The ground is the prepared painting surface and for egg tempera this should be a traditional gesso. This gesso is a mixture of some form of whiting (chalk, gypsum, marble dust or titanium oxide), rabbitskin glue and water.
Acrylic gesso is not so absorbent and is certainly not chemically compatible with egg tempera.
The support must first be sized with a layer of rabbitskin glue which acts as an isolating coat and helps to bind the gesso to the support. This layer of glue sizing is made up of the same rabbitskin glue mixture that is used in making the gesso. (In a ratio of 1 ounce glue to 16 fluid ounces of water.)
There are a number of tried and tested traditional gesso recipes. The amount of ingredients is the only slight variation, but the below recipe is the one most commonly used and with very few problems.
- 16 fluid ounces water
- 1 ounce of glue
- 24 ounces (by volume) of whiting
By volume?……..measure the whiting in a fluid measuring jug.
Most people use rabbit skin glue; some use gelatin. Rabbit skin glue can be purchased in sheets or granules and is readily available from large art suppliers and is the glue most recommended.
Some people use gelatin which is thought to be inferior to rabbit skin or calf skin glue for use in gesso. If you do use gelatin it is not recommended to use the edible variety (grocery store gelatin) because it has a yellowish cast and a weaker strength. Nevertheless, some people have successfully used edible gelatin.
- Add the glue to the water and allow to stand over night or until the
glue granules are completely dissolved.
- Heat the glue mixture slowly on a double boiler. Do not let the glue
boil or come into contact directly with the heat source.
- Sift the whiting into the warm glue mixture. Stir very carefully so as not to
introduce air bubbles into the mixture then let the mixture stand for up to an hour.
- Strain the mixture through a nylon paint strainer or an old pair of tights.
The glue should never exceed about 135 degrees Farenheit or 57 Celsius. Exceeding this temperature will dramatically weaken the glue’s ability to adhere.
If you don’t have a double boiler then improvise with an old saucepan and a paint kettle which stands on spacers placed on the bottom of the saucepan.
During the process you may add 1 tablespoon of Liquitex Flow-Aid to reduce pin holes in the gesso. You may also float a thin layer of denatured alcohol on the surface of the mixture to prevent the gesso from skimming over. You can also place the mixture in the refridgerator over night, this has the effect of dispelling many air bubbles.
When gesso cools to room temperature it turns into a gel. To keep it ‘workable’ it should be reheated on the double boiler.
Do not overheat!!
A number of layers of gesso need to be applied to the support and it is important to apply the same number of coats to the front and rear to minimise the effect of warping. The amount of layers depends on how thick the gesso is applied but the rule of thumb is to apply sufficient which can be sanded to a smooth surface without sanding back to the support.
Anything between four and eight layers will be necessary.
The first layer is very important as it is here where any pinhole bubbles can cause their problem.
Apply the first coat thinly and with a scrubbing action. Successive coats should be brushed on, alternating the direction of the brush strokes between each layer. Allow each layer to be touch dry btween each coat.
This process should be conducted in one session.
Once all the gesso has been applied lay the panel flat and allow to dry for at least 24 hours.
Once fully dry you can then set about scrapping and sanding the surface to a smooth glass like finish. After sanding, brush the surface with a bristle brush and hold the panel at an angle to the light to check for any imperfections.
Start by sanding with 120 grade sandpaper. Rub a stick of willow charcoal all over the panel and rub well into the grooves with your fingers. Then sand with 320 grade paper until all traces of the charcoal are removed. You should have as near a perfect panel!!