Learn how to prepare animal and hide glue for the creation of gesso and bole. Mix a perfect recipe for traditional water gilding and iconography techniques.
Hide Glue is produced from various animal skins and is light in color, has a high cohesion and has no odor. It is highly refined, consistent, processed and formed into small cubes. Hide glue has a bloom strength of 240 and is highly recommended for making gilder’s gesso.
Rabbit skin glue is produced mainly from shaved skin parts of rabbits, has extremely high cohesion properties, and has distinctive color and odor. Similar to Hide Glue, it is highly refined, consistent, processed and formed into small cubes. Rabbit Skin Glue has a Bloom strength of 400 making it much stronger than Hide Glue and is recommended for mixing with gilder’s clay to make gilder’s Bole.
Glues recommended for water gilding can be compared by evaluating their gel strength as defined by the Bloom Gelometer, which is a measurement of the rigidity of the glue.
The Bloom unit is a measure of the force required to create a 4mm depression on the surface of a sample prepared using a specific formula (see below). High Bloom indicates more rigid and stronger glue. Lower Bloom units indicate glue more flexible and weaker.
Place 2 ¾ oz. of Hide Glue in to a pot or enamel container with a quart of cold water. After the glue has soaked overnight it will have swollen to about three times its dry volume. Insure that the pot is not too shallow so that all the glue can be exposed to water and prevent clumping of pieces adhering to each other. If this happens separate these pieces away from the rest of the swollen glue material.
The pot containing the glue and water is then heated until the glue is fully dissolved. Never allow the glue to boil, as the color darkens and the strength is immediately altered. To insure temperature control a double boiler should be used, with occasional stirring if necessary.
The traditional method of testing the glue strength is to allow the glue to cool to room temperature. At this point it should assume a firm but not tough jelly. Downward pressure is applied to the jelly with the thumb and forefinger, at the same time separating the jelly creating a crack or fissure. The side walls of the crack should be rough or granular, if it is smooth than the glue mixture is too strong. You can thin the glue by re-heating in a double boiler and adding one to two oz. of water as necessary. If the glue is too thin, than re-heat and add more pre-soaked rabbit skin glue. Note, this is a generalized method of determining the glue strength and individual requirements may vary.
Once the glue is of correct strength, it is heated again as hot as possible without boiling. It is then added gradually into a pot containing the whiting or chalk. The amount of whiting required will depend on the size project you are working on. Generally, start with one cup whiting or chalk. Slowly add the hot glue and mix thoroughly to produce a paste. The hot gesso should be the consistency of average light cream. Further smoothness is achieved by straining the mixture through cheesecloth (which may be squeezed). This will break up any clumps of whiting and remove any coarse material.
You can make a quick test of the gesso by applying the hot gesso by brush to a piece of wood or Masonite. Allow to dry completely. Using forced hot air (hairdryer) can accelerate this process. Use 320 grit sandpaper and lightly sand the surface. If the sandpaper fills up too quickly and removes the gesso too easily, than it is too thin. If the gesso is hard and obvious scratch marks are left from the sandpaper, the gesso is too hard. Return to the double boiler and add more glue/whiting as needed.
Recipes for making gilder’s glue and gesso vary and experimentation will lead to the perfect formula for your needs. Temperature and humidity will affect the performance of glues and seasonal changes to recipes may be required. Gilders may vary the glue/water ratio, the temperature/time dissolving process, and may mix Hide and Rabbit Skin glues to create the perfect material to create a superb gesso source for water gilding or Fine art application.
Dry pigment may be added to the gesso to create a variety of color options. Use Fine ground dry pigments and add to the whiting before the hot glue mixture is slowly stirred in. Hardness of your gesso mixture will vary when dry pigment is added so experimentation will provide the best results.
Gesso has a limited shelf life and will begin to mold over time. Store unused gesso or glue in a class container in a dark, cool environment. When ready to use again, return to the double boiled to create the hot, smooth gesso material as stated above. If mold or mold smell is present discard and create a new batch.