Preparation of paste. Ordinary paste is prepared either from flour or starch, and according to the raw material used in its preparation, may, therefore, be divided into starch and flour paste.
Starch is an indispensable constituent of certain parts of plants, and plays an important part in the nutrition of the plant. It is chiefly manufactured from potatoes, Indian corn and grain. Examined under the microscope, it is seen to be composed of small granules consisting of layers placed one above the other.
Starch paste. In stirring starch with water to a thin paste and gradually heating it, it will be observed that at a temperature between 140° and 158° F. a peculiar change takes place; the thin milk-white liquid becomes transparent, opalizes, and at the same time becomes thickly fluid, in short, the starch is converted into paste. During this process the separate layers of the starch granules become detached somewhat in the same manner as an opening bud, whereby they absorb water, and the peculiar mass, called paste, is formed. That paste is not a solution is easily proved by the fact that on attempting to filter starch-paste only water drains off, while the starch remains upon the filter and gradually dries to a horny mass.
Paste left to itself soon decomposes, especially during the hot season of the year; it becomes sour through the formation of lactic acid, butyric acid, acetic acid, and other substances, and loses its adhesive power.
In preparing paste, the following rules must be especially observed: Divide the starch in water by constant stirring so as to form a homogeneous, rather thinly liquid fluid, and then add boiling water in small portions, stirring constantly. The conversion of the starch into paste is recognized by the thickening of the entire mass and the appearance of opalescence, when it is only necessary to add the required quantity of water to give the paste the desired consistency.
If white lumps are observed, it is an indication that the starch has not been thoroughly mixed with the water, and that certain portions of it have remained dry. Paste containing such lumps cannot be applied with any degree of uniformity, and besides it possesses less adhesive power. Nothing can be done to remedy the evil except diluting the paste with a considerable quantity of water and boiling, with constant stirring, until the mass is perfectly homogenous.
Starch paste prepared in a proper manner possesses great adhesive power, and, when applied in a thin layer, dries to an almost colorless coating. Pure starch paste is used for many purposes. It serves not only for pasting paper, wall paper, etc., but also for sizing tissues, such as paper-muslin, linen, etc., in order to give them lustre, body, and, under certain circumstances, greater weight. To increase the weight of linen, white lead or heavy spar is frequently mixed with the starch.
Flour paste. The principal constituent of flour, besides starch, is gluten. It is obtained in a pure state by tying flour in a linen bag and kneading it under water so long as the latter is rendered turbid by particles of starch. The gluten remaining in the bag is a light-brown, very tenacious mass, drawing threads between the fingers, and, as regards its chemical properties, is closely allied to albumen and caseine. Gluten, like the last-mentioned substances, shows a tendency to form combinations with lime which gradually solidify, and it can therefore be used for preparing cements. Like albumen and caseine, it speedily putrefies if exposed to the air in a moist state, and in decomposing forms products which have a very unpleasant odor.
Flour paste is prepared in precisely the same manner as starch paste, but while the latter is white, flour paste, even if prepared from the best wheat flour, has always a yellow-brown color. As regards adhesive power it is superior to starch paste, but is less durable.
There are many means to prevent the spoiling of paste. With paste once dry and kept so, there is no danger of spoiling, but if it is alternately exposed to dampness and dryness, as for instance with wall-paper hung on walls not entirely dry, decomposition will unavoidably take place, and the wall paper will become spotted and fall off the wall.