38 
THE YOUNG SCIENTIST. 
mit it to cool slowly, and it will have be- 
come soft again. 
The highest degree to which a piece of 
steel may be hardened is attained by 
heating it to a red heat and cooling it in 
mercury, but it is not found necessary to 
use mercury for the cooling process in the 
case of any ordinary metal-cutting pro- 
cesses, because salt water and generally 
pure water is found to extract the heat 
sufficiently (piiclv to make the steel hard { 
enough. Between this degree of greatest ! 
hardness, however, and the degree of 
greatest softness of any piece of steel , | 
there are a number of degrees of hardness \ 
that are known by the term, temper, and ! 
a piece of steel is tempered when it is 
given any degree of hardness above that | 
it possesses as it is in the forged bar and ! 
less than its greatest degree of hardness. 
All cutting tools that are not left fully ' 
hardened are tempered to a degree that 
has been found most desirable for the 
purpose for Avhich the tool is to be used. 
The necessity for tempering a tool 
arises from the fact that hardening- a 
piece of steel decreases its strength and 
inakes it brittle, while for many cutting- 
purposes strength in the tool is a necessity, 
as is the case with a knife blade. The 
cutting edge of a knife would last much 
longer if the blade was fully hardened, 
but that would weaken it so much that it 
would break very easily ; hence it is first 
hardened and then tempered, the temper- 
ing process consisting of heating it to 
some degree less than that to which it 
was heated to harden it, and serving to 
restore the strength that was lost in the 
hardening. The object is thus to give to 
every tool as much hardness or as great a 
degree of temper as possible, while hav- 
ing it strong enough for its cutting duty. 
In order that the degree of temper may 
be uniform for a given purpose it is neces- 
sary that (supposing them to be all made 
from the same quality and grade of steel) 
they all be heated to the same tempera- 
ture for the hardening process, and then 
reheated to the same degree for the tem- 
pering process. It may be remarked, 
however, that a variation in the heat for 
the tempering process is a greater detri- 
ment than one in the heat for the harden- 
ing, because the effect upon the steel is; 
greater. 
Fortunately, however. Nature has given 
us an excellent gauge whereby we can 
know to a certainty how much a piece of 
steel is heated so long as its temperature 
is not greater than 600 degrees or less 
than 000 degrees, and it is between these 
i \:o temperatures that the degrees of tem- 
per for all cutting tools is obtained. 
If a piece of steel, either hardened or 
quite soft, have its surface brightened and 
is slowly heated, it will, when it is heated 
to GOO degrees, assume a yellow color. As 
it becomes more heated the yellow deep- 
ens, until it passes to a red ; from that it 
passes (as the steel gets still more heated) 
through various shades of purple, then to- 
blue, and finally to a very pale blue, with 
a slight green tinge, which appears when 
tlio steel is heated to 000 degrees, 
It is clear, then, that if a piece of steel 
has been hardened, we may know how far 
its tempering has proceeded ; or, in other 
words, to wdiat extent we have at any 
point of time reheated it by the color that 
Fig. 1. 
has ai)peared upon its brightened surface. 
Hence this process is termed color tem- 
pering. 
Now, since the colors will appear upon 
soft as well as upon hardened steel, their 
appearance does not tell us anything 
more than that it has been heated to a 
certain degree, and that if the steel had 
been previously hardened, then it is tem- 
pered to a degree corresponding to the 
color on its surface. 
The degrees of temper corresponding to 
