SUPPORTS 233
Gesso, in its restricted meaning, the white priming or ground made from
burnt gypsum (plaster of Paris) and glue, which was used by the Italian painters
for preparing wooden panels or other supports for painting and gilding. Today
the term is often applied to grounds made from almost any white inert or pigment
like whiting or zinc oxide (see p. 115). Properly prepared gesso is hard and can
be finished to an ivory-like surface with a proper absorbency for paint. It has
been used on carved wooden moldings which were to be gilded and decorated
because it was easier than the wood itself to carve delicately and to prepare for
gilding (see Thompson, The Materials of Medieval Painting^ p. 31). Gesso sets
more slowly than plaster of Paris but is harder, more strongly adhesive, and can
be smoothed more satisfactorily. Modern painters and picture framers prefer to
use whiting or gypsum mixed with zinc oxide in place of straight gypsum or
plaster of Paris because this addition gives gesso a greater bulking and hiding
power; fewer coats are required. Such formulas call for about equal measures
(equal parts by volume) of pigment (gypsum, whiting, zinc oxide, or mixture)
and glue solution (i part of glue to 15 parts of water by weight).
Glass is a super-cooled liquid made by the fusion of soda, lime, and silica in
varying proportions. Other metal oxides, such as those of lead, barium, potassium,
and iron, may enter into its composition. Although glass was known in Egypt as
early as 3500 B.C., cast plate glass was not made until the latter part of the XVIII
century. G. W. Morey (* A Half Century of Glass . . .,' p. 943) says, c . . .
during all these centuries only one type of glass was known, soda-lime-silica glass,
modified, of course, by the very considerable amount of impurities that crude
manufacturing methods made unavoidable/ Hackh's Chemical Dictionary says
that glass is an amorphous, hard, brittle, often transparent material consisting of
a mixture of the silicates of some of the alkali metals, the alkaline earth metals,
and the heavy metals, and that it is obtained by the solidification of a fused mass
containing:
a. SiC>2 as quartz, flint pebbles or siliceous sand; this can'be replaced by B2O3,
AUOa or Mn2Og.
b. Na2O or K2O as carbonate or sulphate.
r. CaO as limestone or marble; this may be replaced by PbO, MgO, ZnO or
BaO. The constituents a> b, and c are finely powdered, well mixed and
heated to 1000-1100° C.; on cooling to 890° C. the pasty glass is worked
by blowing, drawing, casting, or fusing piece to piece and now annealed
by slow cooling or reheating and cooling. The composition of glass varies
between (K, Na)2O, (Ca, Pb)O, 6 SiO2 and 5(K, Na)2O, 7(Ca, Pb)O,
36 SiO2.
It is well known that glass was used for windows in Roman times. N. H. J.
Westlake (I, 4) cites several examples of window glass from Pompeii and other
Roman sites. Hot glass was probably both cast and blown at that time. Because