MEDIUMS AND ADHESIVES 33
have not added oxygen, any saturated glycerides originally present in the oil,
and small amounts of other compounds formed by slight decomposition and
further oxidation of the first oxygen-addition product. At present, it is widely
held that linoxyn consists, physically, of a continuous lattice-work of oxidized
glyceride molecules enclosing the liquid (unchanged) glycerides, the whole forming
a perfectly homogeneous, solid jelly (Hilditch, p. 391).
Linseed Oil, the most important of the vegetable drying oils, is obtained from
the seeds of the flax (Linum usitatissimum), the same plant that furnishes linen
fibre. The content of oil varies with the source and the season. An average is 35
to 40 per cent. The oil is obtained almost entirely by expression rather than by
solvents, for the oil cake is of great value in cattle-feeding. Cold expression yields
the better oil. This is edible, has a pleasant flavor, and a bright, golden yellow
color. Most of the oil in commerce, however, is hot-pressed. This method yields
a light brown oil which is slightly turbid, owing to albuminous and extractive
substances and to moisture. Artists' oils are generally obtained by cold expression.
Linseed oil may be refined chemically, by washing, or by simply allowing impuri-
ties to settle. The three standard methods of chemical refining are: (i) with
concentrated sulphuric acid, (2) with alkali (sodium hydroxide or carbonate), and
(3) with brine (see Oils, refining). The alkali refined oils are more desirable and
more expensive. If an oil is used as a grinding medium, free fatty acid should be
present, for the neutral oils wet pigments with difficulty (see Oils, relation to
pigments). Artists1 oils are usually bleached by exposure to sunlight. Doerner
(p. 101) objects to bleaching, for he says that it does not last and that it is better
if the inevitable yellow tone is taken into account from the outset. Further
bleaching can be obtained by chemical means.
Linseed oil has a faint but distinctive odor. Its iodine value (see Oils and
Fats) varies from 170 to 195, being highest in Baltic oil which is from the purest
seeds. The iodine number in this is the highest of those of known fatty oils, except
perilla. The chemical composition of linseed oil, like that of the other vegetable
drying oils, is not fully established. It is essentially composed of mixed triglyc-
erides of linolenic, linoleic, oleic, and stearic acids with small amounts of other
acids, e.g., palmitic. It also contains approximately I to 1.5 per cent of materials
classified as unsaponifiable matter. Proportions of the four main types of acids
vary according to origin and to conditions of growth. Long (' Drying Oils') gives
the following percentages in North American and South American seeds: saturated
acids (stearic and palmitic), 4.8 to 9.0 per cent; oleic, 13,2 to 16.0 per cent;
linoleic, 37.9 to 45.0 per cent; linolenic, 36.4 to 40.3 per cent; unsaponifiable
matter, 1.05 to 1.4 per cent. Linseed oil is the one commonly used for grinding oil
colors, as an additional painting medium, and as an ingredient in emulsions and
in varnishes.
Manilla (see also Copal). This name came into use because the Philippines
was the first principal region to ship copal resin to the European varnish manu-