Glue suitable as a joining medium for the above purposes should be of an amber or brown-yellow color, transparent or translucent, clear, dry and hard, and show a glassy fracture which should not be brittle, but somewhat elastic. Placed in cold water it should swell up and absorb as much of it as possible without actually dissolving, even if it remains there for 48 hours. The supernatant water should be free from a putrid odor and contain but a small quantity of foreign substances in solution. Such glue passes into solution at 122° F., and dissolves entirely on heating to 144.5° F. Heating to a higher temperature should be avoided.
Glue as a binding agent. Glue solution is used for bind-together pulverulent substances, such as mineral colors in the manufacture of colored paper and paper-hangings, in painting in distemper, in the size of the gilder; or it is mixed with plaster of Paris or chalk for the manufacture of plastic masses which become hard on drying, such as stucco-work, papier-maché, etc. Generally speaking, it is best to use only good sound glue for these purposes, though it may sometimes be possible to utilize defective and cheap qualities without injurious consequences. For color mixtures, the glue should at all events be free from acids and alkalies, as they exert a decomposing and altering effect upon the colors. The gilder should always use the best quality of glue, as otherwise the work he applies later on to the size will spoil.
A very large quantity of glue is consumed in the manufacture of matches, and much depends on its quality and drying properties. The dipping composition for matches containing phosphorus is a bath of glue of 25 to 50 per cent. strength to which the requisite amount of an oxidizing agent, like potassium nitrate or chlorate has been added, kept at a temperature of 100.4° F. The phosphorus is cautiously put in; it melts, and is stirred to an emulsion, when the sand, glass or other friction-agent is incorporated. The object of the glue is to protect from oxidation, without diminishing the sensitiveness. Glue is also used as the binding material in the heads and rubbers of safety matches.
Book binders require for the better classes of work a glue which should naturally be pale and strong, and without marked odor. Some inferior glues which have been chemically bleached turn almost black in the pot, owing to the bleaching agent not having been properly removed or neutralized.
Sand, glass and emery papers and cloths are made by coating the surface with a thin uniform layer of strong glue, and sifting the powder evenly on.
Glue in sizing. The principal object of sizing goods is to impart to them a certain degree of stiffness, to give them a nice appearance and a good feel.
As glue would injure the color of white goods, it cannot be used for sizing them, but, on the other hand, much is employed for preparing size for the use of hat and cloth manufacturers, weavers, etc. Before the introduction of the paper machine and invention of rosin glue, animal glue was exclusively used for sizing paper, but at the present it is only used for sizing paper manufactured from rags, and for pasteboard, and also by manufacturers producing drawing paper sized with animal substances. The paper, after leaving the machine, is passed through a glue solution and then dried in the air.
For actual sizing purposes good and fine varieties of glue are only used, or sometimes the manufacturers prepare their own size by boiling to glue dried calves’ heads, or rabbit skins deprived of their fur, scraps of parchment, etc. For cheap woollen hats, glue is used in place of shellac. The cloth manufacturer procures his glue mostly in the form of a jelly. This variety of glue deserves special attention and the mode of preparing it will be referred to later on.
Glue for culinary and medicinal purposes. The use of glue for these purposes is based upon three properties:
1: Upon its power of coagulating and inclosing while in this state, substances mechanically dissolved and finely divided in a fluid, which, being specifically as heavy as the fluid itself, render the latter turbid and cannot be got rid off by settling. The glue in this case acts as a clarifier.