g PAINTING MATERIALS
The curd of milk with nearly any alkali like borax, trisodium phosphate, or
sodium carbonate, will yield an adhesive. If the alkali is lime, the adhesive is
highly water-resistant. Nowadays hydrated lime (calcium hydroxide) may be
more convenient to use than quicklime. Sutermeister (c. VII) discusses the
theory and practice of casein glue formulation and says (p. 190) that a casein
glue capable of giving excellent dry strength and water-resistance may be pre-
pared from 100 grams of casein, 300 grams of water, and 16 grams of calcium
hydroxide. The casein must be finely ground and must be allowed to soak thor-
oughly before the lime is added in order that solution may take place as readily
as possible. Since the working life of such a glue is limited to 10 to 45 minutes,
it must be used immediately. Prepared casein glues are now on the market, which
have only to be mixed with water.
Casein yields one of the strongest glues known and has been used for centuries
by joiners and cabinet makers. It has served extensively as a binding medium
for cold-water house paints, and, to a limited extent, for pictorial painting, both
as a binding medium and in the preparation of grounds. Craftsmen of ancient
Egypt, Greece, Rome, and China are considered to have used it. Without doubt
it was a joining adhesive in the cabinet work of the Middle Ages. MSS of the
time give directions for preparing an adhesive out of lime and cheese, very similar
to an adhesive that is now used for putting together the wooden parts of an aero-
plane. (A large part of the casein used as a glue today is consumed by the wood-
working industries.) Ancient Hebrew texts mention the use of curd (casein) in
house painting and decoration. Michelangelo is said to have used a combination
of sour milk, oil, and pigments to produce highlight effects on walls (Sutermeister,
p. 105). The material used in the many well preserved XVIII century ceiling
paintings in upper Bavarian and Tyrolean peasant houses is lime-casein. It is
little used as a painting medium by modern artists, except, possibly, for mural
decorating. The casein film is hard, brittle, and insoluble, and lends itself poorly
to handling and to correction.
Casein Tempera (see also Casein). This medium, made from skim milk and
lime, has been used since very early times. It has great adhesive power and has
long served as a joining glue, as well as for painting on walls. Unless properly
thinned, lime casein is not considered to be suitable for easel painting. It is occa-
sionally used, at present, to make oil color short, and has even been added as a
medium with oil colors. With the addition of one fifth of its volume of slaked
lime, casein becomes liquid, is easily emulsified, and can be thinned with water.
Three to five parts of water, or more, can be added, and the emulsion should be
freshly made before it is used. Lime-casein sets quickly and becomes very hard.
For easel painting, Doerner (p. ai8) recommends powdered casein which is
insoluble in water but is soluble in ammonia. Forty grams of casein are mixed
with a small amount of water, and then 250 cc. of warm water are added. After
the lumps have been pressed out, 10 grams of ammonium carbonate, dissolved
in a few drops of water, are added. The solution is ready for use after the carbonic