Rabbit-skin glue

Rabbit-skin glue is a sizing that also acts as an adhesive. It is essentially refined rabbit collagen, and was originally used as an ingredient in traditional gesso.

History

In traditional oil painting as practiced by the Renaissance painter, skin glue was used to coat the canvas. This is necessary because the linseed oil that forms the base of most oil paint contains an acid, linolenic acid, that will over time destroy the canvas fibers.

Production

Rabbit skin glue can be bought in powder form or in larger chunks. Preparation involves using the correct proportion of water to glue to achieve the correct consistency and strength. It should be heated to just short of the boiling point. Too much heat results in a product with reduced adhesive qualities.

Uses

Adhesive

As an adhesive, Rabbit-skin glue is used in the production of the bellows of concertinas.

izing

When used in painting as a sizing, it is spread thoroughly over a canvas that has been placed on the stretcher. When the glue dries, it tightens the canvas. After this has been allowed to dry, an oil-based primer is then applied. When the oil primer has dried (keep in mind that oil paint dries slowly) the oil painter has an excellent surface upon which to paint. A canvas sized with rabbit-skin glue can be made tighter than with other alternatives—such as an acrylic-based gesso—because of the shrinkage. This type of canvas is also valuable because it can be sanded to a flatter texture, which allows the painter to achieve a finer level of detail than can be achieved with a typical acrylic gesso ground.

This rabbit-skin glue ground is only appropriate for use under oil paint. Acrylic-based media will flake off a canvas prepared with rabbit-skin glue and are therefore not appropriate.cite book
author=Gottsegen, Mark D.
title=The Painter's Handbook
date=1993
publisher=Watson-Guptill Publications
location=New York]

While generally good results can be expected, sometimes complex organic substances can produce unpredictable results, or results that are unexplainable. Rabbit skin glue is, of course, actually derived from components of the animal, found on the underside of its skin. Additionally, oil paint is a pigment "ground" in (generally) linseed oil. Other oils can (less frequently) be used. The interactions of inherently complex materials imply that this is not always as clear cut as we would like it to be.

Many modern manufacturers of pre-stretched canvases use an acrylic gesso. Although this fact is not well publicized, some conservators--such as the head conservator for the Smithsonian--believe oil paint placed on top of an acrylic gesso will not be able to firmly attach itself to the acrylic gesso and, following a period of decades, will flake off. This delamination is considered a firm reason not to apply oil paint over acrylic gesso. However, in actual practice, acrylic grounds have been used under oil paint for many decades without problems. Modern acrylic gesso is designed to have a microscopically porous surface that allows the oil paint to form a mechanical bond. cite web
title=Golden Paints
url=http://www.goldenpaints.com/technicaldata/prepsupp.php]

Rabbit skin glue is considered to be a major cause of cracking in oil paintings by most modern conservators. Because the glue is hygroscopic, it continually absorbs moisture from the atmosphere, causing the glue to swell and shrink. Over time, this constant flexing causes the brittle oil paint to crack. Modern substitutes for rabbit skin glue are available, such as Gamblin’s PVA size [cite web
title=Gamblin Artists Colors
url=http://gamblincolors.com/sizes.grounds/index.html ] and Golden Acrylics’ GAC100 . These substitutes do not have the hygroscopic properties of rabbit skin glue, while still being very slightly hygroscopic, and should not cause the damage to oil paints that rabbit skin glue does. However, these modern replacements do not stiffen and tighten the canvas as well as rabbit skin glue does, so some artists still prefer to use rabbit skin glue.

Acrylic gesso presents another problem that makes rabbit-skin glue preferable as a ground. Oil paint has been found to deliminate from an acrylic gesso ground. Therefore, despite the hydroscopic properties of rabbit-skin glue, it is still preferable to acrylic gesso as a ground. A senior curator at the Smithsonian [cite web
url=http://forums.studioproducts.com/showthread.php?t=28610
title=Cennini Forum ] clearly rejected acrylic gesso for oil paint and extolled the long-term virtues of rabbit-skin glue:"The main reason I switched was a conversation I had with Dr. Marion Mecklenberg of the Smithsonian. In his research on older oil paintings he found that the original canvases had lost their tensile strength and it was the rabbit-skin glue that was basically supporting the aged,brittle paint films. He has been acting as a consultant for various enterprises trying to develop modern sizing materials. As of our conversation of a year ago he said none of the new materials as currently constituted (PVA sizes, acrylic fabric stiffeners, etc.) could match the strength, and as I understood it combination of body and 'good' stiffness of RSG and so provide the support to ageing oil paint films that RSG does. He felt that the support provided by RSG was much more important than the fact that it is somewhat hygroscopic. The hope is that new stuff will come along that can match RSG's support capabilities without being hygroscopic, but evidently we're not there yet."

References

* cite book
author=Mayer, Ralph & Sheehan, Steven,
title=The Artist's Handbook Of Materials And Techniques