^6 PAINTING MATERIALS
The gum or glair would make a water mordant. Frequently with them an inert
material, like bole or slaked plaster of Paris, was added to give bulk. Later, mix-
tures of oil and resin with pigment were used for the same purpose (see Thompson,
op. cit.y pp. 226-228).
Non-Drying Oils (see also Oils and Fats). Vegetable, non-drying oils are
characterized by the preponderance of glycerides of acids absorbing two atoms of
iodine to the practical exclusion of glycerides of acids absorbing 4 or 6 atoms
of iodine. The iodine numbers range from about 100 to 80. These oils thicken at
elevated temperatures but they do not dry to a skin, even on long exposure.
Fryer and Weston (pp. 135-143) list the following oils as non-drying: ravison oil,
rape oil, mustard oil, jamba oil, the kernel oils, almond oil, arachis oil, rice oil,
olive oil, and castor oil.
Oils and Fats. Oils and fats belong to the class of chemical compounds known
as esters. They are essentially the glycerol esters of aliphatic acids (see Fatty
Acids) and, to a large extent, acids of the eighteen carbon series. Fats and oils,
however, being natural products, contain varying amounts of impurities, sub-
stances that occur in the seeds along with the fats; small amounts of these are
squeezed out or extracted along with the fat or oil. Most of such impurities are,
of course, removed during the refining process (see Oils, refining), but small
quantities are retained. The impurities are called ' unsaponifiable matter/ Most
important are phytosterol (C27H45OH) and its isomer, cholesterol. The former
occurs in all oils and fats of vegetable origin; the latter is characteristic of all oils
and fats of animal origin. Chemically, there is no distinction between oils and fats
but, popularly, the term 'oil* is used to denote the substances that are liquid at
ordinary temperatures and the term * fat' is used to denote those that are semi-
solid or solid, and typically greasy to the touch.
When oils and fats are treated with steam under pressure, they split up,
glycerine and a mixture of fatty acids are produced, and the elements of water
are absorbed. This process of splitting and absorption of water is termed * hy-
drolysis ' (see also Hydrolyzed Oils). There are five principal series of fatty acids:
1. The stearic acid series, all saturated.
2. The oleic acid series, one double bond.
3. The linoleic acid series, two double bonds.
4. The linolenic acid series, three double bonds.
5. The clupanodonic acid series, more than three double bonds.
Practically, the saturated fatty acids are distinguished from the unsaturated by
the fact that the unsaturated acids can combine directly with iodine while the
saturated can not. One molecule of an unsaturated fatty acid is able to combine
directly with 2, 4, 6 or more atoms of iodine according as it contains I, &, 3 or
more double linkages. As oils and fats differ chiefly in the amount of unsaturated
fatty acids which they yield, the most discriminative results are obtained by a
determination of this figure. The iodine value or number is usually the percentage