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 per cent. 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 per cent. 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.

Dextrin, or British gum, as met with in commerce, is prepared from starch in one or other of several different ways, and is a variable mixture of at least three varieties of true dextrin, soluble or modified starch, starch, a sugar called maltose, and certain minor ingredients and impurities. It will suffice for the purpose now in view, if we select a commercial dextrin, free from acidity, dissolving nearly completely in cold water, and then yielding a solution which, even when strong, has only a light yellowish or brownish colour. When a filtered cold-water solution of commercial dextrin is allowed to evaporate on a glass plate, and the residue becomes air-dry, the film of dextrin left differs from one of true gum by being less friable. A solution of dextrin is, however, far less ad hesive than one of true gum of the same strength.

Honey now claims our attention. It is a common ingredient in moist water-colours, and was often employed in size-painting. It is used to counteract the brittleness of gum or of size when dry, or, by its absorption and retention of water, to keep a paint moist. Honey consists of nearly equal quantities of two sugars known as dextrose and lævulose, a little sucrose or common sugar, small quantities of non-saccharine compounds, and about 20 per cent. of water. As the useful properties of honey depend entirely upon its lævulose, a solution of this sugar should be employed instead of the raw honey: this may be easily prepared in the following way: Pure pale honey, kept until it has become crystalline and semi-solid from the separation of dextrose, is mixed gradually with four times its bulk of proof spirit, and thoroughly shaken at intervals for a few hours. The pale yellow alcoholic solution is then filtered: the filtrate is a solution of lævulose, accompanied by small quantities of the other sugars of honey and of harmless impurities, and for some artistic urposes is at once available.

Should it be desired to obtain a more concentrated solution of this substance, the liquid may be evaporated to the desired consistency in a porcelain basin, or it may be submitted to distillation in a retort. The aqueous solution of lævulose may be decolourized by filtration through warm animal charcoal.

Lævulose, when free from water, forms a glassy solid; but it is usually obtained as a thick syrup. Although this sugar is capable of assuming the crystalline form, it never does so under ordinary conditions. It has a strong attraction for moisture; on this property its usefulness as a constituent of certain paints depends.

Glycerin was discovered in 1779 by Scheele as a by-product in the preparation of lead-plaster; for a long time the comparatively small quantity of glycerin met with in commerce was obtained in this way. It is now prepared from oils and fats by distilling them in a current of superheated steam, sometimes by first saponifying them with alkalies, or decomposing them with sulphuric acid, and then submitting them to this distillation treatment.

Glycerin generally occurs as a thick syrup with a sweet taste: when pure, it may be obtained in deliquescent crystals. Its empirical formula is C3H803. It is a strongly hygroscopic or water-attracting substance, the pure water-free glycerin being capable of absorbing more than one-third its weight of water from the air. Commercial glycerin always contains water: the specific gravity of the liquid affords a rough method of estimating the amount. For pure glycerin at 15.6° C. has the specific gravity 1.265, while that which contains 20 per cent. of water is reduced to 1.213; with 30 per cent. it is 1.186, and with 40 per cent. 1.157. The presence of sugar, a not uncommon adulterant, may be recognised by the turbidity caused by mixing the glycerin, after evaporation to remove water, with chloroform. Glycerin containing lead darkens when sulphuretted-hydrogen water is added to it, while the presence of acids may be recognised by blue litmus-paper, which is not reddened by pure glycerin. The water-attracting property of glycerin induced me to use it as a substitute for honey in preparing moist water-colours so long ago as 1856, but I am given to understand that it was employed in 1847 by Messrs. Winsor and Newton. Even in cake-colours a trace of glycerin may be introduced with advantage, as it renders them less friable and more easily rubbed down with water.

It prevents size, glue, and white of egg from becoming brittle on drying, and on this account may be used in the preparation of linen, canvas, etc., as painting-grounds. Care must, however, be taken in every case not to add more glycerin than is necessary to effect the purpose in view. It is a useful addition to gum-water, 1 dram to each ounce of gum present being sufficient; some copying-inks contain it. Modelling clay may be kept moist by means of glycerin.