GLUE, an impure gelatine, used as an ad hesive. The substances of which glues are made are ossein of bones and hides, chondri' gen of cartilage, isinglass from the bladders of fishes, and elastin, found in certain liga, meets.
In the modern method of making glue from bones they are first crushed, and then placed in pots of stone in a tank or retort, and their fat is extracted by boiling them in a solvent, usually a cheap grade of naphtha. The heat is supplied by steam coils. The first vapors of naphtha contain the moisture of the bones, and these are carried off from the top of the retort into a condenser. When moisture ceases to come over with the naphtha the re torts are closed, and the naphtha with its dis solved fat is drawn off at the bottom. This operation is continued with new naphtha three or four times; the operation requiring about 12 hours. The naphtha remaining in the bones is blown out by passing high-pressure steam through the retort. The bones are then boiled in water under steam pressure of 15 pounds to the square inch, which is later reduced to four or five pounds, when the glue in the interior of the bone fragments begins to ooze out This is washed down at intervals with a spray of hot water. When the solution in the boiler contains about 20 per cent of glue, it is drawn off, skimmed of any grease which may have escaped the naphtha, clarified with one-half of 1 per cent of potassium alum, agitated at a temperature of 175° F., and then strained through canvas or fine wire gauze. The glue solution then goes to the concentrators where its moisture is removed by the vacuum method at a comparatively low temperature. The glue is bleached by passing sulphur dioxide through it while in the liquid form, and it is then run into troughs to the depth of five inches. When the °jelly* has set, it is cut into thin slices by a wire knife, and placed upon wire nets to dry. As this jelly melts at about 75 degrees, the air currents used in dry ing it have to be cooled below this temperature in summer, and the freezing air in winter has to be warmed to a drying degree. The slices or sheets are dry in four or five days, then still containing 10 to 13 per cent 'of water. When prepared from clippings of hides, these are steeped in lime water for several days to remove the hair and blood, and then drained and dried in a current of air, that the lime may absorb carbonic acid, and thus prevent the injurious effects of the alkali upon the gelatine.
The clippings are then washed first with water and then with dilute hydrochloric acid and again dried. They are then enclosed in sacks and boiled in water until the solution is fodnd to gelatinize firmly on cooling. The itnpurities are allowed to settle, and the residuum to gela tinize in shallow wooden boxes; it is then cut into slices and dried upon nets. Good glue is semi-transparent, and free from spots and clouds. Marine glue, a composition used for cementing materials that are exposed to moist ure, is made by dissolving 1 part of india rubber in 12 parts of mineral naphtha, and adding 20 parts of powdered shellac; it resists wet, and cements glass and metals as well as wood. Fish glue is made from the skins of fluke and other flatfish and the bladders and Offal of any kind of fish. The product is a very strong adhesive, but needs deodorizing; this is accomplished by adding about 1 per cent of sodium phosphate together with one-fourth of 1 per cent of saccharin. White fish-glue, or diamond cement, is made of isinglass dis solved in alcohol. Before use on important work glues are subjected to tests for moisture and ash; for acidity; for contained fat; for gelatine content; for water absorptive capacity; besides several tests of the jelly as to adhesive power, viscosity, tensile strength and tendency to foam (because of included peptones). , The glue industry in the United States was founded by Peter Cooper in 1827, when he established a factory in Brooklyn. About the same time a factory in Philadelphia was started by Charles Baeder and William Adamson. At present glue factories are centralizing near the great slaughter-houses of the Middle West, the sources of raw supplies, and the larger packing concerns, notably the Armours and the Swifts in Chicago and the Cudahy Company in Omaha; have their own glue plants. (See PACKING IN utivrev.) The factories still in the East ate largely supplied with imported hides. The ex. port trade is steadily growing and has passed the $500,000 mark per annum. France alone surpasses America in the quality of its finer glues, and these are imported for use in Maki. ing straw hats. The finest glues made in the United States are prepared from sinews, and h if likely that continual experiment upon them will result in a product equal to the best im ported from France.