AA PAINTING MATERIALS
44
He points out its inconvenience, as compared with tempera, for works that
required careful design, precision, and completeness, and assumes that the Van
Eycks (traditionally credited with the discovery of oil painting) had aimed to
overcome the stigma attached to it as a process fit only for ordinary purposes and
mechanical decorations.
Laurie thinks that stand oil (see Polymerized Oil) was known and used at the
time of the Van Eycks (see' Notes on the Medium of Flemish Painters '). Maroger
and Ruhrmann (Walther Ruhrmann, ' Das Bindemittel der alten Meister/
Technische Mitteilungen fur Malerei, L [1934], PP- 43-47? 52~56> 60-67, 74-76,
81-84) have both written recently on the Van Eyck medium and both explain it
as an emulsion.
Whatever may have been the particular developments of the XV century in
the use of oil as a painting medium, it is apparent that during the XVI and XVII
centuries it became the prevalent film material. Then and for some time later it
was ground with pigment in the painter's workshop. During the XIX century
various means were devised for storing this paint. Bags of skin or small bladders
had already served as containers. Rigid metal tubes with pistons were occasionally
employed for this purpose and then these gave place to the collapsible tube in
which artists' oil paint is now sold.
Oils, refining. In the commercial manufacture and refining of oils from seeds,
the first process is to clean the seeds themselves, and, with modern devices for
this purpose, foreign material can be reduced to a few tenths of one per cent.
The seed is then ground and expressed, either hot or cold, or the oil is extracted
by suitable solvents. Such oil is always more or less impure and must be further
refined. The impurities present consist of suspended matter, including mucilage,
albumenoid matter, and resinous bodies, which may be dispersed as relatively
coarse matter or in exceedingly fine suspensions of colloidally dispersed material.
The impurities also include natural coloring matter, free fatty acids produced by
hydrolysis, and semi-volatile compounds dissolved in the oil and giving it odor
and taste. There are three principal methods of refining linseed oil: (i) with con-
centrated sulphuric acid, (2) with alkali (sodium carbonate or caustic soda), and
(3) with brine. In the first, the oil is agitated in lead-lined tanks with approxi-
mately two per cent of sulphuric acid. The acid dehydrates and coagulates the
albuminous and carbohydrate material which settles out and allows the clear oil
to be drawn off, washed, and dried. The acid value of oil refined in this way is
higher than the acid value of the original oils. This is desirable in those that are
to be used for grinding certain pigments, for the free fatty acids facilitate wetting.
In the second process the oil is agitated with a hot, aqueous solution of sodium
hydroxide or sodium carbonate. The soap formed by the action of the alkali and
the coagulated albuminous and carbohydrate material settles out and the clear
oil is then washed and filtered with the aid of fuller's earth. If the oil is for paints
or varnishes, only enough alkali is used to neutralize most of the free acidity,
about 0.3 to 0.5 per cent of free fatty acid being left The alkali process is the