supports	251
the deterioration of plasters. When plaster in put on, the wall, whether of stone,
brick, or slate, is first thoroughly wetted with lime or baryta water. One or more
coats of rough plaster are then laid on. These are allowed to set, and the rough
plaster is wetted with distilled or lime water before the final coat, one eighth to
one quarter inch in thickness, is applied. In the Greek method there were three
steps. The first coats were often composed of slaked lime with rough sand, pounded
brick, or tile, and sometimes even straw, flax, or cotton. While this rough plaster
was still damp, the second coats of lime and river sand were laid on. The third step
was the application of the final coats. These were three and each was allowed to
dry. The plaster was composed of lime and fine marble dust. The surface obtained
by this process was so smooth and hard that it could be polished.
Paleolithic man undoubtedly knew of burning and slaking lime and, from the
abundant deposits of limestone and gypsum, made plasters that have lasted
through the centuries. In Egypt a clay plaster is found, dating from the pre-
historic period. Two principal kinds were employed: the first, very coarse, of Nile
alluvium with generally a small amount of carbonate of lime and occasionally
gypsum. Being unburnt, this had no binding properties. The second, which is
still used as a finishing coat to sun-dried brick, consisted of a natural mixture of
clay and limestone. Preparatory to painting, these rough surfaces were usually
covered with a gypsum plaster, an exception being found at El-Amarna where
paint was applied directly to the first layers. Though there was plenty of lime in
ancient Egypt, gypsum seems to have been used almost exclusively for covering
temple interiors and exteriors preparatory to painting. A theory has been advanced
that gypsum was used because of a scarcity of fuel, for it requires a far lower
temperature for burning than lime. Analyses of plaster from the pyramid of
Cheops show 82 per cent calcium sulphate and about 10 per cent calcium carbon-
ate. The Egyptians used plaster for coating brick and wood and for covering
mummy cases which were subsequently painted. Herodotus speaks of burnt
gypsum and other ancient sources, notably Vitruvius and Pliny, tell of the use of
both lime and gypsum plaster.
There is a difference of opinion as to whether the fresco paintings at Knossos,
Crete (1500 B.C.), are true fresco or secco. It would be the earliest example of
true fresco. The ground plaster for the palace wall paintings has three layers: the
first is very coarse; the second is a relatively refined plaster; and the final coat is a
fine lime plaster without any intermixture of sand or marble dust. The lime had
probably remained slaked for a long period before being used. As the Egyptians
used fresco secco on their gypsum plaster grounds, it seems probable that the
paintings at Knossos were done in this same method, in spite of having been
executed on a lime plaster. A few centuries later, at Mycenae, a fine, white lime
stucco is found. The Greeks used plaster very liberally to cover both the interiors
and exteriors of temples and public buildings, for flooring pavements, and for
covering statues. The plaster coatings were then painted, though a pavement