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.