Firing--Three Kinds of Kiln--Advantages and Disadvantages--The
Gas-Kiln--Quick Firing--Danger--Sufficient Firing--Soft
Pigments--Difference in Glasses--"Stale" Work--The Scientific
Facts--How to Judge of Firing--Drawing the Kiln.
The way in which the painting is attached to the glass and made
permanent is by firing it in a kiln at great heat, and thus fusing the
two together.
Simple enough to say, but who is to describe in writing this process in
all its forms? For there is, perhaps, nothing in the art of
stained-glass on which there is greater diversity of opinion and
diversity of practice than this matter of firing. But let us make a
beginning by saying that there are, it may be said, three chief
modifications of the process.
First, the use of the old, closed, coke or turf kiln.
Second, of the closed gas-kiln.
And third, of the open gas-kiln.
The first consists of a chamber of brick or terra-cotta, in which the
glass is placed on a bed of powdered whitening, on iron plates, one
above another like shelves, and the whole enclosed in a chamber where
the heat is raised by a fire of coke or peat.
This, be it understood, is a slow method. The heat increases gradually,
and applies to the glass what the kiln-man calls a "good, soaking heat."
The meaning of this expression, of course, is that the gradual heat
gives time for the glass and the pigment to fuse together in a natural
way, more likely to be good and permanent in its results than a process
which takes a twentieth part of the time and which therefore (it is
assumed) must wrench the materials more harshly from their nature and
state.
There are, it must be admitted, one or two things to be said for this
view which require answering.
First, that this form of kiln has the virtue of being old; for in such a
thing as this, beyond all manner of doubt, was fired all the splendid
stained-glass of the Middle Ages.
Second, that by its use one is entirely preserved from the dangers
attached to the _misuse_ of the gas-kiln.
But the answers to these two things are--
First, that the method employed in the Middle Ages did not invariably
ensure permanence. Any one who has studied stained-glass must be
familiar with cases in which ancient work has faded or perished.
The second claim is answered by the fact, I think beyond dispute, that
all objections to the use of the gas-kiln would be removed if it were
used properly; it is not the use of it as a process which is in itself
dangerous, but merely the misuse of it. People must be content with what
is reasonable in the matter; and, knowing that the gas-kiln is spoken of
as the "quick-firing" kiln, they must not insist on trying to fire _too_
quick.
Now I have the highest authority (that of the makers of both kiln and
pigment) to support my own conviction, founded on my own experience, in
what I am here going to say.
Observe, then, that up to the point at which actual fusion
commences--that is, when pigment and glass begin to get soft--there is
no advantage in slowness, and therefore none in the use of fuel as
against gas--no possible _disadvantage_ as far as the work goes: only it
is time wasted. But where people go wrong is in not observing the vital
importance of proceeding gently when fusion _does_ commence. For in the
actual process of firing, when fusion is about to commence, it is indeed
all-important to proceed gently; otherwise the work will "fry," and, in
fact, it is in danger from a variety of causes. Make it, then, your
practice to aim at twenty to twenty-five minutes, instead of ten or
twelve, as the period during which the pigment is to be fired, and
regulate the amount of heat you apply by that standard. The longer
period of moderate heat means safety. The shorter period of great heat
means danger, and rather more than danger.
Fig. 42 is the closed gas-kiln, where the glass is placed in an enclosed
chamber; fig. 43 is the open gas-kiln, where the gas plays on the roof
of the chamber in which the glass lies; fig. 44 shows this latter. But
no written description or picture is really sufficient to make it safe
for you to use these gas-kilns. You would be sure to have some serious
accident, probably an explosion; and as it is absolutely necessary for
you to have instruction, either from the maker or the experienced user
of them, it is useless for me to tell lamely what they could show
thoroughly. I shall therefore leave this essentially technical part of
the subject, and, omitting these details, speak of the few _principles_
which regulate the firing of glass.
[Illustration: FIG. 42.]
[Illustration: FIG. 43.]
[Illustration: FIG. 44.]
And the first is to _fire it enough_. Whatever pigment you use, and with
whatever flux, none will be permanent if the work is under-fired; indeed
I believe that under-firing is far more the cause of stained-glass
perishing than the use of untrustworthy pigment or flux; although it
must always be borne in mind that the use of a soft pigment, which will
"fire beautifully" at a low heat, with a fine gloss on the surface, is
always to be avoided. The pigment is fused, no doubt; but is it united
to the glass? What one would like to have would be a pigment whose own
fusing-point was the same, or about the same, as that of the glass
itself, so that the surface, at least, of the piece of glass softens to
receive it and lets it right down into itself. You should never be
satisfied with the firing of your glass unless it presents two
qualifications: first, that the surface of the glass has melted and
begun to run together; and second, that the fused pigment is quite
glossy and shiny, not the least dull or rusty looking, when the glass is
cool.
"What one would like to have."
And can you not get it?
Well, yes! but you want experience and constant watchfulness--in short,
"rule of thumb." For every different glass differs in hardness, and you
never know, except by memory and constant handling of the stuff, exactly
what your materials are going to do in the kiln; for as to
standardising, so as to get the glass into any known relation with the
pigment in the matter of fusing, the thing has never, as far as I know,
been attempted. It probably could not be done with regard to all, or
even many, glasses--nor need it; though perhaps it might be well if a
nearer approach to it could be achieved with regard to the manufacture
of the lighter tinted glasses, the "whites" especially, on which the
heads and hands are painted, and where consequently it is of such vital
importance that the painting should have careful justice done to it, and
not lose in the firing through uncertainty with regard to conditions.
Nevertheless, if you observe the rule to fire sufficiently, the worst
that can happen is a disappointment to yourself from the painting having
to an unnecessary extent "fired away" in the kiln. You must be patient,
and give it a second painting; and as to the "rule of thumb," it is
surprising how one gets to know, by constant handling the stuff, how the
various glasses are going to behave in the fire. It was the method of
the Middle Ages which we are so apt to praise, and there is much to be
said for practical, craftsmanly experience, especially in the arts, as
against a system of formulas based on scientific knowledge. It would be
a pity indeed to get rid of the accidental and all the delight which it
brings, and we must take it with its good and bad.
The second rule with regard to the question of firing is to take care
that the work is not "stale" when it goes into the kiln. Every one will
tell you a different tale about many points connected with glass, just
as doctors disagree in every affair of life. In talking over this matter
of keeping the colour fresh--even talking it over with one's practical
and experienced friends generally--one will sometimes hear the remark
that "they don't see that delay can do it much harm;" and when one asks,
"Can it do it any good?" the reply will be, "Well, probably it would be
as well to fire it soon;" or in the case of mixing, "To use it fresh."
Now, if it would be "as well"--which really means "on the safe
side"--then that seems a sufficient reason for any reasonable man.
But indeed I have always found it one of the chiefest difficulties with
pupils to get them to take the most reasonable precautions to _make
quite sure_ of _anything_. It is just the same with matters of
measurement, although upon these such vital issues depend. How weary one
gets of the phrase "it's not far out"--the obvious comment of a
reasonable man upon such a remark, of course, being that if it is out
_at all_ it's, at any rate, _too_ far out. A French assistant that I had
once used always to complain of my demanding (as he expressed it) such
"rigorous accuracy." But there are only two ways--to be accurate or
inaccurate; and if the former is possible, there is no excuse for the
latter.
But as to this question of freshness of colour, which is of such
paramount importance, I may quote the same authority I used before--that
of the _maker of the colour_--to back my own experience and previous
conviction on the point, which certainly is that fresh colour, used the
same day it is ground and fired the same day it is used, fires better
and fires away less than any other.
The facts of the case, scientifically, I am assured, are as follows. The
pigment contains a large amount of soft glass in a very fine state of
division, and the carbonic acid, which all air contains (especially that
of workshops), will immediately begin to enter into combination with the
alkalis of the glass, throw out the silica, and thus disintegrate what
was brought together in the first instance when the glass was made. The
result of this is that this intruder (the carbonic acid) has to be
driven out again by the heat of the kiln, and is quite likely to disturb
the pigment in every possible way in the process of its escape. I have
myself sometimes noticed, when some painted work has been laid aside
unusually long before firing, some white efflorescence or
crystallisation taking place and coming out as a white dust on the
painted surface.
Now it is not necessary to know here, in a scientific or chemical sense,
what has actually taken place. Two things are evident to common sense.
One, that the change is organic, and the other that it is
unpremeditated; and therefore, on both grounds, it is a thing to avoid,
which indeed my friend's scientific explanation sufficiently confirms.
It is well, therefore, on all accounts to paint swiftly and
continuously, and to fire as soon as you can; and above all things not
to let the colour lie about getting stale on the palette. Mix no more
for the day than you mean to use; clean your palette every day or nearly
so; work up all the colour each time you set your palette, and do not
give way to that slovenly and idle practice that is sometimes seen, of
leaving a crust of dry colour to collect, perhaps for days or weeks,
round the edge of the mass on your palette, and then some day, when the
spirit moves you, working this in with the rest, to imperil the safety
of your painting.
_How to Know when the Glass is Fired Sufficiently._--This is told by the
colour as it lies in the kiln--that is, in such a kiln that you can see
the glass; but who can describe a colour? You have nothing for this but
to buy your experience. But in kilns that are constructed with a
peephole, you can also tell by putting in a bright iron rod or other
shining object and holding it over the glass so as to see if the glass
reflects it. If the pigment is raw it will (if there is enough of it on
the glass to cover the surface) prevent the piece of glass from
reflecting the rod; but directly it is fired the pigment itself becomes
glossy, and then the surface will reflect.
This is all a matter of practice; nothing can describe the "look" of a
piece of glass that is fired. You must either watch batch after batch
for yourself and learn by experience, or get a good kiln-man to point
out fired and unfired, and call your attention to the slight shades of
colour and glow which distinguish one from the other.
_On Taking the Glass out of the Fire._--And so you take the glass out of
the fire. In the old kilns you take the fire away from the glass, and
leave the glass to cool all night or so; in the new, you remove it and
leave it in moderate heat at the side of the kiln till it is cool enough
to handle, or nearly cold. And then you hold it up and look at it.