While the material used in mosaic wall decoration is sometimes a
natural product, like marble, porphyry, coral, or alabaster, the
picture is composed for the most part of artificially prepared
smalts--opaque glass of various colours, made in sheets and then
cut up into cubes. An infinite variety in gradations of colour
and texture is thus made possible.

The gold grounds which one sees in nearly all mosaics are constructed
in an interesting way. Each cube is composed of plain rather coarse
glass, of a greenish tinge, upon which is laid gold leaf. Over
this leaf is another film of glass, extremely thin, so that the
actual metal is isolated between two glasses, and is thus impervious
to such qualities in the air as would tarnish it or cause it to
deteriorate. To prevent an uninteresting evenness of surface on
which the sun's rays would glint in a trying manner, it was usual
to lay the gold cubes in a slightly irregular manner, so that
each facet, as it were, should reflect at a different angle,
and the texture, especially in the gold grounds, never became
monotonous. One does not realize the importance of this custom
until one sees a cheap modern mosaic laid absolutely flat, and then
it is evident how necessary this broken surface is to good effect.
Any one who has tried to analyze the reason for the superiority of
old French stained glass over any other, will be surprised, if
he goes close to the wall, under one of the marvellous windows
of Chartres, for instance, and looks up, to see that the whole
fabric is warped and bent at a thousand angles,--it is not only
the quality of the ancient glass, nor its colour, that gives this
unattainable expression to these windows, but the accidental warping
and wear of centuries have laid each bit of glass at a different