MEDIUMS AND ADHESIVES 45
most costly but yields a product of superior clarity and color. In the third method,
oil is boiled in tanks containing strong brine with a 10 per cent solution of crude
aluminum sulphate or with a 10 per cent solution of sulphuric acid, and then is
allowed to settle. The settled oil is treated with dry fuller's earth which adsorbs
any remaining mucilage and also bleaches the oil to some extent. Besides these
methods industrially used, there is an old workshop practice, still somewhat
followed, of washing oil with water. The two fluids are shaken together and when
the water, and any water-soluble impurities it carries, has settled out, it is drained .
off. Frequently, also, the whole container, after shaking, is put in a freezing
temperature and when the water part is frozen, the oil is poured off. Laurie
(The Painter's Methods and Materials', p. 128) describes some of these methods
in detail:
Linseed oil is prepared by grinding, heating and pressing the seeds of the common
cultivated flax. As it comes from the press it requires to be refined. This can be done
in various ways. The oldest and the most satisfactory manner for artists' purposes
is to expose it to light and air in covered glass vessels. A variation of this method is
to float it on salt water, introducing as well a certain amount of sand. Large glass
flasks are filled one-third of salt water, one-third of oil, and are then loosely corked
and placed outside. Every day for the first two or three weeks the contents are
vigorously shaken up. The oil is then left for a few weeks to clarify and bleach. By
this process mucilaginous and albumenoid substances are removed from the oil,
and the final product, pale and clear, dries quite quickly enough.... The same result
is obtained on a large scale by the addition of a small quantity of sulphuric acid,
which chars and removes impurities, and subsequent washing.
Linseed oil can also be bleached by the action of ozonized air. The bleached
oils are almost colorless, but a certain amount of oxygen is inevitably absorbed
by the glycerides during the oxidation of the non-fatty coloring matter to color-
less derivatives. Linseed oil which has received one or the other of the foregoing
treatments may be designated as refined, pale, or bleached linseed oil, but in the
paint and varnish trades these oils which have not been further treated (see
Polymerized, Boiled, and Blown Oils) are frequently classed as raw linseed oil,
in spite of- any preliminary refining.
Oils, relation to pigments. The oil required to give a stiff paste with a pigment,
i.e., when each pigment particle is thoroughly wetted by the liquid, is known
as the oil absorption of the pigment and its numerical value is given by the volume
of a standard oil (linseed) required for 100 grams of pigment. There are differences
of opinion on this subject. Thorpe and Whitely (II, 103) say: 'The oil required to
give a stiff paste with a pigment depends chiefly on the specific gravity of the pig-
ment and, also, on its physical condition, e.g.y the shape and size of its particles/
According to Gardner and Levy (p. 531),' The amount of oil required for pigment
saturation or wetting is directly proportional to the specific surface of the pig-
ment mass existing at the point of saturation. As the specific surface of the mass
is relative to its degree of particle subdivision or fineness, it also measures to a