Product Profiles: Rabbit Skin Glue vs. Gelatin

Q: "I've been using Utrecht Rabbit Skin Glue for several years. I would like to know how the product is produced, how it should be stored, and how it differs from food gelatin, as well as other types of animal glue."

A: Rabbit Skin Glue (RSG) and edible gelatin are chemically very similar. Both are derived from animal proteins (collagen). Differences in source and processing, however, make these two materials substantially different from one another.

Rabbit Skin Glue (RSG) is a by-product of fur/hide and meat (rabbits are not raised expressly for glue). Most edible gelatin is derived from pork by-products. Compared to hide glues, food gelatin is processed more intensely for flavor, texture and digestibility. As a result of this intensive processing, the proteins that make up edible gelatin are shorter, so food gelatin would not perform as well as Rabbit Skin Glue if used as adhesive or sizing.

RSG is not the only animal-derived glue used as a painting size. Fish bladder gelatin (isinglass), cow hide glue and casein (whey protein) have all been used in the preparation of artists’ canvas. While we haven’t compared the performance of different animal sizings, makers of traditional musical instruments often prefer specific animal glues for different applications. Some craftsmen report, for instance, that RSG is more flexible than hide glue, and use RSG where a stiffer glue might fail.

In terms of storage conditions, a low humidity environment is best, both for stored glue granules and for RSG-sized supports. RSG will take up moisture from the air and swell (a property called hygroscopy). This can cause dry glue granules stored in a damp environment to clump, and applied sizing to swell underneath primer and paint. RSG is also vulnerable to microorganisms and insects in a way that synthetic (acrylic and PVA) glues are not. RSG imparts more stiffness to fabric supports thanmost synthetic glues, however, so many artists continue to use it, especially in combination with traditional oil-based primers.

Regarding shelf life: the longer glue is stored, the less soluble it becomes, so it's a good idea to "proof" the prepared glue before use. A significant amount of undissolved granules in a prepared batch can indicate reduced solubility. Glue strength can be tested by dipping a cold spoon in the warm liquid to see if it gels. If itdoesn't thicken and coat the spoon, it's probably too old to use and is no longer at optimal strength.

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