g0 PAINTING MATERIALS

tendency to brittleness may be reduced by the addition of Venice turpentine,
elemi, and other substances. It has been observed that films from its alcohol
solution develop fine, hair-like cracks which give the surface a silky sheen.

Sandarac was commonly used as a varnish ingredient in the Middle Ages.
A recipe is quoted by Laurie (Materials of the Painter's Craft, p. 286) in which
sandarac VQ&. pica grcca (colophony) are added to an almost equal amount of oil.
This would make an extremely thick varnish, and one that could be used only
if rubbed on while hot or if thinned in some way. In many of these old recipes
the proportion of resin to oil is very high, indicating the use of a soft resin.

Saponified Oils are oils or fats (see Oils and Fats) which have been split up
by the action of an alkali, glycerine and a soap being formed in the process:

(C15H3iCOO)3C3H5 + 3NaOH = 3d5H31COONa + C3H5(OH)3

tripalmitin sodium soap glycerine

hydroxide

Sarcocolla is a resin from Penaea sarcocolla^ an African Penaeaceae.

Semi-Drying Oils (see also Oils and Fats). Vegetable semi-drying oils are
those containing notable proportions of glycerides of acids which absorb 4
atoms of iodine, and only small amounts (if any) of glycerides of acids absorbing
6 atoms of iodine. These oils have iodine numbers ranging from about 120 to
100. They are characterized by the fact that they form a skin when exposed to
the air at somewhat glevated temperatures. Some very slow-drying oils may be
classed by one writer as drying oils and by another as semi-drying oils. Those
classed as vegetable semi-drying oils by Fryer and Weston (pp. 126-132) are
maize oil, kapok oil, cotton-seed oil, sesame oil, croton oil, and curcas oil. The
semi-drying oils lend themselves especially to the preparation of blown oils.

Shellac is the resinous secretion of the lac insect (see also Lac and Resins).
After the crude lac is gathered from the tree, it is crushed and graded and the
largest particles, called * seed-lac,' are selected for making the best grade of
shellac varnishes. The lac is heated, squeezed through a cotton bag, and worked
into a plastic mass ready for stretching. It is stretched, either on rollers or by
hand, into a thin sheet about four feet square. The sheet is slowly cooled and is
broken into the flake-like pieces which appear on the market. Shellac comes
almost entirely from India, although it is also produced in small quantities in
Burma, Indo-China, and Siam. Its constitution has been investigated, but results
vary because the origin of samples is often indefinite or unknown. Tschirch and
Farner (see Barry, p. 258) give the following data on a sample of unknown origin.

74-5 per cent
Coloring matter ................... 6.5 " "
Wax ............................. 6,0 « «
Moisture. . ....................... 3.$ « «
Residue, ......................... « ««