SOLVENTS AND DILUENTS 215
may be the * pitch from trees/ The distillate, a complex substance containing
terpenes, creosote, acetone, and phenolic bodies, is now sometimes called ' tar
spirit/ It would have good solvent properties but a doubtful value as a thinner or
diluent for resin varnish (see Laurie, Greek and Roman Methods of Painting
[Cambridge, 1910], pp. 27-33). Other references in Pliny, however (XIV, 25;
XVI, 21 and 23; and XXIV, 22), leave little doubt that the distilled volatile
component of coniferous wood or of oleoresins was known and used in ancient
times. (A more complete discussion may be found In A. Lucas, * Cedar-Tree
Products Employed in Mummification,' Journal of Egyptian Archaeology, XVII
[1931], pp. 13-21; and Kenneth C. Bailey, The Elder Pliny's Chapters on Chemi-
cal Subjects [London: Edward Arnold and Co., 1932], II, 238-239.)
In spite of the evidence about the knowledge of turpentine in classical times,
there is nothing to indicate that it was used with paints. In the early part of the
mediaeval period, however, when the art of distillation had been perfected and
when turpentine was a more familiar product, it may well have been common as a
painter's material. Leonardo da Vinci in the late renaissance refers to it occa-
sionally, chiefly in connection with the preparation of c Greek fire.' That there
may be confusion in his writings between the crude oleoresin and the spirit is
possible, but in one place he definitely speaks about a turpentine of the second
distilling (Edward MacCurdy, The Notebooks of Leonardo da Vinci [New York:
Reynal and Hitchcock, 1938], II, 196). Berger (I, 228n, and III, i6on) says that
there is no information about when the distilled product was first used in the art
of painting, though he mentions a description made by Marcus Graecus which
shows that distillation was familiar in the VIII century. By the XVI century
numerous references show that distilled turpentine was a regular ingredient of
varnishes (Charles L. Eastlake, Materials for a History of Oil Painting, 2 vols
[London, 1847-1869], I, 291, 301, and 313; Berger, IV, 191, 193).
V. M. and P. Naphtha (Varnish Makers and Painters Naphtha) is a petroleum
distillate with a boiling range, 100° to i6o°C, between those of gasoline and kero-
sene. It is a common turpentine substitute and is widely used as a diluent and
thinner for oil paint. (See also Petroleum Thinner,)
Water. This, the common solvent or diluent for aqueous mediums, glues, gums,
egg white, casein, and others, the diluent for emulsions, like yolk of egg, the
solvent for dyes and for inorganic salts, is without doubt the most generally used
of any single material in the arts. Chemically, it is considered to be inert and
stable. Most substances that are dissolved in it can be recovered in their original
state after evaporation. To a small extent, however, it does dissociate, forming
H"1" and OH~ ions, and these ions are capable of entering into a type of chemical
reaction called hydrolysis or hydrolytic dissociation. Many organic materials,
the lower organic esters like ethyl acetate, for example, are readily hydrolyzed
by water with the formation of some acetic acid. Moreover, water serves as a
catalyst for many chemical reactions, and the effects of many gases depend on at