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the aerograph process, that is to say, the gum is blown on to the letters
under air pressure; thereafter, powdered enamel is blown on by the
same process. Next, the letters are fused or baked in a furnace, thus
drying the enamel on the copper. After cooling and cleaning, the letters
are ready for use. The spraying or aerograph process is carried out in
specially constructed enclosures, these being ventilated by air ducts on
which a powerful exhaust fan is working. The fumes of the gums and
dust from the dry enamel are quickly drawn off.
Enamel Plates. The plates are cut into shapes by the guillotine
and are pickled in muriatic acid. The first coat of colour is now blown
on to the plates by the aerograph process. This is followed by fusing
or baking, cleaning, and finally re-fusing. The various colours are then
blown on to the plate, and a stencil bearing the wording or design of
the plate required is placed over it; the superfluous colour is then
brushed out. When this operation is completed, the plate is once again
fused, and then Anally cleaned.
Embossed Glass Tablets. The tablet required is cut from a sheet
of coloured glass and the edges are either ground smooth or bevelled.
The surfaces of the tablet are completely covered by an acid-resisting
preparation called asphaltum. The lettering or design is now traced by
stencil upon the surface of the tablet, the pattern, etc., being actually
traced in the asphaltum. Next, the tablet is placed in a bath of hydrofluoric
acid and the acid attacks the portion of glass not protected by
the asphaltum, thus embossing or eating into the glass, so forming the
desired words or pattern. After 20 hours' exposure to the acid, the
tablet is removed and then passed through weaker acid baths until
eventually all the acid is washed off. By this time the lettering or
design stands out white upon a coloured background. In other tablets
on clear glass, the design is embossed in the same way, but is gilded or
coloured as required. In further types the design is transferred to the
tablet by means of stencil and is brushed out.
Silvering Glass. The prepared tablets are placed upon a hot plate
the temperature of which is about 150° Fahr. Silver nitrate solution
is then poured over the surface of the glass. The heat passing through
the glass from the hot plate below causes evaporation of the water, and
a coat of silver nitrate is left on the glass. Glass letters are cut from
sheets of glass either by hand or by sand-blast; they are then shaped,