Mouth glue forms a very convenient portable cement of considerable adhesiveness. For some purposes, especially for attaching drawing-paper to a board, it is the most convenient form, but for ordinary desk use, the mucilage bottle is to be preferred. Mouth glue may be purchased in cakes from the dealers in artists' supplies. Those who attempt to make it themselves should use a very pure form of glue or gelatine, quite free from smell or taint, as this will prove very disgusting when the glue is moistened with the lips. Sugar is generally added, not for the purpose of sweetening the article, but to render it more soluble. It will be found that a pure, but dark brown sugar is better than a white article, and a little syrup or molasses better than either. When molasses is substituted for sugar, the quantity employed may be greatly diminished. Phin uses a nice article of common glue instead of the so-called mouth glue. It requires a little more rubbing than the mouth glue, but it holds more strongly, and resists better the wetting to which mechanical and architectural drawings are subjected.
(1) Soak 4 oz. best glue and 1 oz. isinglass in water until soft. Pour off the superfluous wafer, and add 1 oz. brown sugar. Melt the whole together with a gentle heat, and allow it to evaporate until quite thick. Pour into a flat-bottomed dish that is quite cold; if placed on ice, so much the better, as it will prevent the glue sticking to it. When solid, cut into cakes. (2) Glue, 5 oz.; sugar, 1 oz.; dissolved in water, boiled down, poured into moulds, and dried. (3) Isinglass and parchment glue, each 1 oz.; sugar candy and gum tragacanth, each 2 dr.; add to them 1 oz. of water; boil the whole till the mixture appears, when cold, of the consistence of glue. Then form it into small rolls for use. This glue, wetted with the tongue and rubbed on the edges of the paper, silk, etc, to be cemented, will, on their being laid together and suffered to dry, unite them as firmly as any other part of the surface.
Put a pinch of shredded gelatine into a wide-mouthed bottle; put on it a very little water, and about J of glacial acetic acid; insert a well-fitting cork. If the right quantity of water and acid be used, the gelatine will swell up into worm-like pieces, quite elastic, but at the same time, firm enough to be handled comfortably. The acid will make the preparation keep indefinitely. When required for use, take a small fragment of the swelled gelatine, and warm the end of it in the flame of a match or candle; it will immediately "run " into a fine clear glue, which can be applied at once direct to the article to be mended. The thing is done in half a minute, and is, moreover, done well, for the gelatine so treated makes the very best and finest glue that can be had. This plan might be modified by dissolving a trace of chrome-alum in the water used for moistening the gelatine, in which case, no doubt, the glue would become insoluble when set. But for general purposes, there is no need for subsequent insolubility in glue.