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.
[CHAPTER VIII]
The Second Painting—Disappointment with Fired Work—A False Remedy—A Useful Tool—The Needle—A Resource of Desperation—The Middle Course—Use of the Finger—The Second Painting—Procedure.
And when you have looked at it, as I said just now you should do, your first thought will be a wish that you had never