Recipe for gesso
Add 5 tablespoons of bone glue to 450ml of cold water.
Wait 1 hour for the granules of glue to dissolve. Stir every 10 minutes. After that let it soak fo seveal more hours.
To prepare gesso you need a steam bath: the pot with mixture needs to be put into another, larger one, so that the mixture won’t boil or stick. Heat the mixture up until it’s liquid. The temperature in the larger pot cannot exceed 70 C.
For every tablespoon of the glue add 10 tablespoons of chalk (ca 50 spoons in total). Add it gradually and stir to dissolve any clumps.
You can add 2-3 tablespoons of flax seed oil to make the mixture more flexible. Be careful not to add too much of it or your gesso will be hard to sand.
Stir patiently to dissolve any clumps. When the mixture is uniform you can start applying it.
Apply gesso with a wide, flexible spatula. The layer should not be more than 2-3 mm thick. To thick a layer can make the shield too heavy and crack with impacts.
When the layer dries completely sand it with sandpaper (40-100). For best results use a polishing machine. When the gesso is smooth and even you can paint it with temperas or acrylics.