Rabbit Skin Glue
This is a high-grade glue made in the U.S. It is a fine mesh granular glue that is easy to dissolve in water. The high-quality grade means that it is the lightest in color and most translucent glue available. This makes it ideal for use in gesso and as a medium for distemper painting.
Stronger than most modern adhesives, rabbit skin glue is used in traditional woodworking, gilding, and painting techniques. First soaked in water and then heated in a water bath, it is applied warm, and gels when left to cool. In woodworking, rabbit skin glue’s solubility in water makes it reversible, while its “open time” allows for repositioning. In painting and gilding techniques, it is used as a size for canvas and boards, in recipes to make traditional gesso, and in distemper paints.
Animal glues vary in strength, but rabbit skin glue usually offers the highest strength, viscosity, and elasticity. Genuine rabbit skin glue tends to gel at lower temperatures, making it easier to use in gesso applications. Otherwise, glue made from bovine collagen is comparable.
Origin and History
Animal glues have been in use since ancient times. Paintings and murals between 1500–1000 BCE show details of wood gluing operations. A casket removed from the tomb of King Tut shows the use of glue in its construction. Many art objects and furnishings from the tombs of Egyptian pharaohs are bonded or laminated with some animal glue. The first references in the literature concerning glue providing simple procedures for making and using animal glue were written about 200 BCE.
Much of the original development of adhesives based on natural products has come in the woodworking and paper industries. Before World War I, there were simply no other options. The five classes of adhesives used most were animal glues, liquid glues (lower strength variety of fish or animal glue stabilized with acid for long-term storage), casein and vegetable protein glues, starch glues, and blood albumin glues. They are also used less in adhesive formulations than sodium silicate, mucilage, asphalts, gums, shellacs, and natural rubber.
Initially, the hides are kept in a lime slurry pit for one to three months for lime curing. This process helps to loosen collagen bonds in hides so that it can be extracted easily. After lime curing, the hides are washed several times to remove excess lime, and then the glue is extracted by cooking in boiling water. The extracted glue is then concentrated with the help of an evaporator. The concentrated glue is dried in drum driers and pulverized for final packing.
Rabbit Skin Glue in Modern Commerce
The modern process of preparing animal glue has resulted in greater conformance of physical properties derived from different mammals' hides, bones, and connective tissues. Today, rabbit skin glue denotes a grade of animal glue rather than its source. While it is possible to obtain animal glue derived from rabbits, the properties of the collagen glue from these sources are similar, if not the same.
Animal glues are adhesives that are high molecular weight polymers in organic colloid form from hydrolyzed collagen found in animal hides, connective tissues, and bones. Glue contains two groups of proteins: chondrin, which accounts for its adhesive strength, and gluten, which contributes to jelling strength. Animal glue is derived from the simple hydrolysis of collagen, the principal protein constituent of animal hide, connective tissue, and bones.
Hide and bone glues make up the two major types of animal glue. Hide glue, the superior of the two, yields a relatively neutral pH in solution, usually in the range of 6.5 to 7.4, although wider variations are possible. Bone glue is generally acidic, having pH values of 5.8 to 6.3. A glue with a high acidity absorbs less water and tends to set more slowly than a glue with low acidity.
Animal glue is soluble in water and insoluble in oils, greases, alcohols, and other organic solvents. When placed in cold water, the glue absorbs water and swells to form a gel. When heated, the glue dissolves to form a solution. When the solution is cooled, the glue forms an elastic gel once again. This property is thermally reversible, and upon application of heat, the gel liquefies. An animal glue solution’s gelling or melting point can vary from below room temperature to over 48.9° C (120° F), depending upon the grade, concentration, and presence of modifiers in the glue.
|Source:||Bovine and porcine hide, connective tissue, and bones|
|Chemical Name:||Hydrolyzed collagen|
|Viscosity, 17.7% solution °E:||6.16 (See Note 1)|
|Jelly Strength, Bloom Gram:||450|
|Specific Gravity:||1.27 @ 25° C.|
Always make the minimum concentration required; as a guide, a set jelly should be somewhere between hard set and liquid. For a canvas or panel size, try 40 to 70 grams of rabbit skin glue for every quart of water. For distemper paints, 60 to 100 grams for every quart. As an adhesive, check the consistency by dipping a piece of wood into the glue pot. If the glue runs off smoothly, you’ve got it right. If it is too thick, add a little water. Use the glue hot.
Alteration of Animal Glue
Collagen glue can be modified with a wide variety of additives. Adding at least 1% by weight (to the dry glue) of alum (aluminum sulfate) or formaldehyde effectively makes it more water-resistant. Increasing the amount of alum may be needed to increase the resistance of the glue to water. Some authors recommend 5% by weight of the dry glue, while others as much as 20%, depending upon the application. Potassium chloride and potash (potassium carbonate) can prevent brittleness and crazing. An addition of 5% glycerin increases the flexibility of the glue but also increases its hygroscopicity. Adding 5–10% or more by weight of urea extends the gel time and increases flexibility, producing a liquid glue at room temperature.
7 parts or 70 grams of rabbit skin glue (dry)
100 parts or 1 liter of water
7 grams alum (optional)
1. Prepare rabbit skin glue by soaking 70 grams of rabbit skin glue in 1 liter of water (or 800 ml of water, if adding the alum solution) for several hours or overnight.
2. Warm the swollen glue in a double boiler or glue pot to 57° C (135° F) to melt and dissolve entirely in water. Animal collagen should never be heated over 65° C (150° F) as this weakens or destroys the protein.
3. Add the alum to 200 ml of water and let it dissolve. Add the alum solution to the warm glue before applying it to the panel or canvas. The alum will make the sizing more water-resistant.
How to Use
1. Apply the glue at room temperature as a loose jelly in a single, thin layer to canvas, panel, or paper with a brush.
2. Let the size dry for about 24 hours.
Distemper Paint Recipe
Rabbit skin glue, hide glue, and technical gelatin provide a low-cost, easily formulated paint called ‘distemper.’ Diluted with water, it is suitable for color sketching and painting.
1 part or 100 grams of technical gelatin, hide glue, or rabbit skin glue (dry)
10 parts or 1 liter of water
1. Soak the glue in water for several hours or overnight to cause the glue to swell.
2. Warm this swollen glue in a double boiler or glue pot to 57° C (135° F) to melt and dissolve entirely in water. Animal collagen should never be heated over 65° C (150° F) as this weakens or destroys the protein.
How to Use
1. Work the dry pigments with water into a heavy paste with a palette knife.
2. Grind the pigment into the warm solution of glue.
3. Keep the paints warm enough to remain a solution while painting with them, and use warm water to dilute them.
Use a bristle brush for painting, applying the paint in thin layers to sized paper, cardboard, panel, or canvas. This method is excellent for alla prima painting and thin underpainting. To harden and make the paint film more water resistant, spray the dried painting with a 10% solution of water and alum or a 4% formaldehyde solution.
1: The viscosity measurement of a 17.7% percent solution at 20° C (68° F). In Great Britain, a scale was used as a conventional measure of kinematic viscosity. The Engler scale is based on comparing the flow of the substance being tested to the flow of another substance, namely water. Viscosity in Engler degrees is the ratio of the time of flow of 200 cubic centimeters of the material whose viscosity is being measured to the time of flow of 200 cubic centimeters of water at the same temperature (usually 20° C [68° F] but sometimes 50° C [122° F] or 100° C [212° F] ) in a standardized Engler viscosity meter.
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