
heat and sudden cooling is directly opposite 
in these two respects. Soft steel is the most 
tenacious, but ihe least hard. The opera- 
tion of hardening diminishes its tenacity, 
and increases its hardness progressively ftp 
the maximum of heat the steel is formed to 
bear. There will be no difficulty then in 
conceiving, that the best state of steel, re- 
lative to any particular use, will be at some 
precise degree between the softest and 
hardest qualities. Thus for springs, muck 
tenacity and moderate hardness are re- 
quired. For chissels and similar tools, which 
operate by a blow, a greater hardness may 
be admitted. Razors, knives, and such 
tools as effect the intended purpose by a 
gradual stroke, will be still more valuable 
the harder they are ; but even in these the- 
tenacitv must not be too much diminished, 
otherwise tire edge will be liable to break. 
They must all be capable of having the 
edge turned or bended on one side in the 
operation of whetting. Files are perhaps 
of all tools such as require the greatest hard- 
ness; but in these, it is far from being the 
utmost the steel is capable of receiving. It 
is found, that the tenacity of steel is consi- 
derably increased by continued hammering 
to a certain point. But the whole effect 
of this hammering is taken off by strong 
ignition. Good steel by hardening at a 
white heat may be rendered so brittle, that 
it will break full as easily as a stick of the 
same dimensions, and its texture is then 
found to be coarse and large grained. As 
the subsequent annealing does not restore 
the effect of the hammering, nor bring the 
grain of the steel to the state it would have 
possessed if a lower heat had been used at 
first; it is evident, that the most useful 
hardness is produced by that degree of heat, 
which is just sufficient to effect the purpose. 
And accordingly, tools made of cast steel, 
and intended to sustain a good edge for 
cutting iron and other metals, are not after- 
ward annealed, but the ignition is carefully 
regulated at first. Annealing ought only to 
be used where considerable softness is re- 
quired. 
Iron may be hardened to a certain de- 
gree by ignition and plunging in water, but 
this effect is confined to the surface; except, 
as it very often happens, the iron contains 
veins of steel. These are no small impedi- 
ments to the filing and working this mate- 
rial. It sometimes likewise may happen, 
that the iron may have undergone a process 
of the nature of case-hardening from the 
fuel. We have been informed by a workman, 
that ignited iron, suddenly plunged into the 
soft leather of a shoe, becomes very hard 
on its surface, which must arise from an in- 
stantaneous effect of this kind. 
