MEDIUMS AND ADHESIVES 77
with a solution of viscose, then immersed in an acid bath, when the fabric becomes
cemented together with regenerated cellulose.
Walnut Oil is expressed from the nuts of the walnut tree (Juglans regid) which
contain about 65 per cent. The best is from cold expression which yields a pale-
colored oil with an agreeable taste and the odor of walnuts. The hot-pressed oil
is greenish in color and has an acrid taste. It may also be separated by boiling
the kernels in water, a process recommended by Leonardo da Vinci. Nut oil has
an iodine number of 140 to 150 and dries more slowly than linseed oil but does
not turn yellow so readily. Laurie (The Painter's Methods and Materials, p. 133)
says that it dries very slowly, but, if exposed to light or air over water, it can be
obtained very pale in color and will then dry quite as quickly as the linseed oil
prepared for painters' use. It is said to have less tendency to crack than linseed
oil. As early as the V century it was recommended by Aetius for varnishing wax
pictures and gilt surfaces. Its use was formerly more widespread than it is today,
It is recommended for all light pigments in the treatises on painting by Leonardo
da Vinci, Vasari, Borghini, Lornazzo, Armenini, Bisagno, Volpato, and others.
It is said to have fallen into disuse because it easily becomes rancid, and manu-
factured tube colors ground in it are difficult to keep in storage for any length of
time. (See also Oils and Fats.)
Water Color. The modern term is used to describe a standard preparation of
pigment ground in water-soluble gums, usually from Acacia Senegal or from Acacia
arabica Willd. (see Gums). The typical water color painting is executed with this
paint thinly on a support of paper.
Water-Glass is a thick, syrupy, clear liquid, an aqueous solution of potassium
or sodium silicate. It differs from ordinary insoluble glass in that it contains no
calcium, barium, or aluminum. Water-glass serves as an inorganic binding medium
because, on evaporation of water, it leaves an amorphous, coherent film. By
virtue of its strong alkalinity, water-glass can be used only with a limited number
of pigments. Church (p. 85) says that as early as 1648 a silicate of potash, called
' fluid silica,' was made but that the commercial production of these salts dates
from 1825. Water-glass is used for mineral painting or so-called stereochromy (see
Wolfgangmuller, * Erfahrungen mit Keim'scher Mineralfarbe " A," ' Technische
Mitteilungen fur Makrei, XLVIII [1932], pp. 124-126, 134-135). In the practice
of painting, water-glass is ordinarily used as a fixative for colors that are put
on merely with water. Potassium silicate appears to be preferable to sodium
silicate for this purpose. It is only useful on plaster supports, since a chemical
combination of the alkali silicate with lime of the plaster is essential to the for-
mation of a durable film. Laurie (The Painter's Methods and Materials, p. 215)
says that certain murals in the House of Lords which were executed in this medium
have suffered from disintegration. Water-glass is used extensively as an adhesive.