The invention of M. E. Godard, of Paris, has for its object the
reproduction of images and drawings, by means of vitrifiable colors on
glass, wood, stone, on canvas or paper prepared for oil-painting and on
other substances having polished surfaces, e. g., earthenware, copper,
etc. The original drawings or images should be well executed, and drawn
on white, or preferably bluish paper, similar to paper used for ordinary
drawings. In the patterns for glass painting, by this process, the place
to be occupied is marked by the lead, before cutting the glass to suit
the various shades which compose the color of a panel, as is usually
done in this kind of work; the operation changes only when the glass
cutter hands these sheets over to the man who undertakes the painting.
The sheets of glass are cut according to the lines of the drawing, and
after being well cleaned, they are placed on the paper on the places for
which they have been cut out. If the window to be stained is of large
size and consists of several panels, only one panel is proceeded with
at a time. The glass is laid on the reverse side of the paper (the side
opposite to the drawing), the latter having been made transparent by
saturating it with petroleum. This operation also serves to fix the
outlines of the drawing more distinctly, and to give more vigor to the
dark tone of the paper. When the paper is thus prepared, and the sheets
of glass each in its place, they are coated by means of a brush with
a sensitizing solution on the side which comes into contact with the
paper. This coating should be as thin and as uniform as possible on
the surface of the glass. For more perfectly equalizing the coating, a
second brush is used.
The sensitizing solution which serves to produce the verifiable image is
prepared as follows: Bichromate of ammonia is dissolved in water till
the latter is saturated; five grammes of powdered dextrin or glucose are
then dissolved in 100 grammes of water; to either of these solutions
is added 10 per cent. of the solution of bichromate, and the mixture
filtered.
The coating of the glass takes place immediately afterward in a dark
room; the coated sheets are then subjected to a heat of 50° or 60° C.
(120° to 140° Fahr.) in a small hot chamber, where they are laid one
after the other on a wire grating situated 35 centimeters above the
bottom. Care should be taken not to introduce the glass under treatment
into the hot chamber before the required degree of heat has been
obtained. A few seconds are sufficient to dry each sheet, and the wire
grating should be large enough to allow of the dried glass being laid in
rows, on one side where the heat is less intense. For the reproduction
of the pictures or images a photographic copying frame of the size of
the original is used. A stained glass window being for greater security
generally divided into different panels, the size of one panel is seldom
more than one square meter. If the picture to be reproduced should be
larger in size than any available copying frame, the prepared glass
sheets are laid between two large sheets of plate-glass, and part after
part is proceeded with, by sliding the original between the two sheets.
A photographic copying frame, however, is always preferable, as it
presses the glass sheets better against the original. The original
drawing is laid fiat on the glass of the frame. The lines where the lead
is to connect the respective sheets of glass are marked on the drawing
with blue or red pencil. The prepared sheets of glass are then placed
one after the other on the original in their respective places, so that
the coated side comes in contact with the original. The frame is then
closed. It should be borne in mind that the latter operations must be
performed in the dark room. The closed frame is now exposed to light. If
the operations are performed outdoors, the frame is laid flat, so that
the light falls directly on it; if indoors, the frame is placed inclined
behind a window, so that it may receive the light in front. The time
necessary for exposing the frame depends upon the light and the
temperature; for instance, if the weather is fine and cloudless and the
temperature from 16° to 18° C. (60° to 64° Fahr.), it will require from
12 to 15 minutes.
It will be observed that the time of exposure also depends on the
thickness of the paper used for the original. If, however, the weather
is dark, it requires from 30 to 50 minutes for the exposure. It will be
observed that if the temperature is above 25° C. (about 80° Fahr.), the
sheets of glass should be kept very cool and be less dried; otherwise,
when exposed the sheets are instantly metallized, and the reproduction
cannot take place. The same inconvenience takes place if the temperature
is beneath 5° C. (41° Fahr.). In this case the sheets should be kept
warm, and care should be taken not to expose the frame to the open air,
but always behind a glass window at a temperature of from 14° to 18°
C. (about 60° Fahr.). The time necessary for the exposure can be
ascertained by taking out one of the many pieces of glass, applying to
the sensitive surface a vitrifiable color, and observing whether the
color adheres well. If the color adheres but slightly to the dark, shady
portions of the image, the exposure has been too long, and the process
must be recommenced; if, on the contrary, the color adheres too well,
the exposure has not been sufficient, the frames must be closed again,
and the exposure continued. When the frame has been sufficiently
exposed, it is taken into the dark room, the sensitized pieces of glass
laid on a plate of glass or marble with the sensitive surface turned
upward, and the previously prepared vitrifiable color strewed over it by
means of a few light strokes of a brush. This powder does not adhere to
the parts of the picture fully exposed to light, but adheres only to the
more or less shady portions of the picture. This operation develops
on the glass the image as it is on the paper. Thirty to 40 grammes
of nitric acid are added to 1,000 grammes of wood-spirit, such as is
generally used in photography, and the prepared pieces of glass are
dipped into the bath, leaving them afterward to dry. If the bath becomes
of a yellowish color, it must be renewed. This bath has for its object
to remove the coating of bichromate, so as to allow the color to adhere
to the glass, from which it has been separated by the layer of glucose
and bichromate, which would prevent the vitrification. The bath has also
for its object to render the light parts of the picture perfectly
pure and capable of being easily retouched or painted by hand. The
application of variously colored enamels and the heating are then
effected as in ordinary glass painting. The same process may be applied
to marble, wood, stone, lava, canvas prepared for oil painting,
earthenware, pure or enameled iron. The result is the same in all cases,
and the process is the same as with glass, with the difference only that
the above named materials are not dipped into the bath, but the liquid
is poured over the objects after the latter have been placed in an
inclined position.
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