24 PAINTING MATERIALS
Fish Oil shows wide variation. The oil and the films made from it have bad
odors and the films suffer from non-uniformity. Although it is ordinarily called a
non-drying oil, it will dry when boiled with a siccative. Doerner (p. 114) says that,
in spite of all efforts at prevention, it occurs here and there in artists' colors.
Fixative. Any film material, which can be dissolved in low concentration and
low viscosity, may be sprayed upon drawings or pastels for the purpose of holding
the pigment granules in place. Such a sprayed film, or material capable of being
sprayed, is called a ' fixative/ As fixatives are commonly used, however, they
include only the natural resins such as mastic, dammar, and bleached shellac
(see also Resins).
Flour Paste (see also Starches). Since flour consists of a mixture of gluten and
starch, flour pastes differ materially in their working properties from starch pastes,
and pastes made from different varieties of flour also differ among themselves.
Wheat flour and rye flour are the ones most commonly used, but rice flour is
often made into paste and some mixtures contain corn, barley, or buckwheat.
Besides the simple preparation of paste by cooking flour in water, there are other
methods which result in a partial breaking down of the flour molecules by means
of fermentation or heat. For example, in one process mentioned by Alexander
(Walton, p. 177) flour and water are mixed to form a dough which is fermented
at 110° F. and then is cooked, dried, and pulverized. The powder may be kept
indefinitely without deterioration and be used for a paste when desired. Ferment-
ing pastes of flour are known to be used for mounting in Japan. In another process
flour is heated under steam pressure with about five times its weight of water
and, when partly cooled, has a quantity of raw flour added. Dextrin may be used
in place of the cooked flour.
For certain purposes, the flour can often be advantageously combined with
other materials. For securing paper, leather, etc., to metals, Alexander (loc. cit.)
gives the following directions. Ten pounds of animal glue are melted in 3 gallons
of water at a moderate heat. Twenty pounds of rye flour are then mixed with 4^
gallons of cold water and 8 pounds of acetic acid are added; the whole is poured
into the melted glue and boiled. Doerner (p. 226) says that many tempera recipes
used in commercial art are based on rye-paste emulsion, which, for such a purpose,
is combined with a glue solution and boiled linseed oil. He gives a trade recipe
which has been found useful by different artists in the painting of large, decorative
surfaces: Rye flour, 125 g., is mixed with 50 cc. warm water and to this are added
100 cc. of cold water. After these are thoroughly mixed, 300 cc. of boiling water are
put in and then 125 cc. of boiled linseed oil. This is followed by loo cc. of cold
water and 125 cc. of boiled linseed oil. The whole mixture is then given an op-
tional thinning with water.
Fprmogelatin (see Gelatin). When gelatin solutions, especially concentrated
solutions or those containing free alkali, are treated with formaldehyde, the gelatin
is converted, upon drying, into an insoluble substance known as formogelatin,