Size and Glue

Size and glue may be considered together. They consist of two distinct yet similar compounds, known respectively as gelatin and chondrin. These bodies consist of carbon, hydrogen, nitrogen, and oxygen; and, when pure, they contain no sulphur. They are soluble in hot water, yet are coagulable by tannin and by some other compounds, organic and inorganic. Chondrin is thrown down from its solution by alum, and, indeed, by several compounds which do not precipitate gelatin. The latter body is obtained from skin, tendons, and bones. These organized structures contain a substance called ossein, or collagen, which, under the influence of boiling water, dissolves, becoming changed into gelatin. This conversion occurs more quickly when the process is performed under a pressure somewhat greater than that of the atmosphere, and, therefore, at a temperature rather higher than 100° C. In this way the transformation of the organic tissue of ivory, bone, vellum, parchment, fish-bladder, etc., into gelatin may be readily effected. The purity of the product depends, in part, upon the care with which the raw materials have been selected and cleansed, in part upon the temperature and the duration of the extraction.

If the temperature be too high, or the boiling be much prolonged, the gelatin produced is transformed partially into a substance which does not gelatinize when its aqueous solution is cooled, Chondrin is obtained from cartilage, which consists mainly of cartilagin, or chondrigen, by the same process which changes collagen into gelatin. A hot solution of chondrin gelatinizes on cooling just like one of gelatin; but it does not yield, with the same amount of substance, so firm a jelly. Size, glue, and commercial gelatin, consist of mixtures of gelatin, chondrin, and the non-gelatinizing substances produced by the long-boiling or the over-heating of their solutions. Isinglass, vellum, and ivory-dust yield a size which contains nothing but gelatin and a little mineral matter; the darker and stickier kinds of glue contain many impurities, having been made from very varied materials, such as ox-hoofs, horseflesh, old leather, etc.; they often contain sulphuric acid.

In selecting a size for artistic use, the special purpose in view will indicate whether an insoluble (in cold water) and strongly-gelatinizing, or a partially soluble and very adhesive one should be selected. The former is less liable to crack when dry than the latter. The very fine gelatins used in photography will often be found suitable. A few experiments, with cold water and then with hot, will soon reveal the peculiarities of the samples submitted to examination. As caustic lime, caustic soda, chloride of lime, sulphurous acid, and certain mineral acids, are frequently employed in the manufacture of size, glue, and gelatin, it is absolutely necessary to ascertain, before using these materials in any process of painting, their freedom from free acids, free alkalies, or bleaching agents. A hot-water solution of the material must not redden blue litmus-paper, nor bleach dahlia-paper, nor embrown tumeric-paper.

Glue and size may sometimes be purified and improved by cutting up the solid or gelatinous mass into small pieces, soaking them in distilled water for a few hours, and then pouring off the liquid before dissolving them.

The temporary preservation from putrefaction of the solutions of the substances described in the present chapter, may be effected in several ways. A lump of camphor, or a few drops of eugenol (from oil of cloves), is generally sufficient. I have preserved the egg-yolk medium for tempera-work for many days in an agreeable condition for use by the following plan: A saturated solution of eugenol in 5 percent acetic acid is first made, then this is added, drop by drop, with constant agitation, to the required number of yolks in a wide-mouth bottle, the point at which to stop further addition being learnt by the change of colour of a slip of turmeric-paper. When this paper just regains its original yellow colour, which was turned brownish-red by the yolks, no more acetic acid is wanted. Any water needed for thinning the medium may now be added, together with a lump of camphor, which will remain floating on the surface.