0^4 PAINTING MATERIALS
of obvious shortcomings, glass has never been an important support for paintings,
although the large and rigid flat surfaces it offers must have suggested its use. It
has, however, been made a support for miniatures and for decorated clock faces.
The smoothness of its surface has presented a certain difficulty in the application
of paint. Painting on glass or painted glass may be of different kinds: the applica-
tion of a pigment with a medium, in the ordinary sense, or the kind that is
commonly called ' stained glass ' where vitrifiable pigments are applied and fired
in. J. D. LeCouteur (p. i) contends that the expression ' stained glass/ so often
applied to colored glass whether mediaeval or modern, is misleading and erroneous.
It should be ' painted glass/ He goes on to say, however, that the method, dis-
covered in the XIV century, whereby glass was colored yellow by painting with a
silver solution and retiring, produces a glass that may properly be called 'stained.'
Painting and gilding on glass appear to have been frequently done in the Renais-
sance. In Thompson's translation of Cennino Cennini, // Libro delT Arte^ there is
shown a diptych with panels of gilded and painted glass and Cennino Cennini
describes (pp. 111-113 °^ tn^s translation) a method for gilding on glass and
scratching in designs with a needle. Leonardo da Vinci made a suggestion (see W.
Ostwald, p. 137) that paintings be made directly on glass and seen through it so
that there would be no diffusion of light from the surface of the paint. The sug-
gestion may have been acted on; paintings done in this way have survived from
somewhat later times. For the most part, they are of provincial origin, and doubt-
less large numbers have been lost entirely.
Although glass is generally considered to be a stable material, it is subject to
internal changes and decay which depend upon environment. There is some
tendency to crystallization or * devitrification' on the part of the super-cooled
liquid alkali silicates from which it is made, and this causes glass to become more
brittle with age. Ancient glass, which has long been buried in the soil, exhibits a
dull surface with well known iridescence. The cause is said to be the action of
moisture, oxygen, and carbonic acid in the atmosphere. They dissolve the alkali
from the glass and leave silicic acid in the form of minute scales that produce
iridescence by interference. In ancient glasses there was often an excess of alkali
over and above the proportion required to combine with the silica present. The
excess is soluble in water and attracts moisture and carbon dioxide from the atmos-
phere. There have been seen ancient glazes on pottery where practically all the
alkali has been leached out by ground waters to leave only a residue of porous
silica or silica hydrate. Potash glasses, like Venetian glass, are liable to become
moist (' sweating glass ') because of the hygroscopic nature of the potassium
carbonate that is formed in the initial stages of decay. The silica does not reach a
powdery condition, for it is dissolved by the moist" alkali. It may become opales-
cent but not iridescent. These are, however, extreme cases of deterioration, A
glass that still retains its paint film has probably changed little in the course of
centuries, except to become more brittle.