Glue is prepared from waste pieces of skin, 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. Good glue should be hard in the cake, of a strong, dark color, almost transparent, free from black or cloudy spots, and with little or no smell. The best sorts are transparent and of a clear amber color. Inferior kinds are sometimes contaminated with the lime used for removing the hair from the skins of which they are made. The best glue swells considerably (the more the better) when immersed in cold water, but does not dissolve, and returns to its former size when dry. Inferior glue made from bones, will, however, dissolve almost entirely in cold water.

How To Prepare Glue

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 to keep the outer vessel 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 melted. Too large a quantity should not therefore be made at a time. Glue may be freed from the foreign animal matter generally in it by softening it in cold water, washing it with the same several times, till it no longer gives out any color, then bruising it with the hand, and suspending it in a linen bag beneath the surface of a large quantity of water at 66° 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 1220 without water, and filtered, some other impurities will be retained by the filter, and a colorless solution of glue be obtained.

The addition of a little bichromate of potash will render glue impervious to moisture after exposing to the light, and a small quantity of methylated spirits will greatly improve its keeping qualities.

Mixing Glue

A minimum amount of glue should be used in good work, and it should be applied as hot as possible. The surfaces of the 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, is four thousand pounds. The strength of common glue for coarse work is increased by the addition of a little powdered chalk. The hotter the glue the greater its cohesion; therefore in all large and long joints the glue should be applied immediately after boiling. Glue loses much of its strength by frequent re-melting; that glue, therefore, which is newly made, is much preferable to that which has been re-boiled.