200 PAINTING MATERIALS
Saponin is a glucoside obtained from the soapwort and other plants. It is a
white, amorphous powder which forms a solution with water that foams like soap
when shaken. It has found some little use in cleaning paintings, and it is thought
to be less harmful than soap for this purpose (see Mayer, p. 398). A. Lucas
(Antiques > Their Restoration and Preservation, ad ed. [London: Edward Arnold
and Co., 1932], pp. 166-167) recommends a I per cent aqueous solution of saponin
for removing accumulated dirt and smoke from a painting, but cautions that if
used in excess the water may penetrate to the canvas and cause damage.
Soap, Commonly the word is used to define the sodium or potassium salt of
palmitic or stearic acid. It may be given to the mixed salts of several acids ob-
tained from ordinary fats. Although other metals can form soaps, these are not
soluble in water. Alkaline hydrolysis of fats with strong alkalis, producing a soap,
is called saponification. A soap is hard if made with soda, soft if made with potash.
Sodium carbonate and olive oil make Castile soap.
Occasionally it has been an addition to painting mediums, but the principal
use of soap is as a detergent. It can aid in the removal of grease or dirt from paint
or fabric. In doing so, it acts partly as an emulsifying or surface-active agent
which, by lowering interfacial tension, cuts fatty films and releases and disperses
particles of dirt they contain. In part, also, it appears that dirt particles are
adsorbed to the surface of colloidal soap particles. In its relation to paint, soap
has a number of defects. When put with emulsions, it is said to cause a later
darkening of the paint (Doerner, p. 226). Mayer (p. 373) says that when soap is
dissolved in water and used as a cleaning agent for paint, a minute quantity of
alkali may be produced by hydrolysis and may alter the surface color. He also
warns (p. 406) that the oil paint film can be weakened by soap solution, its adhe-
sion destroyed, and a permanent blush left on it. Undoubtedly part of the ill
effects of soap solutions on paintings is from the seepage of water into the ground
and support.
Historically, soap appears to have been known in classical times and small
quantities have been found at Pompeii and other sites dating from the I century
A.D. Early treatises on painting frequently mention mixtures of fat and lye for
use in the refinement of granular blue pigments, and De Mayerne, in the early.
XVII century (Berger, IV, 292), has a brief description of the cleaning of a picture
with soap and fine ashes, followed by washing with water.
Solvent. This name is given to any of the volatile fluid organic compounds
which can, without chemical change, convert a solid or semi-solid organic material
into a technically useful solution. Generally such a solution is a mobile liquid
capable of application in thin films. The organic substances converted are usually
colloidal, oils, waxes, resins, gums, and cellulosic bodies, and are used as varnishes
or mediums for paints.
Solvents, classification. A chemical basis serves best to afford an arrangement
of the various useful solvents in painting and restoration. Most of them are