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Moderator Answer
(george o'hanlon)
1. Regarding labeling laws in the European Union, that may be the case, but based on my experience marketing products in Europe, many consumer laws are unevenly applied or enforced throughout the EU. The labeling of animal glue in the U.S. is mostly based on the traditional usage of these products. Some producers in the U.S. use different names (i.e., rabbit skin glue, hide glue, bone glue, gelatin, etc.) to distinguish the gel or Bloom strength of the glue. It is a common practice among resellers to use the term “rabbit skin glue” for bovine and porcine glue since the name is more familiar to customers. (This is practice is not unusual—the word “gesso” to market acrylic-based dispersion grounds is a well-known example of products that do not contain gesso or gypsum.) Glue producers usually classify glues they produce with the following terms: Hide Glue, Bone Glue, and Industrial Gelatin. Since they use bovine, porcine, and fish bones, cartilage and skin, to produce glues of a wide range of Bloom values, the source of the animal protein is less important to their naming conventions.
2. It does not necessarily follow that the gel or Bloom strength of the animal glue is the most important property in making traditional glue grounds. The cohesive strength of a animal glue is determined by its molecular structure and intermolecular bonding, as expressed in Bloom values. To produce strong animal glue films with great stiffness and a high resistance to impact in the dried state, the same rules generally apply as obtaining high gel strength. Hide glues generally have greater cohesive strength than strongly denatured and degraded bone glues, which display lower tensile strength and are much more brittle. Mammalian collagen tends to yield stronger glues than most aquatic sources. Cold water fish gelatin in particular shows comparatively low tensile strength which is comparable with that of bovine bone gelatin while a high tensile strength, similar to that of hide glue, has been reported for mildly prepared sturgeon isinglass. Fish gelatin such as sturgeon isinglass does not form gels at the temperatures used to measure Bloom values, yet the strength can be quite high in the dried state.
3. The Bloom strength of modern animal glue can range from 80 to 512. On a purely numerical value high Bloom strength would be 512. For diffeernt applications, the Bloom strength of 200 may be high for gilding and wood working, but not for glue grounds or painting.
How is the Bloom value or gel strength of animal glue measured? A water solution consisting of 6.67% glue is placed in a specified glass bottle, which is then placed in a chilled water bath and held at 10º C for 17 hours. After chilling, the rigidity of the gel is measured as the force, in grams, required to press a half-inch diameter plunger to a depth of 4 millimeters into the surface of the gel.
The rigidity or strength of the gel depends upon glue concentration, the intrinsic strength of the glue, pH, temperature, and the presence of any additives. The intrinsic strength of glue is a function of both structure and molecular mass.
The gel strength of edible and technical gelatins range from 50 to 300. Technical gelatins differ from edible and pharmaceutical gelatins principally in that it is not essential that they meet the rigid specifications for human consumption.
The gel strength of bone and hide glues tpyically range from 250 to 512. Glues are classified in the industry as Type A (porcine hide), Type A (bovine hide), and Type B from bone.
The overall properties of the ground is based on a number of factors—not only on the gel strength of the glue. The composition of the mineral filler, the organic binder concentration, and the water/calcium sulfate mixing ratio (in the case of gesso or gypsum), grounds with a wide range of mechanical properties can be obtained. Grounds with a high concentration of animal glue is not recommended because they could lead to a highly hygroscopic ground, which could undergo drastic dimensional changes and lose structural stability at high relative humidity (RH).
4. Rather than looking toward the source of the mammalian protein, artists should base their decision on the Bloom value of the glue and other properties of the ground as noted in the bold text above.
As a note, modern gelatins and glues are more highly processed (denatured and degraded) than older glues. It has been noted that old glues contain small amounts of fatty acids (lipids) that can help to distinguish them in grounds and paintings.
A factor that often discourages artists from working with animal glue as a paint binder is the inconvenience of working with a warm solution. This can be overcome by allowing the solution to sit for a time and partially decompose. As an early medieval treatise stated, “after a few days it will stay liquid without heating. It may smell bad, but it will be very good”.
You may find this article regarding the use of amimal glue in plasters useful: Kerstin Elert, Cristina Benavides-Reyes, and Carolina Cardell, "Effect of animal glue on mineralogy, strength and weathering resistance of calcium sulfate-based composite materials", Cement and Concrete Composites 96 (2019) 274–283.