a large.
A head as big as the palm of your hand is not a very severe test of your
powers; but in one as large as the _whole_ of your hand, say a head
seven inches from crown to chin, the problem is increased quite
immeasurably in difficulty. The real test is being able to produce in
glass a real facsimile of a head by Botticelli or Holbein, and when you
can do that satisfactorily you can do anything in glass-painting.

Do not aim to get _too much_ in the first painting, at any rate not till
you have had long practice. Be content if you get enough modelling on a
head to turn the outline into a more sensitive and artistic drawing than
it could be if planted down, raw and hard, upon the bare, cold glass.
After all it is a common practice to fire the outline separately, and
anything beyond this that you get upon the glass for first fire is so
much to the good.

But besides the quality of the _gum_ you will find sometimes differences
in the quality or condition of the _pigment_. It may be insufficiently
ground; in which case the matt, in passing over, will rasp away every
vestige of the outline, so delicate a matter it is.

You can tell when colour is not ground sufficiently by the way it acts
when laid as a vertical wash. Lay a wash, moist enough to "run," on a
bit of your easel-slab; it will run down, making a sort of
seaweed-looking pattern--clear lanes of light on the glass with a black
grain at the lower end. Those are the bits of unground material: under a
100-diameter microscope they look like chunks of ironstone or road
metal, or of rusty iron, and you'll soon understand why they have
scratched away your tender outline.

You must grind such colour till it is smooth, and an old-fashioned
_granite_ muller is the thing, not a glass one.

Now, after all this, how am I to excuse the paradox that it is possible
to have the colour ground _too_ fine! All one can say is that you "find
it so." It can be so fine that it seems to slip about in a thin, oily
kind of way.

It's all as you find it; the differences of a craft are endless; there
is no forecasting of everything, and you must buy your experience, like
everybody else, and find what suits you, learning your skill and your
materials side by side.

Now these are the chief processes of painting, as far as laying on
colour goes; but you still have much of your work before you, for the
way in which light and shade is got on glass is almost more in "taking
off" than i