252 PAINTING MATERIALS
where the plaster has been stained with pigment during the slaking process is
found in several temples. The thickness of plaster walls in ancient times was much
greater than now. The Greeks employed a method of beating the stucco with an
implement called a baculi until the texture became so close and fine that it re-
sembled marble.
Cowper (p. 4) says that there was much plaster work in Rome and that it was
the common external finish for private houses. Examples of the decorative modelled
stucco of Rome and Pompeii are well known. Vitruvius (II and VII) gives ex-
plicit working instructions for the choice and use of lime, sand, pozzolana, and
other materials for mortar and plaster, and he gives, also, details for the applica-
tion of fine six-coat marble dust and lime plaster on reed laths for internal
decorative work. The old Roman wall paintings seem to have been executed on a
very smooth plaster. The pigment has probably been preserved because of the
wax film used for protection. In mediaeval times the art of making fine lime and
hydraulic mortars was lost, although poor lime mortars continued to be used. In
the Renaissance, however, interest in the ancient arts of decorative plaster and
stucco work was revived. Cowt>er (p. 5) says that Vitruvius was still accepted as
the authority on plaster.
Linen stiffened with plaster was used for decorating purposes in Egypt, and
Cennini, writing in 1437, speaks of fine linen soaked in glue and plaster and laid
on wood as a painting ground. Canvas and plaster were in general use in Great
Britain up to about 1850.
Wall paintings recovered from Central Asia by Stein, and dated from the end
of the III century A.D. to the X century or slightly later, show supporting walls
of a mud plaster containing, in some instances, an admixture of fibre or coarse
straw. The mud surface has been smoothed and a coat of whitewash applied over
it. The thickness of the slabs is from one and a half to two inches. Plaster and
decorated plaster work reached a high development in Asia Minor; Persian,
Moorish, and Saracen work, however, depended mainly on gypsum.
Painted plaster has been found in excavations in both the American continents,
in Asia, and in the Near East. Lime and gypsum plasters and Portland cement
are used today much as they were by the ancients. Modern mural painting, in
either true fresco or tempera, follows the same rules established centuries ago.
Plaster, deterioration and treatment. Clay plasters are perhaps the weakest,
lime and gypsum are stronger and pozzolanic plasters are the strongest. In the
drying-out and shrinking process they are subject to cracking in the order named.
This defect is counteracted by the proper proportion of filler and binder. Plaster
is subject, also, to slight stresses from thermal expansion and contraction. Walls
may suffer disfigurement through large settling cracks which run from corner to
corner* These are caused by movement of the building after the plaster has
set and are not necessarily defects in the plaster itself. Except for the pozzolanic
types, plasters are seriously affected by continued or alternate exposure to water,