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Starch

Starch comes next in our list. This important food-substance occurs in commerce in a condition so nearly pure that there is no need to describe its character. For the limited uses to which it is put in artistic practice the uncoloured or white starch should be selected. The starch from rice, wheat, maize, or potatoes may be employed indifferently. Arrowroot may also be used. The preparation of starch-paste does, however, require some care. The best plan is to thoroughly agitate 50 grams of the dry powdered starch with enough cold water to produce a liquid of creamy consistence, and then to pour this mixture slowly into a vessel in which about 300 cubic centimetres of distilled water is kept in steady ebullition. All but 2 percent of the starch will dissolve into a nearly transparent homogeneous paste: the quantity of starch must be reduced if a thin liquid be required.

¶ Although starch has not hitherto been much employed in painting, its merits are such as deserve a more extended use. As its constituent elements include neither nitrogen nor sulphur, it is, on the one hand, more stable and less liable to the attacks of micro-organisms than size, white of egg, or casein; while, on the other hand, its chemical inertness is such that there is no fear of its exercising any injurious effect on colouring matters. But ordinary starch paste, owing to its viscous character, is not very suitable as a binding material for pigments. However, by means of certain treatments, as with ozone, glycerin, or volatile acids, starch can be brought into a more soluble and liquefiable form of great adhesiveness, and admirably fitted as a binding material in water-colour painting. Moreover, the various preparations of soluble starch possess in a high degree the property of becoming insoluble in cold water after they have once become dry. In consequence, a pigment laid on in admixture with a soluble starch vehicle becomes, after it is dry, irremovable by water, so that further washes of colour may be added without disturbing the previous layers.

¶ Soluble starch may be obtained by dissolving 10 grams of caustic alkali in 400 cubic centimetres of water, and then stirring in 100 grams of starch previously ground into a paste with a little water. The mixture should then be carefully and uniformly warmed until it has become transparent. After heating for about fifteen minutes, hydrochloric acid is added to the paste until it no longer shows an acid reaction to litmus paper: the addition of a little β-naphthol will protect the product from mould. Similar preparations of starch can be bought under various fancy names (such as vegetable glue). They are produced in the way just described, and are used in the preliminary priming of canvas instead of ordinary animal size; but before the artist employs any of them, the sample should be tested with reddened litmus paper to see that it has no alkaline reaction, and with blue litmus to learn if an excess of acid be not present. For the careful and necessary neutralization of the product is not unfrequently omitted.

The drawback to the preparation of soluble starch by treatment with caustic alkali lies in the presence of much alkaline chloride in the product; it is not desirable to introduce sodium chloride, and still less potassium chloride, into a coloured drawing. An entirely satisfactory variety of soluble starch is obtained by the limited action of fresh malt-extract, in very small quantity, upon starch-paste at 75°. Or dilute sulphuric acid may be used, in the same way, to produce the desired transformation, the action being stopped directly the liquid becomes clear, by stirring in an excess of precipitated barium carbonate, which is subsequently removed by filtration. Still another method of preparing soluble starch is by heating it with glycerin. It is recommended to employ 6 grams of dry potato starch and 100 grams of glycerin, heated together for about half an hour to 190° C., and then cooled down to 120° C. The soluble starch may be thrown down from this liquid by adding to it three times its bulk of strong alcohol. It must be remarked that the so-called 'soluble starch' prepared by the several methods just described is not precisely an identical product.

In its most characteristic form it dissolves freely in hot water, but is deposited as a white powder during the cooling of the solution; cold water holds about 3 percent in solution. It is stated that the variety prepared by means of sodium peroxide is much more soluble in cold water.

Starch contains carbon, hydrogen, and oxygen only, and is a carbohydrate having the empirical formula nC6H10O5. It is a stable compound. Commercial starch always contains some water, generally from 12 to 18 per cent.


Last Update: 2011-01-23