Gesso is a white, pasty medium generally used for priming canvases. However, it's also used to sculpt shapes that will later be painted over. Ornate gold frames, for example, usually have gesso sculpture underneath all that gold leaf. It's possible to make your own gesso from glue and an inert white pigment such as chalk or gypsum, and do-it-yourself gesso is just as effective as the store-bought kind. Because gesso does not lend itself well to three-dimensional sculpture, most people use some sort of backing, such as wood panel, to sculpt on.
Prepare the hide glue the day before you plan to use your gesso. Hide glue, often marketed as rabbitskin glue, comes in powder form and must be mixed with water. Simply add 11 parts water to one part powder, and let the mixture sit for 24 hours.
Prepare the base of your sculpture. If it is wood or wood panel, sand it down until the surface is smooth. Warm the glue in a double boiler until it is the consistency of milk. Paint the warm glue over all surfaces of the panel, allowing half an hour to dry when you finish. Rourke Visual Art suggests adding extra layers to the back of the panel to offset the weight.
Put the rest of the glue back into the double boiler and gradually add the chalk or gypsum. You should use one and a half times the amount of powder as there is glue. Many artists also add a small amount of titanium white pigment to make the gesso brighter. You can add any pigment you want if you want colored gesso. Allow the glue to soak into the powder and stir carefully with a brush.
Apply the gesso to your base. It's best to start out with a few smooth layers of gesso, building them up gradually on top of each other. Use paintbrushes and whatever other tools you wish to create shapes out of the gesso as it hardens. Keep the gesso in the double boiler warm while you work.
Finish your sculpture by sanding it lightly with fine sandpaper, if necessary. If you will be painting on top of your gesso sculpture, you may want to go over it with another layer of hide glue or with a varnish to reduce its absorbency.
Avoid making bubbles when you stir the glue and the gesso, as this will make it more difficult to work with.