Glue is an inferior kind of gelatin, and is of two kinds,—animal glue and fish glue. Animal glue is made of bones and trimmings, cuttings and fleshings from hides and skins of animals. Sinews, feet, tails, snouts, ears, and horn pith are also largely used. Cattle, calves, goats, pigs, horses, and rabbits, all yield characteristic glues.
The best glue is made from hides of oxen, which are soaked in lime water until fatty or partly decayed matter is eaten out and only the glue is left. The product is cleaned, boiled down and dried.
The best and clearest bone glues are obtained by leaching the bones with dilute acid which dissolves out the lime salts and leaves the gelatinous matters. Such leached bone is sold as a glue stock, under the name of "osseine." This material together with hides, sinews, etc., has the gelatin or glue extracted by boiling again and again, just as soup stock might be boiled several times. Each extraction is called a "run." Sometimes as many as ten or fifteen runs are taken from the same kettle of stock, and each may be finished alone or mixed with other runs from other stock, resulting in a great variety of commercial glues.
Manufacturers use many tests for glue, such as the viscosity or running test, the odor, the presence of grease or of foam, rate of set, the melting-point, keeping properties, jelly strength (tested between the finger tips), water absorption (some glues absorb only once their weight, others ten or twelve times), and binding or adhesive tests. This latter varies so much with different materials that what may be good glue for one material is poor for another.
Putting all these things together, glues are classified from grade 10 to 160, 10 being the poorest. The higher standards from 60 and upwards are neutral hide glues, clear, clean, free from odor, foam, and grease. The lower standards are chiefly bone glues, used for sizing straw hats, etc. They are rigid as compared with the flexibility of hide glues. For wood joints the grade should be 70 or over. For leather, nothing less than 100 should be used, and special cements are better still.
The best glue is transparent, hard in the cake, free from spots, of an amber color, and has little or no smell. A good practical test for glue is to soak it in water till it swells and becomes jelly-like. The more it swells without dissolving the better the quality. Poor glue dissolves. Glue is sometimes bleached, becoming brownish white in color, but it is somewhat weakened thereby.
Fish glue is made from the scales and muscular tissue of fish. Isinglass is a sort of glue made from the viscera and air bladder of certain fish, as cod and sturgeon.
Liquid glue may be made either from animal or fish glue. The LePage liquid glue is made in Gloucester, Mass., one of the greatest fish markets in the country. Liquid glue is very convenient because always ready, but is not so strong as hot glue, and has an offensive odor. Liquid glues are also made by rendering ordinary glue non-gelatinizing, which can be done by several means; as, for instance, by the addition of oxalic, nitric, or hydrochloric acid to the glue solution.
To prepare hot glue, break it into small pieces, soak it in enough cold water to cover it well, until it is soft, say twelve hours, and heat in a glue-pot or double boiler, Fig. 243. The fresher the glue is, the better, as too many heatings weaken it. When used it should be thin enough to drip from the brush in a thin stream, so that it will fill the pores of the wood and so get a grip. Two surfaces to be glued together should be as close as possible, not separated by a mass of glue. It is essential that the glue be hot and the wood warm, so that the glue may remain as liquid as possible until the surfaces are forced together. Glue holds best on side grain. End grain can be made to stick only by sizing with thin glue to stop the pores. Pieces thus sized and dried can be glued in the ordinary way, but such joints are seldom good. Surfaces of hard wood that are to be glued should first be scratched with a scratch-plane, Fig. 111.
To make waterproof glue, add one part of potassium bichromate to fifty parts of glue. It will harden when exposed to the air and light and be an insoluble liquid.8
Footnote 8: For recipes for this and other glues, see Woodcraft, May '07, p. 49.
General directions for gluing.9 Before applying glue to the parts to be fastened together, it is a good plan to assemble them temporarily without glue, to see that all the parts fit. When it is desirable that a certain part, as the panel, in panel construction, should not be glued in place, it is a wise precaution to apply wax, soap, or oil to its edges before insertion. Since hot glue sets quickly, it is necessary after the glue is applied to get the parts together as soon as possible. One must learn to work fast but to keep cool. To expedite matters, everything should be quite ready before the process is begun, clamps, protecting blocks of wood, paper to protect the blocks from sticking to the wood, braces to straighten angles, mallet, try-square, and all other appliances likely to be required.
Footnote 9: For special directions, for particular joints, see under the various joints, (Chap. VII.)
Whenever it is possible to break up the process into steps, each step can be taken with more deliberation. For example, in assembling framed pieces that are doweled, it is well to glue the dowels into one set of holes beforehand, making tenons of them, as it were. Time is thus saved for the final assembling when haste is imperative. The superfluous glue around the dowels should be carefully wiped off.
Likewise in gluing up framed pieces, sections may be put together separately: as, the ends of a table, and when they are dry then the whole may be assembled. When the pieces are together the joints should be tested to see that they are true, and that there are no twists.
A good way to insure squareness, is to insert a diagonal brace on the inside, corner to corner, as in Fig. 294. Such a brace should be provided when the trial assembly is made. Another good way to insure squareness is to pass a rope around two diagonally opposite posts, and then by twisting the rope, to draw these corners toward each other until the frame is square.
The superfluous glue may be wiped off at once with a warm damp cloth, but not with enough water to wet the wood. Or by waiting a few minutes until the glue thickens, much of it can readily be peeled off with an edge tool. Either of these ways makes the cleaning easier than to let the superfluous glue harden.
The work when glued should remain at least six hours in the clamps to harden.