MEDIUMS AND ADHESIVES 63
the mucilage coagulates and separates out from drying oils. (See Oils and Fats
and Polymerized Oil.)
Starches (see also Dextrin and Flour Paste) are carbohydrates occurring in
plants and synthesized by them from carbon dioxide and water by means of
energy derived from sunlight and absorbed by chlorophyll, their green coloring
matter. This process is known as photosynthesis. Starch occurs as white granules
in nearly all plants, the granules from different plants differing in size and shape.
Because of the difference, a microscopic examination will reveal the source of the
starch. The size ranges from a diameter of ip (p = i/iooo millimeter) or less to
one of 150/4, and, although in some starches the granules are nearly all large (e.g.y
canna), and in some nearly all small (<?.£., rice), there are starches consisting of
both large and small granules. The granules of most starches exhibit diversity of
form. They are classified by Eynon and Lane as round, lenticular, elliptical, oval
or ovate, truncated, and polygonal. The percentage of starch present varies from
plant to plant. Rice is about 75 per cent starch; corn, 50 per "cent; and potatoes,
20 per cent. Starch is composed of carbon, hydrogen, and oxygen and has the
empirical formula, (CeHioOs)^ where the exact value of * n' is unknown. It is
prepared commercially from potatoes, corn, rice, etc., by the simple, mechanical
process of grinding and washing. When a suspension of impure starch in water is
allowed to settle slowly, the impurities settle out first and can be removed. It
is almost insoluble in cold water but, when a suspension in water is heated, the
granules swell and form a viscous solution which sets to a jelly on cooling. If
sufficiently diluted, it will remain liquid at room temperatures. Dextrin is pre-
pared by heating dry starch at 200° to 250° C. On hydrolysis in the presence of
dilute hydrochloric acid, starch is converted to the sugar, dextrose (CeHiaOe);
in the presence of the enzyme, diastase, it is converted to the sugar, maltose
(C12H22Oii). A characteristic and delicate test for starch is the deep blue color which
it gives with iodine.
For the preparation of starch paste the starch from rice, wheat, corn, potatoes,
or arrowroot may be employed. It is mixed with enough cold water to make
a creamy liquid which is then poured into the requisite amount of boiling water.
Starch, being free from nitrogen and sulphur, has no effect on pigments but it
holds poorly on fat grounds and is, therefore, not to be recommended in connec-
tion with oil colors. Ordinary starch paste is too viscous to be very suitable as a
binding material for pigments.
Since paste contains a very large percentage of water, it tends to soften or to
* wet up * the material to which it is applied, and to shrink or swell cloth or paper.
The moist, organic film encourages mold growth, and the changing moisture con-
tent of the atmosphere makes for instability. These disadvantages can, however,
be largely eliminated by proper handling. The paste films become brittle with age,
probably from a gradual loss of moisture and from development of mold and
micro-organisms. An added hygroscopic material would be advantageous if it
did not increase the tendency to mold growth. Stout and Horwitz say: