FIRING
AFTER enamel has been charged on a piece of metal, it is “fired,” that
is, heated until the enamel is fused, after which, on cooling, it
becomes hard. It is best fired in a muffle furnace, although with care a
nice job may be done with a blowpipe, a Bunsen burner, or even an
alcohol lamp.
In case a Bunsen burner or lamp is used, it is well to have a metal
tripod supporting a sheet of metal or wire mesh on which to set the
work, as holding it by a pair of tongs becomes very tedious.
[Illustration:
FIG. 5. METAL TRAY OR TILE.
]
A muffle may be heated by electricity, gas (with or without an air
blast), oil, coal, or wood. Electricity or gas, however, is preferable,
as they are not accompanied by dirt. The muffle itself, which is simply
a small oven, is generally made of fire clay, although both fused quartz
and nickel are sometimes used. A door to the muffle is not necessary,
although it is an aid to cleanliness. If, however, a door is used it
should have a hole in it large enough to allow the enameler to watch his
work constantly. It is very important to keep the enamel work away from
the oxidizing effect of the fire; so that any cracks that appear in a
muffle must be immediately repaired. It will be found very advantageous,
too, to have the doors of the furnace on the side of it opposite to that
containing the mouth of the muffle.
The pieces to be fired should be laid upon a tray of a size convenient
to pass easily through the mouth of the muffle. These trays are made
preferably of sheet nickel for this metal will not scale or flake off
when heated and so contaminate the enamel. As these trays become quite
hot it is advisable to handle them by means of a pair of long-handled
tongs.
Before placing a tray of pieces in the muffle to be fired, it is well to
leave it near the mouth of the muffle for a minute or two so that the
enamel may become entirely dry before it is placed in the muffle, for if
damp the enamel will be blown off of the metal by the rapid evaporation
of the water which it contains.
When the pieces are fully dry, the tray may be placed in the muffle,
which should be heated to a bright red heat of about 1400° F. With such
a heat it should take from half a minute to two minutes to “fire” or
melt the enamel. The time required will vary with different enamels and
can only be learned by experiment. The enamel should be kept in the
muffle until it begins to melt and its surface becomes fused and
comparatively smooth and glossy, when it should be removed from the
furnace and allowed to cool in the air. It is not necessary or even
desirable to fire the enamel perfectly smooth in the first firing. It
generally takes at least two coats, that is two chargings and two
firings, before a piece is ready to be stoned. If, however, only one
coat is to be used, it should be fired as smooth as possible.
When the pieces cool, the metal (unless it is fine, _i.e._, pure,
unalloyed) will be found covered with a black scale caused by oxidation.
This may be removed by simply brushing it, which operation in most cases
will clean the piece sufficiently for the second charging. Otherwise the
piece may be “pickled,” or cleaned with acid.
Subsequent firings do not differ from the first one except that in the
last firing the enameler should try to obtain as smooth a surface as
possible.
After the last firing the pieces should be submerged for a minute or two
in a mild pickle consisting of one part sulphuric acid and one part
water. This will clean off the oxide and make the metal bright.
If red enamel is used it is best to throw the piece into a cup of thick
heavy machine oil to cool instead of letting it cool in the air. This
will give the red a greater brilliancy. Red enamel, too, often requires
a greater heat than other colors, although too low a heat tends to
destroy the gloss on any enamel. Many delicate colors which are opaque
when fired at a low heat, will become opalescent and in some cases
transparent if fired at a higher temperature. Red enamel loses its color
if fired too many times, and a large number of firings have a weakening
effect on high-karat gold.
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