39. Before we commence to frost,
the cock should be perfectly finished, with all the holes made, the
regulator cap in position, the screw hole made for the Howard regulator
and the index arc engraved with the letters S and F.

[Illustration: Fig. 39]

It is not necessary the brass should be polished, but every file mark
and scratch should be stoned out with a Scotch stone; in fact, be in the
condition known as "in the gray." It is not necessary to frost any
portion of the cock _C_, except the upper surface. To protect the
portion of the cock not to be frosted, like the edges and the back, we
"stop out" by painting over with shellac dissolved in alcohol, to which
a little lampblack is added. It is not necessary the coating of shellac
should be very thick, but it is important it should be well dried.

HOW TO PREPARE THE SURFACE.

For illustration, let us suppose the back and edges of the cock at Fig.
39 are coated with shellac and it is laid flat on a piece of paper about
a foot square to catch the excess of mastic. Holes should be made in
this paper and also in the board on which the paper rests to receive the
steady pins of the cock. We hold the sieve containing the mastic over
the cock and, gently tapping the box _A_ with a piece of wood like a
medium-sized file handle, shake down a little snowstorm of mastic dust
over the face of the cock _C_.

Exactly how much mastic dust is required to produce a nice frosting is
only to be determined by practice. The way to obtain the knack is to
frost a few scraps to "get your hand in." Nitric acid of full strength
is used, dipping the piece into a shallow dish for a few seconds. A
good-sized soup plate would answer very nicely for frosting the bottom
plate, which, it will be remembered, is 6" in diameter.

HOW TO ETCH THE SURFACE.

After the mastic is sifted on, the cock should be heated up to about
250 deg. F., to cause the particles of mastic to adhere to the surface. The
philosophy of the process is, the nitric acid eats or dissolves the
brass, leaving a little brass island the size of the particle of mastic
which was attached to the surface. After heating to attach the particles
of mastic, the dipping in nitric acid is done as just described. Common
commercial nitric acid is used, it not being necessary to employ
chemically pure acid. For that matter, for such purposes the commercial
acid is the best.

After the acid has acted for fifteen or twenty seconds the bra