_2g	painting materials
Willd. is inferior to that produced by Acacia Senegal, and is little used for artistic
purposes. The good gum is variously known as Kordofan, picked Turkey, white
Senaar, and Senegal gum. The tree, called by the natives, 'hashab/ grows chiefly
in the Sudan and Senegal, sometimes attaining a height of twenty feet. The gum
exudes naturally and is slightly darker and less valuable than is that from the
cultivated trees. Gum arabic—Kordofan or Senegal—appears on the market in the
form of rounded 'tears/ either colorless or slightly yellow. The lumps are brittle
and break with a vitreous fracture, exposing a transparent interior, colorless in the
finer grades. Senegal gum, which Church (p. 79) considers the only sort that
ought to be employed in painting, should leave no appreciable residue when dis-
solved in cold water. It should be clear, giving no color with tincture of iodine.
If a reddish purple color results, the gum has probably been adulterated with
dextrin.
To prepare gum arabic for use, according to Church (p. 80), it is finely pow-
dered and slowly stirred into boiling, distilled water, the proportions being one
measure of the powdered gum to two of water. The solution should stand for at
least a day, and then be decanted from any sediment into a wide-mouthed bottle
covered with a glass cap. The addition of a lump of camphor, a few drops of
eugenol, or 0-naphthol, makes an effective preservative. Gum solutions may be
emulsified by a fatty oil (see Emulsions), and a small amount of glycerine (not
more than 5 per cent) may be added to the emulsion to eliminate brittleness. Gum
emulsions are sometimes used in the manufacture of water colors.
Gum Tragacanth. (see also Gums) is produced by leguminous shrubs belonging
to the genus Astragalus. It consists of a small quantity of gum soluble in water, a
little starch and cellulose, and a large proportion of a mucilaginous substance
which swells in cold water but does not dissolve. This latter constituent is a com-
plex compound of carbon, hydrogen, and oxygen, called'bassorin.'Gum tragacanth
contains 12 to 15 per cent of water, and leaves a to 3 per cent of ash when burned.
It may be prepared for use by placing some finely powdered gum in a bottle, wet-
ting it with alcohol, and then adding the required amount of water, with shaking
at intervals. Only 2 or 3 per cent of the gum makes a thick solution. Unlike gum
arabic, which dissolves in water, gum tragacanth swells, forming a mucilaginous
mass which must be strained through a cloth. A uniform consistency is difficult
to obtain. Gum tragacanth may be used as a medium for painting on linen. It
must be applied thinly and the painting left unvarnished for some time. Its
principal use, however, is as a binder in the manufacture of pastel crayons.
Gums (see also Tempera). A group of non-crystalline, structureless materials,
occurring widely in plants, composed mainly of carbon, hydrogen, and oxygen,
and forming viscous solutions or mucilages, is given this general name. Their chief
characteristic is that they dissolve in water, forming a clear solution, or swell
when they are soaked in water. This differentiates them from resins which are
sometimes misleadingly called by the same name. Gums differ, also, from gelatins,