Rabbit Skin Glue

​​​Whoops – in the second qeustion I meant to say “Rabbit, cow, pig”;  saying “cow, bovine” is a bit redundant!​​

​​​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.​

​Thanks so much for this comprehensive answer – it’s very helpful.  

The transition in the word “gesso” from denoting a gypsum + animal glue ground to its current status (any kind of ground – chalk or gypsum, mostly synthetic polymer-based) is a done deal, I believe (for better & worse); very few artists think of gesso in the traditional sense.  I appreciate this is problematic for those still using a traditional ground – i.e. what to call it?  I switch between the vague terms “true gesso” and “traditional gesso”, or the mouthful “a chalk or gypsum plus animal glue ground”.  None works as well as the original word, “gesso” – but the general populace no longer knows the original meaning, and it feels pointless to insist upon it.  So it goes, language evolves. (I realize not everyone agrees, as Mark Gottsegen’s frustrations with the word “gesso” on the now defunct AMIEN forum made clear!)
I think it’s more problematic when a term is still evolving and common usage is unsettled; i.e. artists don’t realize “ivory blacks” are now from bone, or that many so-called “alizarin crimsons” are now quinacridones.  It’s not merely a matter of semantics, it’s about understanding materials; i.e. quinacridones have different properties – smaller particle size, higher tinting strength, purer chroma – from traditional alizarin.  This is how I think of glue nomenclature – the lack of settled usage means artists are confused or uninformed.  Even after much reading and study myself I remain, to a degree, uncertain as to what constitutes a good ground glue and what to call that glue.

So, with the understanding that the industry’s terminology is messy, what is a simple but clear, complete definition for the best glue to use for gesso?  Would it be…
Use a hide glue (not bone), from any mammalian source (rabbit, pig, cow), with a high gel strength (minimum 300, but higher numbers are better).
Is that accurate?
I appreciate the point, George, that a good ground is dependent on more than just the glue; percentage of glue is very important (too much glue and the ground is, as you note, too hygroscopic; also, in my experience, too much glue makes a too hard, less receptive ground, prone to cracking) whereas too little glue results in a crumbly, friable ground.  Solids content matters too.  I wasn’t aware of the water/calcium sulphate ratio topic – thanks for that article. 
Finally, I agree that having to work with a warm solution deters some artists from homemade gesso (although, in my experience, it’s much more about the entire process – where to get ingredients, what ratios to use, multiple layers, etc.). But I’d like to counter the idea that letting a hydrated glue decompose, in order to keep it liquid without heat, is a practical solution.  First, in an age where everyone has countless gizmos I don’t think it’s much of a stretch to have an electric kettle or single electric burner in a studio.  Second, decomposition degrades/weakens a glue.  I once experimented with letting glue/gesso sit out, unrefrigerated, over the course of the several days it took to hydrate the glue, make gesso, then coat one side of a panel.  By the fourth day, when I went to coat the opposite side, there was a distinct smell of rot in the air.  When the gesso dried on that second side, it was slightly discolored in places, very flakey, delaminating (see photo attached).Rotten Gesso Image.jpeg  ​​I’ve also seen animal glue, when left out, grow mold fairly quickly.  Given these experiences, I always tell students to refrigerate hydrated glue and gesso.  For glue painting (distemper) it may not be as consequential, but for gesso I wouldn’t feel confident working with a rotten glue.
As always, I’m grateful for your in depth replies, George.  There’s ever more to learn and update, and the conversation continues. 
Koo Schadler

​High gel strength​ is likely desirable for glue grounds, as I call all grounds made with animal glue. Still, considerations may offset this recommendation, such as the viscosity of the resulting glue solution, which will affect the proportion of water to solid materials. All of these factors will influence the final mechanical properties of the ground. The optimum concentration of glue to filler, ratio of water, and gel strength would make an interesting study to determine the best formula for glue grounds.​
This is an excellent review of animal glue:
Nanke C. Schellmann (2007) “Animal glues: a review of their key properties relevant to conservation”, Studies in Conservation, 52:sup1, 55-66, DOI:10.1179/sic.2007.52.Supplement-1.55​

​​Thanks, George. Downloaded the article and already made a few changes to my book.   A study of ground glues would be really helpful – let’s hope someone does it someday.  Koo