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.
Large quantities of isinglass and gelatine, specially prepared for the purpose, are used for clarifying and fining beer, wine and other liquids, as well as for preparing jellies. The material to be used for jellies and other culinary purposes must, of course, be colorless and entirely free from odor. Jellies are made palatable by flavoring with spices, sugar, essences, etc., before congealing, A vegetable gelatine, Agar-Agar, which will be referred to later on is now brought from China, and being cheaper and perfectly free from odor, has become quite a competitor with isinglass and gelatine.