One of the best chemical tests of the quality of glue is to ascertain the proportion of pure gelatine in the mass. This may be done by means of a solution of binitrate of mercury, in water acidulated with nitric acid. The test should be compared with a solution containing a known amount of gelatine.
The usual chemical test to distinguish the presence of glue in any liquid is tannic acid, which forms, in a solution of any kind of gelatine, a copious leathery precipitate, of which the particles, however, cannot be made to adhere together like leather, the fibrous structure of the " histose," from which the gelatine is derived, being absent.
The imports of glue and glue clippings into the 'United Kingdom, in 1870, were 30,293 cwt., valued at 36,046/. ; and the exports were 2439 cwt., valued at 4937/.
Liquid Glue.—(a) Ordinary glue, 5 lb., is dissolved in water, contained in a vessel which may be heated by means of a water bath, care being taken to stir from time to time. When all the glue is melted, 1 lb. of commercial nitric acid is gradually added in small doses. This addition creates an effervescence, and a disengagement of red nitrous fumes. When all the acid has been poured in, the vessel,is withdrawn from the fire, and the contents are left to cool. This glue may be kept for a long time, even in uncorked vessels. It is used cold, by means of a brush. It may aho be employed as a lute, by spreading it on strips of linen. (b) Ordinary glue, 100 oz., is dissolved in a water bath with 250 oz. vinegar ; when the whole has become liquid, 250 oz. ordinary alcohol, and 10 oz. alum are added, the mass being kept over a fire for a quarter of an hour. It is very tenacious, and does not become putrid. When too thick, a little water may be added, and the mixture may he heated. It is very useful for cementing, in the cold, a variety of small objects, and is much employed by the makers of false pearls. (c) Four parts by weight of gelatine or ordinary glue are dissolved, in the cold, or better, at a gentle heat, in ten parts of cominercial acetic acid. (d) Glue in fine pieces, 6 parts, macerated for some hours in water, 16 parts, adding hydrochloric acid, 1 part, and sulphate of zinc, part ; the whole is then exposed, for ten to twelve hours, to a temperature of 80° or 90° (176°-194° F.); the mixture will keep unaltered for a long time.
Parchment Glue.—Parchment, 10 parts, is cut into small pieces, and boiled in 128 parts of water, until the liquid is reduced to 80 parts. The decoction is filtered through linen, and evaporated over a gentle fire, until it presents the desired consistence.
Size.—A recently introduced size which, for the paper-maker's purpose, is said to be 50 per cent. cheaper aud much better than the ordinary size, is made in the following way :—In a copper pan heated by steam (waste steam will do) from 45 to 50 lb. of soda is dissolved in 200 to 240 lb. boiling water ; while still boiling, 300 lb. perwdered rosin is added, and thoroughly stirred in till it is entirely dissolved, an operation generally requiring three to four hours. This soda-rosin compound is dissolved in water, in the proportion of 1 lb. of the former to 30 to 40 lb. of the latter ; it is then thoroughly incorporated with a glue solution, made by dissolving 100 lb. glue in 300 to 400 lb. water. The two solutions arc then boiled together for about ten minutes, after which the mixture is run through a fine sieve or filter, and is then ready for use. The best proportions for mixing the vegetable and anirnal sizes are, for part of rosin, 1 part of glue; for some puiposes, equal parts of each may be taken. The addition of starch, if required, can be made as usual.
Waterproof Glue.—In order to render glue insoluble in water, even hot water, it is only necessary, when dissolving the glue for use, to add a little biehromate potash to the water, and to expose the glued part to light. The proportion of bichromate will vary with cilcumstances ; but for most purposes, about A of the amount of glue used will suffice.
Glutinous Cements.—By glutinous cements, are understood those whose base is a substance containing a large proportion of gluten, such as the flour of wheat, rye, rice, &c. They are com monly known as " paste," of which the chief varieties are the following:— Japanese Paste.—The Japanese malKe a very fine paste from rice flour. The flour is mixed with a little cold water, and into it is gradually poured boiling water, till the mass has attained the proper consistence ; it is then boiled for one or two minutes. It is beautifully white and trans parent, as well as very strong, and is consequently well adapted for fancy work requiting a colourless cement.