1.
Flint glass (powdered) 10 parts. } moderately
White Arsenic 1 " } hard.
Nitre 1 " }
2.
Red Lead 1 " } soft.
Flint glass (powdered) 3 " }
3.
Flint glass 6 "
Red Lead 8 "
(Mixed with four parts of the first flux, soft.)
The use of very soft fluxes is attended with this inconvenience, that
the boracic acid contained in them is generally acted upon by moisture
and becomes hydrated, and in this condition often causes the painting
to peel away. Harder fluxes, although they have the disadvantage of
necessitating the glass to be submitted to a much higher temperature
for a longer time in the kiln or muffle, are the best, and, with
judicious management, can be used without any injurious consequences
to the work on which they are employed. Lead fluxes, containing oxide
of lead, are sufficiently fusible for all ordinary purposes, and are
not liable to the same objection as fluxes containing borax. Suppose,
then, it is desired to paint the outlines of a face, the glass is cut
to the shape of the face in the cartoon; it is then laid upon it, and
the painter, seeing the lines through the glass, is able to trace them
with his brown paint upon its surface. He generally uses gum water as
his vehicle, and puts on the shading also with the same mixture,
though sometimes it is found necessary to use a substance which is not
affected by moisture, as for instance, tar-oil. It is impossible, in
the short space of this article, to indicate those occasions on which
one should be used in place of the other; a knowledge of this can only
be obtained by consulting authorities in which details are more
minutely given, or by watching the operations of the glass painter in
his workshop. When the face is finished, it is removed, and another
portion of the figure, say a piece of the drapery, is proceeded with
in exactly the same way; and so, by a repetition of this process in
all parts of the figure, it is completed, and looks very much like a
puzzle, the parts being put together on the cartoon before the work is
finished, in order to see that the whole is harmoniously treated. In
shading the face, hands, and those parts of the drapery which require
it, a glass easel is used, on which the figure is put together, and
the parts made to adhere by wax, so that the artist is able, while
painting, to form an idea by transmitted light of the effect which
will be produced when the window is finished. The ornament is painted
in a similar manner, but usually not with the same care in the details
of its execution.
When all the glass is painted, it is fired in a muffle, upon the
proper construction of which a great deal depends. It is usually made
of iron, and should not be more than 15 inches from its bottom to the
top, though its width may vary. It is never well to have muffles for
firing glass for painted windows larger than about 2 feet wide, by 2
feet 6 inches deep. The top of the muffle is usually slightly arched
from side to side, and it is placed in the furnace on a tolerably
thick stone floor, so that the bottom may not get too hot. The fire,
which is lighted below, is allowed to play up its sides and over its
top, the flue being so built as to draw the flames in that direction,
for a top heat is the best heat for firing glass regularly. The muffle
is arranged with ridges in its sides, passing from front to back
parallel to one another on one side, and exactly opposite to
corresponding ridges parallel to one another on the opposite side.
These metal ridges are intended to receive iron plates, and there is
generally about an inch or rather less between the top of one plate
and the bottom of another, when the muffle is perfectly filled. The
plates are covered over with perfectly dry powdered chalk or whiting,
and the pieces of glass are laid upon them with their painted sides
uppermost. When the plates are charged, they are put into a muffle
with an iron door, in the centre of which is a hole, and a conical
tube with the base attached round it. It is larger than the opening at
the other end, which projects some 6 or 7 inches from the surface of
the muffle-door at right angles to it. A second door is then placed at
a short distance from the first, the tube passing through a hole made
for the purpose in it. The orifice is usually stopped by a piece of
fire-clay, which can be removed at pleasure. The use of the tube is,
to enable the manager of the kiln to look into the muffle, from time
to time, to see that the glass does not get too much heated. When the
firing is completed, the fire is raked out and the muffle is allowed
to cool very slowly, and by this process the glass becomes annealed.
When it is desired to apply to any portion of white glass some yellow
silver stain, this can be done either in the first firing, by floating
it on to the places to be stained, and allowing it to run in a sort of
stream from the brush, so that it will evenly cover the surface and
cause the heavier portions of the stain, namely, the mixed metallic
silver and antimony, to sink regularly to the bottom, and come fairly
in contact with the glass. Not very long ago, it was mentioned to me
by a glass painter of note, that the workmen much prefer using the old
stain made with silver and antimony, to that which is produced by
using nitrate of silver. This really is a mistake on their part, for,
when properly managed (and the knowledge of how to manage this stain
can be acquired with very little trouble), the nitrate of silver stain
is by far the best, and produces much better tints, with less chance
of what the men call sulphuring when the glass is fired. This
sulphuring is simply the result of opacity, obtained by heating the
glass to too high a temperature. If the staining is to be performed in
the same firing as that by which the painting is to be fixed, it is
quite clear that the outlines of the part to be stained must be
painted in, with tar-oil, or with some such substance which is not
affected by the moisture of the stain. However, in general, the
staining operation is performed after the first firing, that is to
say, those pieces of glass to which the silver is to be applied are
stained in the method above described after the first firing, and are
then fired again, because the heat required to produce a good stain
from silver is of a somewhat different character from that which is
required simply to fuse the flux that binds the pigment to the glass.
A longer and less intense heat, technically called a "soaking," is the
best for producing an even and pure yellow tint. If the temperature be
allowed to rise too high, the oxide of silver, which alone can stain
the glass, gets reduced wholly or in part, and when this happens to
only a slight extent, it destroys the transparency of the stain; and
when it happens to a great extent, it destroys its colour altogether,
making the glass opaque.
It is a matter of astonishment to me that glass painters do not use a
ruby stain, which, with a little practice, can be managed quite as
successfully as the yellow silver one. It is true that it would be
impossible to fire the ruby and the silver stains together, and it
would not be at all convenient to fire the ruby stain at the first
firing of the painted glass. The method of staining ruby is as
follows: grind up carefully some black oxide of copper, mix it with
water (or with a small quantity of gum added), float it on the parts
to be coloured, place it in a kiln and heat it. Black oxide of copper,
when mixed with glass and melted in a glass-pot, makes the glass
green; suboxide of copper, which contains less oxygen than the black
oxide, when treated in the same way, makes it red. Now, if it can be
reduced to the lower oxide of copper, while the black oxide of copper
on the surface of the glass is heated, it will then colour the glass
red. The best way of reducing the black oxide, is to connect the
muffle with a gas-supply pipe, and allow coal gas to pass during the
whole time that the heating process goes on. The action of the gas,
which contains hydrogen and carbon, is to take away oxygen from the
black oxide of copper, when it is at a high temperature; and, as soon
as sufficient is taken away by the hydrogen to reduce the black oxide
to the state of suboxide, it stains the glass red. It does not matter
if the reducing action be continued longer, so that the oxide of
copper be reduced to the metallic state; for at that temperature, the
stain produced by the red oxide of copper is not removed by the
continued action of hydrogen gas. The employment of this process would
certainly enable artists who paint in the later styles of glass
painting, to very much enrich their draperies, and to produce, more
easily, effects which now can only be obtained by a complicated system
of lead-work.
When the pieces of glass which have been fired are perfectly cold, the
next process is to unite them altogether by peculiarly shaped strips
of lead, which are of various kinds, according to the character of the
subject required. The lead has a thick part or core, and at right
angles to the top and bottom of this are thin plates called the
"leaves." The core is milled with little ridges running at right
angles to them, so as to enable the workman to bend the lead about
with facility. The edges of the piece of glass to be leaded are placed
between the leaves and resting upon the core, and the lead is thus
arranged all round the glass, and is then laid in its proper situation
upon another cartoon, prepared from the one from which the figure was
painted, and indicating simply, by lines, where the lead-work is to
come. The first piece is fixed by means of nails temporarily placed
through the lead. Those pieces which touch it in the design are put in
their proper positions, so that the edge touching the next piece will
be underneath the opposite leaves to those which confine the first.
This operation is repeated, till all the parts of the design are
surrounded by lead, and by it united to one another; the joints being
secured by solder, generally applied by gas. Nothing now remains but
to fill in the interstices between the lead and the glass, so as to
make the window firm, solid, and water-tight; and this is done by
rubbing into them with a scrubbing brush a cement, usually made of
white lead, oil, and plaster of Paris. This composition varies in
different stained glass works, nor is it material, provided that the
substance hardens, does not crack, and is waterproof.
From this description it will be seen, that the various colours in the
different parts of the window are put in as pieces, and that no
colours, properly so called, are applied by the brush to the surface.
There are, however, certain tints of the "tracing brown," which can be
obtained by the addition of black oxide of manganese, or by a
different method of preparation of the oxide of iron, to give it its
body. Sulphate of iron, when heated, loses its sulphuric acid, and the
oxide, which was, as sulphate, in the state of protoxide, becomes, by
heating, the red or peroxide of iron; its tint, when made in this way,
being generally lighter than the tint of that form of oxide which is
employed as ordinary tracing brown. It is sometimes called flesh tint,
though this is decidedly an objectionable name for it.
It has been suggested to me, that I should give some receipts for the
manufacture of the enamel colours used in mediA|val glass painting; I
have therefore added a few which are easily prepared. Others of a more
complicated nature had much better be obtained from the makers of the
enamel used in porcelain painting. And here again, let me remark, that
in ordering fluxes from these manufacturers, it should be stated
especially that a flux is required which does not contain borax, nor
should the painters in any establishment be allowed to use these
softer fluxes, which they are almost certain to do, unless forbidden;
for though they are easier to work with, they will infallibly lead to
calamitous results.