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.