The next stage in making our gilded frame is to mix up and apply the gesso. As with the size coat, rabbit skin glue is mixed with water, this time the ratio is 1:10 (1 part glue to 10 parts water). The glue/water is left to soak for 24 hours and is then heated gently in a double boiler. Once it is all melted and warmed the glue is taken off the heat and whiting (calcium carbonate) that has been sieved is added to the glue. I use a spoon to carefully add the whiting to the glue. Usually I use a large metal saucepan, but using a large coffee jar illustrates how the whiting drops to the bottom and absorbs the glue, whiting is added until there is just a small amount of glue left on the surface, basically until no more whiting can be absorbed. Gypsum (calcium sulphate) and some other less common chalks can be used instead of whiting. In times past whiting was mainly used in Northern Europe, while gypsum was favoured in Italy and the south.
This glue and whiting is left for a while before being given a good stir, a few good taps on the jar or container can help eliminate some air bubbles. Some people would now sieve or filter the gesso to remove any last lumps, and sometimes people add other ingredients such as linseed oil, tallow, and alcohol. I have also heard of garlic, salt, and other additives being used, which alter the properties of the gesso.
The rabbit skin glue we use nowadays is a slightly different, modern version of the traditional hide glues used centuries ago, parchment clippings can also be used and is probably a more traditional ingredient, just not that convenient to use. Some relatively new and valid types of gessos are made from PVA adhesive or conservation adhesives such as Paraloid B72. I think the study of gesso is actually quite complex, and I only wish I had time to experiment more with different recipes and types.
So once the gesso is mixed it can be brushed onto the frame, the next coat is applied when the surface has dried and is no longer glossy, apply the next coat to soon and you can get nasty cracks, although you should not wait until the surface is 'white' dry. Timing is key.
The number of coats depends on the quality and type of finish your aiming for, but we usually apply 6 to 8 coats. The gesso can be brushed on or sprayed, if I am spraying I always brush on the first one or two coats and then spray on the final coats, I find the finish is just better doing it this way.
We aim to complete gessoing a frame in a day, doing half one day and the rest the next is never good, as the time between applying each layer is important to building a strong bond between each coat. Another thing to remember is not to overheat the gesso, to much heat breaks the glue down and putting warm gesso on a layer that is cold and dry will result in lots of air bubbles in the finished surface. Once the gessoing is finished the frame is left to dry for at least 12 hours before the next stage, which is the sanding.
