"For the use and good and profit of anyone who wants to enter this profession."
-- Cennino Cennini, sometime before 1437
Make Chalk Gesso
Chalk gesso is a white plaster-like priming that is applied to a rigid substrate (i.e. some sort of board) to make a smooth, hard yet absorbent surface for egg tempera painting. It can also be used under oil paintings. Chalk gesso was the standard underpainting surface for European art during the middle ages and early Renaissance.
You will need:
- Dry titanium white pigment (see notes below)
- Powdered chalk or Marble dust or Whiting (the filler)
- Dry granulated rabbitskin glue
- Distilled water (tap water will probably do, but it's better not to risk mineral contamination causing probems down the line)
- A large, clean jar with a lid
- A double boiler
- A candy thermometer (optional)
- Measuring cups and spoons
- A sturdy stirring stick
- A dust mask
The basic ratio of chalk gesso is equal volumes of pigment, filler, and glue. First you make the glue, then you add it to the blended pigment and filler. This recipe makes about 1 1/2 cups (355 ml), enough to gesso one largeish (say 24"x36", or 60x90 cm) panel.
1. Mix the rabbitskin glue: Sprinkle 1 1/2 tablespoons (22 cm3) of dry glue granules into one cup (236 ml) of water. Stir briefly, then allow to soak overnight. If you have leftover rabbitskin glue from the earlier sizing step, you can use that straight away. Just make sure that you have the same amount by volume -- not weight -- as each of the pigment and the filler.
2. Blend one cup (236 ml) of titanium white pigment and one cup (236 ml) of filler in your large jar. Be sure to wear your dust mask.
3. Warm up the glue mix in the double boiler until it is quite hot and melted. Do not allow it to boil, or the glue will weaken. Folks at Realgesso have determined that about 127 degrees fahrenheit (52 degrees celsius) is the optimum temperature for hot rabbitskin glue; you can measure this with a candy thermometer, if you like. Stir it smooth; this should not take long.
4. Carefully pour the hot glue into the dry ingredients. Stir thoroughly but gently; you do not want to mix bubbles into the gesso.
5. Keep the gesso warm for use by setting the jar in a bath of hot, but not boiling, water. Never put a jar of cold gesso in hot water; put the cold jar in cold water and warm them up together.
NOTES: I used to suggest either titanium white or zinc white pigment for this recipe, but recent (2007) research at the Smithsonian Institution has suggested that zinc white as a pigment has a lot of problems. It is certainly catastrophic in any amount in oil paints. At any rate, there is no need to take chances with the longevity of your art when you can use titanium white pigment perfectly well.
If you want to make a more authentic medieval recipe, leave out the titanium white (discovered in 1821, first mass-produced in 1916) and use twice as much filler. The gesso will be less brilliant and slightly more translucent (you will need more layers), but will do the job just fine.
The next step is to prime the panel.