Glue is prepared from waste pieces of skins, horns, hoofs, and other animal offal.
These are steeped, washed, boiled, strained, melted, reboiled, and cast into square cakes, which are then dried.
The strongest kind of glue is made from the hides of oxen; that from the bones "and sinews is weaker. The older the animal the stronger the glue.
The best sorts are transparent, and of a clear amber colour.
Inferior glue, made from bones, will, however, dissolve almost entirely in cold water.
To prepare glue for use it should be broken up into small pieces, and soaked in as much cold water as will cover it, for about twelve hours.
It should then be melted in a double glue pot, covered, to keep the glue from dirt. Care must be taken that the outer vessel is full of water, so that the glue shall not burn, or be brought to a temperature higher than that of boiling water.
The glue is allowed to simmer for two or three hours, then gradually melted, so much hot water being added as will make it liquid enough just to run off a brush, in a continuous stream, without breaking into drops.
When the glue is done with, some boiling water should be added to make it very thin before it is put away.
Freshly made glue is stronger than that which has been repeatedly remelted. Too large a quantity should not therefore be made at a time.
"Glue may be freed from the foreign animal matters generally in it by softening it in cold water, washing it with the same several times till it no longer gives out any colour, then bruising it with the hand and suspending it in a linen bag beneath the surface of a large quantity of water at 60° Fahr."
By doing this the pure glue is retained in the bag, and the soluble impurities pass through. If the softened glue be heated to 122° without water, and filtered, some other impurities will be retained by the filter, and a colourless solution of glue obtained.1
Glue is used chiefly by the joiner for joints, veneering, etc.
The precautions to be attended to in using glue have already been mentioned in Part II., p. 295.
A minimum amount of glue should be used in good work, and it should be applied as hot as possible. The surfaces of wood to be united should be clean, dry, and true; they should be brought together as tightly as possible, so that the superfluous glue is squeezed out.
"The cohesion of a piece of solid glue, or the force required to separate one square inch, Mr. Bevan found to be 4000 lbs."
From other experiments Mr. Bevan found that the adhesion of two pieces of ash glued end to end amounted to at least 715 lbs. per square inch.
"The lateral adhesion of a piece of board cut out of Scotch fir, which had been quite dry and seasoned, was 562 lbs. to the square inch. Therefore, if two pieces of this board had been well glued together the wood would have yielded in its substance before the glue."
"The strength of common glue for coarse work is increased by the addition of a little powdered chalk." 2
"Mix a handful of quicklime in 4 oz. of Unseed oil; boil them to a good thickness, then spread it on tin plates in the shade, and it will become very hard, but may be easily dissolved over the fire as glue." 3
"Skimmed milk, in the proportion 1 lb. glue to 2 quarts of milk, is sometimes used to dissolve glue, with the view of increasing its capability of resisting moisture."3
"Ordinary glue can be rendered insoluble in water by adding to the water with which it is mixed a small quantity of bichromate of potash; the exact proportion must be ascertained by experiment, but for most purposes 1/50th the amount of glue will be sufficient." 4
One part of indiarubber is dissolved under gentle heat in 12 parts of mineral naphtha or coal tar. "When melted, 20 parts of powdered shellac are added, and the mixture is poured out on metal plates to cool. It is applied by a brush in a melted state, and is specially suitable for all work exposed to wet or moisture.