Various attempts have, as already stated, been made, with the intention of retaining the glue in a liquid form, and of thus avoiding the inconvenience attending the use of a cement which requires to be liquefied by heat whenever it is to be used. The addition of a little nitric acid will prevent the glue from gelatinizing or becoming solid, and the same effect is produced by the addition of a little vinegar, or of pyroligneous acid, which will also prevent it from moulding. It is supposed that the latter is substantially the formula for making the well-known Spaulding's glue. The addition of these substances injures the glue, however. Spaulding's glue may be, more convenient than common glue, but it is far inferior to it in strength. More recently it has been proposed to add sulphate or chloride of zinc to common glue for the purpose of keeping it liquid. A solution of shellac in alcohol has been used and highly extolled as a substitute for common glue. It forms a tolerable liquid cement, but is far inferior to glue.
Any of the following recipes will afford a liquid glue which will answer well enough for purposes where no great strength is required; but there is no cement which is more convenient than common glue, and yet which will unite wood with anything like the efficiency of that article.
(1) Dumoulin's. This is one of the oldest forms and one of the best; it is prepared as follows: Soak 8 oz. of best glue in 1/2 pint of water in a wide-mouthed bottle and melt by heating the bottle in a water-bath. Then add slowly 2 J oz. of nitric acid, sp. gr. 1.330, stirring constantly, Effervescence takes place under escape of nitrous acid gas. When all the acid has been added, the liquid is allowed to cool. Keep it well corked, and it will be ready for use at any moment. It does not gelatinize, nor putrefy, nor ferment. It is applicable to many domestic uses, such as mending china, wood, etc. (2) A very strong glue may be made by dissolving 4 oz. of glue in 16 oz. of strong acetic acid by the aid of heat. It is semi-solid at ordinary temperatures, but needs only to be warmed, by placing the vessel containing it in hot water for a short time, to be ready for use. (3) Dilute officinal phosphoric acid with 2 parts, by weight, of water, and saturate with carbonate of ammonia; dilute the resulting liquid, which must be still somewhat acid, with another part of distilled water, warm it on a waterbath, and dissolve in it enough good glue to form a thick, syrupy liquid.
It must be kept in well-closed bottles. (4) Spaulding's. This is simply good glue prepared with strong vinegar instead of water. Dilute, rectified pyroligneous acid, which is a coarse form of vinegar containing a very little creosote, may be used. It prevents mould and fermentation. (5) Glue, water and vinegar, of each 2 parts. Dissolve in a water-bath, and add alcohol 1 part. (6) A solution of shellac in alcohol is often sold under the name of "liquid glue." See Chinese Glue. (7) Macerate 6 parts glue in 16 of water, until the glue is swollen and soft. Add 1 of hydrochloric acid, and 1 1/2 sulphate of zinc, and let the mixture be kept for 10 or 12 hours at a temperature of 154° to 158° F. (68° to 70° C). Answers admirably for attaching labels to tin and to glass when exposed to damp. (8) The writer of the following claims to have a personal knowledge of its excellence: " An excellent liquid glue is made by dissolving glue in nitric ether. The ether will only dissolve a certain amount of the glue: consequently, the solution cannot be made too thick. The glue thus made is about the consistency of molasses, and is doubly as tenacious as that made with hot water.
If a few bits of indiarubber, cut into scraps the size of a buck-shot, be added, and the solution allowed to stand a few days, being stirred frequently, it will be all the better, and will resist dampness twice as well as glue made with water." (9) Puscher states that a clear liquid glue may be obtained by dissolving 1 part of sugar in warm water, adding 1/4 part of slaked lime, and keeping at 145°-165° F. for several days, with shaking at intervals. From 4 to- 5 parts of the resulting solution of sugar-lime are then used to . dissolve 1 part of glue, the whole being gently warmed. The addition of 2 to 3 per cent, of glycerine improves the glue, and a few drops of lavender oil remove the peculiar odour. (10) 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 be 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. (11) 100 parts of ordinary gelatine are dissolved in 400 parts of water containing 6 to 7 parts of oxalic acid. The solution is kept for 5 or 6 hours on the water-bath in a porcelain infusion pot, after which it is neutralized with carbonate of calcium, the insoluble precipitate filtered off, and the clear filtrate evaporated at a moderate temperature, until about 200 parts are obtained. The product is a durable, slightly-tinted, but clear liquid glue.