erent
materials can be indicated by the character of the lines, shown in Fig.
120.

CHAPTER IX

TREATMENT AND USE OF METALS

ANNEALING.--A very important part of the novice's education is a
knowledge pertaining to the annealing of metals. Unlike the artisan in
wood, who works the materials as he finds them, the machinist can, and,
in fact, with many of the substances, must prepare them so they can be
handled or cut by the tools.

Annealing is one of the steps necessary with all cutting tools, and it
is an absolute requirement with many metals for ordinary use, as well as
for many other articles like glass. This is particularly true in the use
of copper.

TOUGHNESS AND ELASTICITY.--It means the putting of metals in such a
condition that they will not only be less brittle, but also tougher and
more elastic. Many substances, like glass, must be annealed before they
can be put in condition for use, as this material when first turned out
is so brittle that the slightest touch will shatter it, so that it must
be toughened.

Malleable or wrought iron, if subjected to pressure, becomes brittle,
and it is necessary to anneal it. Otherwise, if used, for instance, for
boiler plates, from the rolled sheets, it would stand but little
pressure.

The most immediate use the boy will have is the treatment of steel. He
must learn the necessity of this process, and that of tempering, in all
his cutting tools, and in the making of machinery where some parts are
required to be constructed of very hard metal.

THE PROCESS.--To anneal steel it must be heated to a bright cherry red
and then gradually cooled down. For this purpose a bed of fine charcoal,
or iron filings and lime, is prepared, in which the article is embedded,
and permitted to remain until it is cold.

There are many ways of doing the work, particularly in the use of
substances which will the most readily give up their carbon to the tool.
Yellow prussiate of potash is an excellent medium, and this is sprinkled
over the cherry-heated article to be annealed. The process may be
repeated several times.

TEMPERING.--This is the reverse of annealing as understood in the art.
The word itself does not mean to "harden," but to put into some
intermediate state. For instance, "tempered clay" means a clay which has
been softened so it can be readily worked.

On the other hand, a tempered steel tool is put into a condition where
it is hardened, but this hardness is al