STONING
AFTER a piece of enamel has been fired, it is often found that too much
enamel has been used, that is, that the enamel may have run over the
edge of the part to be enameled, or it may not present a smooth surface
but instead consist of a series of humps, or, especially in the case of
transparent enamels, it may be on in such layers that the color is
deepened or even lost entirely.
This can be remedied by what is known as “stoning,” or filing the enamel
down to a smooth surface. In the case of work that is to be polished it
is always considered best to overcharge the piece and then file it
smooth rather than to try to “flush” it evenly.
Stoning is usually done by means of emery “stones” or “sticks,” which
are made by mixing powdered emery with shellac and heating the same
until the mixture coheres, when it may be pressed into any shape desired
by means of a flat piece of steel. On cooling the stones become very
hard. They are generally from six to twelve inches long and from
three-eighths to one inch square, tapering at both ends to a rather
blunt point. When a stone wears out or loses its shape it can be
remelted and remodeled. The emery used in these sticks runs from No. 70
to No. 180 grit.
[Illustration:
FIG. 6. FIRING.
]
Carborundum stones are often used in place of emery stones. They are
rather more expensive than emery stones and wear out quicker, but they
cut the work down much faster. No. 120 and No. 180 grits are the most
satisfactory. These stones sometimes get filled up with metal, but in
this case the metal can be eaten out with acid.
It is convenient when stoning to hold the piece to be stoned on a small
block of wood, on which a mold of shellac has been made into which mold
the piece of work will snugly fit. If the block is then set on a pivot
either on a bench or bench-pin, so that it will turn easily, it will
then be found that the stoning process has been made considerably
easier. A piece of hard felt or leather or just a plain piece of wood
will, however, answer the purpose. The idea is simply to hold the piece
firmly and conveniently.
The stone should now be rubbed back and forth across the enamel as a
file is used. It is necessary when stoning to keep both the stone and
the piece to be stoned quite wet. A bowl full of water kept near by will
be found very convenient for the purpose. Care must be taken not to make
any deep scratches on the metallic parts of the piece, as these
scratches cannot easily be obliterated.
The stoning should be continued until the surface is smooth or level
with the edges, or until in champlevé and cloisonné enamel the metal
parts all show evenly, or until the desired depths of color are obtained
in the case of transparent enamels. This of course can only be judged by
experience, as the colors will look much brighter when fired again.
When the stoning is finished it will be found that the surface of the
enamel presents a rather dull or dead appearance, which will be removed
by again firing the piece. Before this firing, however, the piece should
be submerged for about a minute or less in hydrofluoric acid and then
brushed in clean water. This may be done with any small stiff-bristled
hand brush. The piece will dry readily in the air, but more quickly if
first dipped in scalding hot water or brought in contact with steam. It
may also be dried in hot air or sawdust. When dry it is ready to be
fired.
Sometimes after stoning, low places or pit-holes are found in the enamel
surface, which may be remedied by re-charging these places with enamel
and then firing the piece.
When only a very small amount of enamel is to be stoned off, or when one
does not want to fire the piece after the stoning, a soft soapstone
called a “Scotch stone” may be used to advantage.
If the piece to be stoned is small it is desirable to have a small
wooden stick the size of a pencil with which to hold the piece steady
and keep it from slipping out of its mold. Of course a mold is not
always necessary, but will often be found very convenient.
A sharp steel tool, such as a three-cornered scraper or a slightly dull
awl, is very useful to clean out pit-holes or to chip enamel off from
places where it is not desired.
When a large number of pieces are to be stoned, it is advantageous to
use an emery or carborundum wheel of No. 90 to No. 120 grit, from two
and one-half to six inches in diameter and from three-eighths to
three-fourths of an inch thick, revolving on a lathe at a low rate of
speed. A stream of water should be kept constantly dropping on the wheel
in order to keep both the wheel and the work wet.
Rubber cots are a great protection to the fingers, which are gradually
worn away by the stones.
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