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 brass is
rinsed in pure water to remove the acid, and dried by patting with an
old soft towel, and further dried by waving through the air.
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