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Question asked 2019-02-02 12:27:08 ...
Most recent comment 2019-02-03 07:57:52
Grounds / Priming
I recently received an email from a monk living on Mt. Athos; he works in the Iconograpy department. He has many question, including trying to address a recurring problem with their icons. I'm in the process of editing his query to a manageable size. Here is the first of several questions from him that I'd like to pose to MITRA:
Is the most important thing in a gesso glue that it is genuine collagen and good quality, or that it has a high bloom strength? We are currently using Lucas brand, bloom 100-110 and 180-200. I understand that the recommendation for gesso is 450 bloom. So can RSG be good quality but also a lower bloom?
Thanks for helping me to help the Mt. Athos monks.
Answers and Comments
There are few companies or individuals today (we know of only one in Europe) producing glue from rabbit collagen. The name "rabbit skin glue" is no longer used in commerce to designate the origin of the collagen, but rather the Bloom Strength or Jelly Strength. Rabbit skin glue is now derived from bovine and porcine bones, cartilage and hides with a Bloom Strength of 400 to 450 grams. The term "hide glue" is applied to glue of the same origin of 200 to 300 grams, and "technical gelatin" to glue of 100 to 200 grams Bloom Strength. So, the name rabbit skin glue is synonymous with animal collagen glue of 400 to 450 grams Bloom Strength.
Since animal collagen glue does not bind the solid particles of a traditional gesso or chalk ground by enveloping them in collagen polymers, but rather forms strong “protein bridges” between particles, it would be important to use a high strength glue. However, the higher the Bloom Strength, the shorter the set time, and is likely why gilders often prefer lower strength glues for gilding, where Bloom Strength is not critical to adhering metal leaf to substrates.
We agree with George that most rabbit skin glue sold today is misnamed and is not truly sourced from rabbits but is rather bovine or porcine. These also rarely have the bloom strength of true RSG. However, just on a side note, I do not want to leave the impression that actual rabbit skin glue is only available in Europe. Certainly, genuine RSG is sold in the US - I believe George sells one, and we carry a genuine rabbit skin glue as well (bloom strength 550). So there are sources of RSG domestically although that might not help this particular monk located in Greece
While not speaking directly to his question, I would also generally recommend the following article as a good overview of the properties of various animal glues, including RSG:
Animal Glues: A Review of Their Key Properties Relevant to Conservation
One interesting idea shared in the above, which I have not further researched, is the impact of the fat content in true RSG and its impact of cohesive strength. From pg 61:
"Although rabbit skin glue has a high gel strength, it has been stated as having lower cohesion and bonding strength than other hide glues [23, 39, 78]. This is thought to be due to its high fat content [9, 23]."
But if that is a downside, the fat content also imparts some benefits:
Pg 61: "A high proportion of fat also improves elasticity, although it simultaneously reduces the gel "strength of the glue and final bond strength [23, 84]."
Pg: 62: " It has also been suggested that a high fat content, such as in rabbit skin glue, accounts for better stability in moist conditions [5, 39]"
So, like all things, never quite as simple as one hopes.
As a final note, generally the lower the gram strength the slower it takes to dry/set and the greater the shrinkage making lower strength glues generally better for cabinet or instrument makers, who generally prefer 120-200 gram strength. Which sounds closer to what he has. Would think something in the 400-500's range would be more preferable for chalk gesso grounds.
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