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. A little
turpentine on a rag will remove the mastic, but turpentine will not
touch the shellac coating. The surface of the brass will be found
irregularly acted upon, producing a sort of mottled look. To obtain a
nice frosting the process of applying the mastic and etching must be
repeated three or four times, when a beautiful coarse-grain mat or
frosting will be produced.

The shellac protection will not need much patching up during the three
or four bitings of acid, as the turpentine used to wash off the mastic
does not much affect the shellac coating. All the screw holes like _s s_
and _d_, also the steady pins on the back, are protected by varnishing
with shellac. The edges of the cocks and bridges should be polished by
rubbing lengthwise with willow charcoal or a bit of chamois skin
saturated with oil and a little hard rouge scattered upon it. The
frosting needs thorough scratch-brushing.

[Illustration: Fig. 40]

At Fig. 40 we show the balance cock of our model with modified form of
Howard regulator. The regulator bar _A_ and spring _B_ should be ground
smooth on one side and deeply outlined to perfect form. The regulator
cap _C_ is cut out to the correct size. These parts are of decarbonized
cast steel, annealed until almost as soft as sheet brass. It is not so