This is a technical grade of gelatin made in Italy of pork skin. Sheet or plate gelatin is sometimes preferred over granular or powdered animal glue, because it results in a clearer, more transparent product. This makes it ideal for use in gilding. Gelatin is preferred by many professional gilders over hide glue, because it can take a burnish easier than hide glue when mixed with gilder's clay, chalk or gesso.
Gelatin glue is graded and sold by its Bloom Value or Bloom strength. Bloom value is a measurement of the strength of a gel formed by a 6 and 2/3% solution of the glue that has been kept in a constant temperature bath at 10� C for 18 hours. A device called a Texture Analyzer or Bloom gelomater is then used to measure the weight in grams required to depress a standard plunger 4 millimeters into the gel. If this procedure requires 200 grams, then the glue is a 200-bloom value glue. Glue is also tested for its viscosity at this same 6 and 2/3% concentration. A standard viscosity range is associated with each bloom level.
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.
Source and Properties
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 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 120� F, depending upon the grade, concentration and presence of modifiers in the glue.
Bloom: 100 - 110 grams
The bloom measurement refers to the elasticity of a gelatinous mass. The higher the number the greater the elasticity.
Viscosity: 23 - 28 millipascal seconds
The millipascal seconds is a measurement of viscosity which is measured by the flow velocity of the glue solution through a funnel.
Always make the minimum concentration required; as a guide, a set jelly should be somewhere between firm gel and jelly. For a canvas or panel size, try 50 grams of gelatin for every liter of water. For distemper paints, 80 to 100 grams for every liter. 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
Gelatin can be modified with a wide variety of additives. To make it more waterproof the addition of 1% by weight of alum (aluminum sulfate) or formaldehyde is effective. Potassium chloride and potash prevent brittleness and crazing. An addition of 5% glycerin increase the flexibility of the glue. Adding 5-10% or more by weight of urea extends the gel time and also increases flexibility. Adding more than 15% of urea to the dry weight of the glue produces a liquid glue at room temperature.
First soak in water for several hours and then gently heat in a water bath until completely dissolved. Apply with a brush while warm or with a spatula when allowed to cool to room temperature (it will gel when cool). For gilding, use as an adhesive for metal leaf, such as gold leaf. Mix with gilder's clay or whiting to make a base for gilding. Add calcium carbonate (calcite, chalk or marble dust) for chalk grounds or calcium sulfate (gypsum) for gesso grounds. Always make the minimum concentration required; as a guide, a set gel should be somewhere between hard set gel and liquid -- about the consistency of jelly. For gilding size, use 50 grams for every liter (quart) of water.
Glue Size Recipe
50 grams gelatin
1 liter water
5 grams alum (optional)
- Prepare glue solution by soaking 50 grams of glue in 800 ml of water for approximately 2 hours. You can also leave it overnight.
- Add 5 grams of 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 water resistant and form a jelly-like consistency once the glue has had time to cool down.
- Apply the glue as a jelly, in a single, thin layer by using a spatula.
- Let the size dry for approximately 24 hours.
Distemper Paint Recipe
Hide glue, bone glue and gelatin both 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. Distemper paintings have lasted for centuries without change.
1 part gelatin
10 parts water
How to Use
Work the dry pigments with water into a heavy paste with a palette knife. Then grind the pigment into the warm solution of glue. Keep the paints warm enough to remain in 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 glue-sized paper, cardboard, panel, or canvas. This method is excellent for alla prima painting and for thin underpainting. To harden and preserve the paint film, spray the dried painting with a 10% solution of water and alum.
- Leave the glue in water overnight or for a full day.
- Let the glue absorb as much water as it can, then pour off the excess water.
- Warm this swollen glue in a double boiler or glue pot. This will cause it to melt. All animal glues should never be heated over 65� C (150� F).
|Source: ||Pork skin |
|Chemical Name: ||Hydrolyzed collagen |
|Chemical Formula: ||C102H151O39N31 |
|Viscosity, 17.7% solution �E: ||6.24 (See Note 1) |
|Jelly Strength, Bloom Gram: ||100-110 |
|pH: ||6.4 |
|Water: ||3.0% |
|Fat: ||- |
|Ash: ||1.0% |
|Foaming: ||- |
|Specific gravity: ||1.27 @ 25� C |
Note 1: The viscosity measurement of 17.7% percent solution at 20�C 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 but sometimes 50�C or 100�C) in a standardized Engler viscosity meter.