Gum

The term gum is properly applied to a number of noncrystalline, structureless substances, of vegetable origin. They consist essentially of so-called hydrates of carbon, and are either soluble in cold water, or swell up when left therein for some time. The only gum of any importance in painting is gum-arabic. This name is not, however, exclusively applied to one variety only; it is given to the gums which exude from several species of Acacia. For instance, Acacia arabica furnishes the Morocco, Mogador, Brown Barbary, and East Indian gums of commerce. But it should be noted that, although A. arabica is a native of India, and is grown to some extent in many parts of that empire, the gum it yields is rarely, if ever, exported thence, the so-called East Indian gum-arabic being really taken from Red Sea ports to Bombay, and thence re-shipped to Europe. Acacia arabica, however, does not furnish a strong and durable gum, and it is from another species, A. Senegal, that we obtain the gum employed as a binding material for water-colours. This gum is known commercially as Kordofan, picked Turkey, white Sennaar, and Senegal gum. The tree which yields it is a native of Senegal and the Sudan; it grows to a height of twenty feet.

The supplies which come from Kordofan are of the finest quality, but all the grades of gum from A. Senegal are superior to the produce of A. arabica in their greater dryness, density, and adhesiveness, as well as in the smaller amount of mineral matter which they contain. It may be added in this place that, according to some authorities, a part of the gum Senegal of commerce is produced by other species of Acacia besides A. Senegal, such as A. Adansonii, A. albida, A. dealbata, A. nilotica, A. Verek, etc., and even from species of Kaya, Spondias, and Sterculia. Suakim gum, the produce of Acacia stenocarpa and A. Seyal, varies greatly in quality, but is largely imported into England, and much used, though not generally available for fine work.

Gum consists mainly of arabin, a mixture of the salts of an acid called arabic acid, and of the free acid itself. The salts are those of the three bases - potash, lime, and magnesia; water is also present. It is probable that, in all varieties, even of the finest gum Senegal, other organic acids, besides arabic acid, are present. An analysis of a fine specimen of picked 'Turkey gum' gave 15 percent of water, and 2.8 percent of ash, leaving 82.2 percent for the arabic and other allied acids and organic matters. The arabic acid was formerly expressed by the formula C12H22O11, but the experiments of O'Sullivan indicate a much more complex composition (C89H142O74).

Gum from Acacia Senegal, the only sort which ought to be employed in painting, should be nearly free from colour, and should dissolve in cold water without leaving an appreciable residue. Its watery solution should be clear, and should give no colour with tincture of iodine, but an abundant precipitate with ammonium oxalate solution. If iodine produce a purplish colour, adulteration with dextrin is indicated; the white precipitate thrown down by the oxalate shows the presence of calcium, a constant constituent of the genuine gum. I have found that the samples of gum sold to me as gum Senegal were of a more pronounced yellowish colour than those bought as gum-arabic and best Turkey: the lumps varied more in size, often contained air-bubbles, and were less fissured. The adhesiveness and toughness of these samples, moreover, compared favourably with these properties as exhibited by the finest and whitest 'Turkey gum' obtainable.

For the preparation of water-colours, and for occasional use in the operations of painting, it is convenient to have at hand a standard solution of gum. This may be prepared by dissolving 1 ounce of the selected gum reduced to fine powder in 2 measured ounces of boiling distilled water. The powdered gum should be very slowly added, with constant stirring, to the boiling water. When the whole is dissolved, the liquid is allowed to stand for at least a day; then it is decanted from any sediment that may have been deposited into a wide-mouth bottle without cork or stopper, but covered with a glass cap. It is well to allow a lump of camphor to float in it, or to add to it a couple of drops of eugenol, the active antiseptic constituent of oil of cloves: a still more effective preservative is β-naphthol.

Gum tragacanth is produced by certain leguminous shrubs belonging to the genus Astragalus. Amongst these may be named: A. gummifer, A. eriostylus, A. brachycalyx, and A. adscendens. Its constituents include a small quantity of a gum soluble in cold water, a little starch and cellulose, and a large proportion of a mucilaginous body which swells up in cold water, but does not dissolve. The substance having these properties is a compound of carbon, hydrogen, and oxygen of very complex constitution, which has been called bassorin. Gum tragacanth contains from 12 to 15 percent of water, and leaves 2 to 3 percent of ash when burnt. A mucilaginous medium made with gum tragacanth may be used for painting on linen: it is not very easy to prepare so as to be of uniform consistency. A fairly good plan is to place the finely-powdered tragacanth in a bottle, and to add enough spirit of wine to moisten it: then add the required amount of water, and shake the mixture gently at intervals. Water containing no more than 3 or 4 percent of the gum constitutes a moderately thick mucilage.

¶ Tragacanth mucilage containing from 1/2 to 2 percent of this gum serves as a binding medium in the making of crayons for pastel work.

Other gums are of small importance. They commonly contain much bassorin and but little arabin. The Australian wattle gums from several species of Acacia are perhaps thus constituted; but if this be the case, the bassorin present in them seems to present some points of difference from the bassorin of tragacanth. Cape gum is produced by Acacia horrida: it is inferior to gum-arabic, as a substitute for which it is used in Cape Colony.