Make the Gesso
Today the best glues are sold under the names of rabbit skin and calfskin glues. Glues are made from the skins and bones of animals; skin or hide glues are very much more suitable than bone glues for our purposes, the optimum being a glue which will set to the strongest jelly. The kind which is made for laboratory and technical use is obtainable in very pure water-white grades mostly imported from France and Belgium.
Glue and gelatin are hygroscopic and are extremely variable products. Because of their variability, especially in connection with the wide range of inert materials, supports, methods of application, and atmospheric and other conditions which accompany their use, it is quite impossible to quote precise formulas which will give universally successful results. Partly for these reasons, and partly to avoid repetition elsewhere, the following instructions have been made rather complete and detailed.
Procedure
Place 2 1/2 ounces of rabbit skin glue in a glue pot, can, or enameled saucepan, with a quart of cold water. If the French variety is used, the sheets must first be broken into small pieces; a cloth wrapped around it will prevent its flying about.The American rabbit skin glue referred to under Some Sources of Materials is of approximately the same strength as the French variety, and may be used in the same proportions; the best calfskin glues have a higher jelly strength and a lesser amount can be used, as determined by making a comparative test. Melting Glue. The pot containing the glue and water is then heated until the glue is dissolved. Glue must never boil, especially for use in such an accurately balanced composition as gesso; the color darkens and the strength is immediately altered in an erratic way. To ensure against this, a glue pot or water bath (double boiler) may be used, although direct heating on an electric hot plate or gas stove covered by a metal sheet is perfectly suitable, provided that the glue is carefully watched and occasionally stirred so that it does not stick to the pot and so that no boiling takes place. This latter method is employed by persons who melt an occasional batch of glue; electric glue pots are used in shops and by those who employ glue continually. Possible further accuracy in the proportions of water and glue may be had by soaking 24 ounces of glue in 26 fluid ounces of water, melting it, pouring it into a quart measure, and making the volume up to a quart by adding hot water- thus eliminating error due to evaporation of water during soaking. The traditional method of testing the strength of the glue at this point is to allow it to cool to normal room temperature in the pot, when it should assume the form of a firm but not tough jelly. Downward pressure is applied to the jelly with the thumb and forefinger, spreading them at the same time so that the jelly is broken apart. By the feel of the strength of the jelly, its degree of resistance to fracture, and most of all by the nature of the fissure produced, one can tell very accurately the strength of the glue. The side walls of the crack should be rough or granular; if they are smooth, the glue is too strong.
If the solution is found to be too dilute, melt into it some of the extra pieces of soaked glue; if it is too concentrated, add hot water. A quick checkup can be made by mixing a little of the glue solution with chalk or whiting and brushing it out on a scrap of wood or Masonite; its drying may be hastened by warming if desired. After this, a few rubs with No. 2/0 sandpaper will indicate the hardness or softness of the gesso. If the sandpaper fills up too rapidly and takes down the gesso too easily, the gesso is too soft; if the sandpaper works with too much difficulty, the gesso is too hard. The glue solution of correct strength is then heated as hot as possible without boiling, and poured gradually into a pot or can containing the chalk or whiting. The mixture is stirred constantly during the pouring to produce a small paste.