MEDIUMS AND ADHESIVES 17
* Dammar as a Picture Varnish/ Technical Studies, II [1934], pp. 149 ff.). The
proportions sometimes given are, I part of resin to 3 parts of turpentine. Sabin
(p. 135) suggests 5 or 6 pounds of dammar resin dissolved in I gallon of turpentine,
and allowed to settle for sixty days. In tempera emulsions, I part of resin is
dissolved in 2 parts of oil of turpentine. Two per cent dammar in petroleum spirit
may be used as a pastel fixative.
Often the varnish is cloudy, probably owing to the presence of insoluble
resenes. According to Barry (p. 94), cloudiness does not necessarily mean reduc-
tion in durability. If the resin is * run' before it is dissolved in the turpentine,
a clear varnish results, but it is naturally darker. It can also be cleared by adding
rosin, though this detracts from its quality. There is recent evidence that dammar
varnish is very resistant to blooming, even in a moisture-laden atmosphere. It
is used to some extent with nitrocellulose in making clear lacquers. Reid and
Hofmann (p. 498) have given a formula for dewaxing dammar resin for commercial
purposes. The resin solution obtained is clear and not milky. Their formula is:
Dissolve 80 pounds of Batavia dammar in a mixture composed of 20 pounds of
ethyl acetate and 40 pounds of petroleum distillate having a boiling range, 80-
180° C. When completely dissolved (in a mixer equipped with a mechanical agi-
tator), add 100 pounds of denatured alcohol, agitate for a time, and then allow to
settle overnight. The waxy precipitate forms a cake in the bottom of the vessel,
and when the clear supernatant solution has been drawn off, the wax cake is re-
moved. This wax has not yet found any special use and is generally burned.
Dammar is mainly composed of dammarolic acid (C54H77O3(COOH)2) and two
resenes. Its melting point is from 100° to 150° C.; its specific gravity, 1.062; and its
acid number, 18 to 16.
Dextrin (see also Starches) is commonly prepared from starch by heating the
dry material at 200° to 250° C. It is less commonly prepared by moistening starch
with dilute nitric or hydrochloric acid and heating it, when air-dried, to about
110° C. Dextrin, as prepared, is a mixture of soluble starch, at least three varieties
of true dextrin, and sugar (maltose and dextrose). It dissolves in water and yields
a syrupy solution with strong adhesive properties. With iodine it gives a color
which varies from red to violet. Its discovery is said to have followed the obser-
vation that starch, which had been roasted during a fire in a Manchester ware-
house, yielded a sticky, gummy solution when wetted with water.
Distemper (see also Tempera) is a term common in the painting trades,*
particularly in England, and indicates a paint made with a glutinous medium.
It is ordinarily used on walls or'in scene painting.
Dragon's Blood (see also Resins) was known by this name in mediaeval times
when it was used as a pigment in manuscript illumination. * Leave it alone/ says
Cennini, *for it is not of a condition to do you much honor* (Thompson, The
Materials of Medieval Painting, p. 124). It seems to have been used for medicinal