2i6 PAINTING MATERIALS
least a trace of moisture. Changes in the chemical nature of lime plaster in the
process of fresco painting require water.
Water has a higher heat capacity than any solid or than any other liquid at
ordinary temperatures and pressures. In other words, more heat is required to
raise the temperature of a given mass of water by a given amount than for any
other substance. Water has also the highest latent heat of fusion (80 calories)
and vaporization (536 calories). Heat conduction is less than that of metals but
is higher than that of other liquids and non-metallic solids. Its maximum density
is at 4°C. It expands when cooled beyond this point to zero and further on solidi-
fication. Surface tension, 73 dynes, is higher than that of any other liquid except
mercury. Because of this many surfaces are hard to wet with water.
In relation to the various film materials and adhesives used in the art of paint-
ing, water may have a solvent action, a swelling action, or practically no action
at all. The waxes and resins are little affected by it. Certain materials like the true
gums, egg white, and fish glue are soluble in cold water; the water causes complete
dispersion (peptization) of the material into particles of colloidal dimensions.
Skin and bone glues and, also, starches are swelled by cold water to form hydro-
gels. Moderate heat is required to convert these gels to a fluid condition.
Natural impurities common in water may be contributing causes of deteriora-
tion of paint materials in which water is an original component. All ground waters
contain various gases and salts. The gases are chiefly those of the air—oxygen,
nitrogen, and carbon dioxide. The salts are chiefly sulphates and carbonates of
calcium, magnesium, and iron. These cause what is called * hardness ' in water. In
addition, there is apt to be organic matter, algae and bacteria. Formerly works on
painting techniques frequently called for distilled water in formulas and for
mixing aqueous paints. Today many municipal supplies, particularly in the
eastern seaboard of the United States, are comparatively free from injurious
impurities and distilled water is not necessary. When, however, the local supply is
not good, distilled water, now easilv available, had better be used in connection
with important work.
Wetting Agent, any material which lowers the interfacial tension between
liquids and solids, and serves thus to aid in wetting a surface (see Surface-Active
Agent).
White Spirits (see Mineral Spirits).
Xylene (xylol) is dimethyl benzene (C6H4(CH3)2). The commercial product,
xylol, is a mixture of three isomeric xylenes, chiefly meta-xylene. Like benzene and
toluene, it is a clear liquid derived from the destructive distillation of coal tar and
fractional distillation of the' light oil/ Less volatile than toluene, it boils at I39°C.,
and its lower volatility favors it as a diluent for brushing lacquers. It is not so
dangerous a toxic as benzene.