Gesso is a powder made from calcium carbonate and glue. Applied to wood, it hardens a brilliant white and can be used a primer or sculpting material. Gesso was traditionally used as a base for tempera and was a favourite of Renaissance panel painters. It can also be used on furniture and mouldings, where it is be built up and carved. Gold leaf is normally applied to wood on top of a layer of gesso. Here are some tips.
To Make gesso begin by slaking your plaster of Paris. Mix together 1 part plaster of Paris and 4 parts water. Let the mixture sit until the plaster settles to the bottom. Pour off the water and remix. Repeat this process three times. The plaster reacts with the water. If the mixture gets warmer than when you started, repeat the process. If it appears to be the same temperature as when you started, then your plaster is slaked.
Scoop out the plaster of Paris and let it dry completely. Break up any clumps that appear as they may contain areas of moisture. Use a common kitchen grater to grind it into a fine powder.
Mix three parts plaster of Paris to 1 part glue. Add several drops of honey to every tablespoon or so of this mixture. Mix thoroughly and pound out any lumps. Your mixture should have the consistency of pancake batter. The gesso is now ready to use.
To colour your gesso, simply mix in a colouring agent. You can use watercolours or gouache. Store your gesso in an airtight container. Add a little water if the gesso becomes too dry.
You can also make gesso with chalk instead of plaster of Paris. Just make sure the chalk is real chalk and not a synthetic substitute.
Do not dry slaked plaster of Paris in an oven. Too rapid drying can cause it to turn back into regular plaster of Paris.