This is a high-grade glue made in Italy of pure rabbit collagen. It is a granular glue that is easy to dissolve in water. 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 both 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. True rabbit skin glue tends to gel at lower temperatures, making it easier to use in gesso applications. Otherwise, glue made from bovine collagen are comparable.
Origin and History
Animal glues have been in use since ancient times. Paintings and murals from the period 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 type of animal glue. The first references in literature concerning glue providing simple procedures for making and using animal glue were written about the year 200 BCE.
Much of the original development of adhesives based on natural products has come in the woodworking and paper industries. Prior to 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 a long term storage), casein and vegetable protein glues, starch glues, and blood albumin glues. Also used to a lesser degree in adhesive formulations at the time were sodium silicate, mucilage, asphalts, gums, shellacs, and natural rubber.
Initially the hides are kept in a lime slurry pit for 1-3 months for lime curing. This process helps loosening of collagen bond in hides so that it can be extracted easily. After lime curing the hides are washed several time to remove excess lime and than the glue is extracted from 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.
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 jelling strength. Animal glue is derived from the simple hydrolysis of collagen, which is the principle protein constituent of animal hide, connective tissue and bones.
Hide and bone glues make up the two major types of animal glue. Hide glue, which is by far the superior of the two, yields a fairly 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 having a high acidity absorbs less water and tends to set more slowly than a glue having low acidity.
Animal glue is soluble only 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 once again forms an elastic gel. This property is thermally reversible and upon application of heat the gel liquefies. The gelling or melting point of an animal glue solution 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: ||Rabbit hides |
|Chemical Name: ||Hydrolyzed collagen |
|Chemical Formula: ||C102H151O39N31 |
|Viscosity, 17.7% solution °E: ||6.16 (See Note 1) |
|Jelly Strength, Bloom Gram: ||350 |
|pH: ||6.2 |
|Water: ||12.6% |
|Fat: ||5.6% |
|Ash: ||1.2% |
|Foaming: ||1 ml |
|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. To make it more resistance to water the addition of 1% by weight (to the dry glue) of alum (aluminum sulfate) or formaldehyde is effective. Potassium chloride and potash (potassium carbonate) can prevent brittleness and crazing. An addition of 5% glycerin increase the flexibility of the glue, but also increases its hygroscopicity. Adding 5-10% or more by weight of urea extends the gel time, and also increases flexibility, producing a liquid glue at room temperature.
7 parts or 70 grams rabbit skin glue (dry)
100 parts or 1 liter 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 completely 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 on 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 which is called ‘distemper.’ Diluted with water, it is good for color sketching, as well as for painting.
1 part or 100 grams technical gelatin, hide glue or rabbit skin glue (dry)
10 parts or 1 liter 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 completely 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 for 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% solution of formaldehyde.
1: The viscosity measurement of a 17.7% percent solution at 20° C (68° F). In Great Britain, a scale used as a conventional measure of kinematic viscosity. The Engler scale is based on comparing a 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.