25 PAINTING MATERIALS
perceptible degrees. The difference is one of purity: the more impure form is
termed * glue ' and is used only as an adhesive; the purer form, termed * gelatin '
or "size/ is used when an especially fine adhesive or a medium is required, and it
has other uses—in foods, in photography, etc. Generally, it is customary to use
the word * size' to indicate a nearly pure gelatin. Glue is an organic, colloidal
substance of varying appearance, chemical constitution, and physical properties.
It occurs in commerce in a wide variety of forms and colors. The colors range
through all shades of white, yellow, and brown and glue may be transparent, trans-
lucent, or opaque. Gelatinous or glue-forming tissues occur in the bones, skins, and
intestines of all animals. These agglutinating materials are removed by extraction
with hot water, and the solution, on evaporation and cooling, yields a jelly-like
substance—gelatin or glue. Glue is prepared from fish bones, skins, or bladders
which give impure forms of bone gelatin, skin gelatin, and isinglass. Parchment
size is made from parchment waste.
When glue is soaked for some time in cold water, it softens and swells without
dissolving, and, when again dried, should resume its original properties. When
gently heated, it dissolves entirely in water, forming a thick, syrupy liquid with a
characteristic but not disagreeable odor. Remelted glue is not so strong as that
which is freshly prepared; and newly manufactured glue is inferior to a glue which
has been in stock for some time. Glue loses strength continuously under the action
of heat, and it is better to heat successive small amounts rather than to have a
large lot cooking for a relatively long period. All glue solutions putrefy with time
and lose their adhesive power. The following table ((/., Encyclopedia Eritannica^
nth ed., p. 143) shows the holding power of glued joints with various kinds of
wood:
WOOD POUNDS PER SQUARE INCH
With grain Against grain
Beech................ 852 434.5
Maple................ 484 346
Oak.................. 704 302
Fir.................. 605 152
The word * glue * has been extended to many other substances that are not
glue at all. Solutions of gums, dextrins, converted starches, etc., are often called
glues, generally modified by the adjective,' vegetable/ Silicate of soda is called
'mineral glue'; solutions of rubber, asphaltum, and the like, in benzene or
naphtha,^are called ' marine glue'; and those of casein are called ' casein glue/
^Glue js used extensively in painting grounds. Gesso is a thick, white, water
paint, with chalk or gypsum as the inert material, and glue or gelatin as the
binding medium. In mediaeval painting, size was sometimes used as a medium in
books, especially for blues which often had to be laid quite thickly, and it was
important to have a strong binding medium which would not be too brittle when