Period gesso is a ground for oil and egg tempera painting. It consists of calcium carbonate (slaked Plaster of Paris), a whitening agent, and a hide glue. Gesso dries to a hard, inflexible and absorbant finish. Gesso is not to be confused with modern polymer emulsion ground (which inaccurately also bears the name gesso) which is what is readily available in almost all art supply stores.
The three components of period gesso are discussed below, along with preparation information.
In period, this substance was known as slaked plaster. Plaster is created from heating gypsum to a high temperature. When dried plaster is combined with water, it reforms into gypsum. However, when plaster is combined with a large quantity of water, a chemical reaction occurs that robs the plaster of its setting properties. What is left is calcium carbonate. This is the process to make slaked plaster and has remained largely unchanged since period times:
- Take dried plaster (Plaster of Paris) and mix it in with a large amount of clear water in a bucket. A 1:50 plaster to water ratio is sufficient.
- Let the plaster stand for a day. Most of the plaster will settle to the bottom of the bucket.
- Pour off the water and fill the bucket with clear water. Stir the plaster. Let stand for a day.
- Repeat these steps each day for 30 days.
- At the end of the 30 days, you will have slaked plaster.
- Pour off the water and strain the slaked plaster with a cheesecloth.
- Squeeze the slaked plaster in the cheesecloth to extract as much water as possible.
- Shape the slaked plaster into small cakes and let them air dry.
Period glues were derived from many sources: fish, calf skins and even cheese. The most widely acknowledged best glue for this purpose was been calf-skin glue or rabbit-skin glue. Rabbit skin glue can be obtained in many fine art stores today. This is the process to make hide glue and has remained largely unchanged since period times:
- Take 1 part dried hide glue and mix it with 10 parts clear water. Let this solution sit for at least an hour to soften the glue particles.
- Heat the glue very gently over a double boiler taking great care not to boil the glue.
- The hot glue is very liquid and watery. If allowed to cool, the glue sets to a jelly-like consistency. Hot glue is needed to create gesso.
This is optional and does not affect the working properies of the gesso. However, a white pigment is often added to gesso to create a very bright white working surface desireable to painters. Powdered white lead was often used in period, with titanium oxide and zinc oxide being used in modern times.
The steps below outline the process of preparing a wooden panel with gesso and has remained largely unchanged since period times.
Wooden panels must first be sized with hide glue. Brush the hide glue thinly onto the wooden panel. The glue should soak into the wooden fibers, not sit on top. Two layers are sufficient. Let dry between coats.
Making the gesso
If a whitening agent is used, mix 5 parts slaked plaster with 1 part whitening agent.
In a heat-resistant bowl or pot, mix 1 part dry whiting (calcium carbonate alone, or the whiting mixture above) with 1 part hide glue. Stir very gently to avoid making froth or bubbles. Place the bowl over a pot of hot water to keep the gesso warm, otherwise the gesso will set in the bowl. This gesso is now ready to use.
Applying the gesso
After the hide glue has dried, apply the gesso in thin layers to the panel with a wide, soft brush using straight brush strokes parallel to one side of the panel. After the first layer is applied, run your fingertips gently over the entire surface of the panel to remove any air bubbles. Let this layer dry. Apply subsequent layers of gesso in strokes perpendicular to the previous layer, always allowing the previous layer to dry just enough so that the gesso will not be taken up when a new layer is applied. Three to eight layers of gesso are applied this way to the panel. Allow the panels to dry overnight.
Finishing the gesso
In period, a flat and straight knife or scraping tool was applied on the surface of the finished panel to remove any bumps and uneven spots. In modern day, sandpaper is used to even out the surface to a smooth finish. After this process, completely clear the surface of the panel of dust and debris. Using a very well wrung out damp, smooth cloth, gently rub the entire surface of the panel with small circular motions. Care must be taken to not rub too much gesso away and expose the wood underneath. Rubbing the panel this way evens the surface out further, getting rid of scratches and surface imperfections from the last step. This final step is akin to polishing. The finished gessoed panel has a slightly reflective, glasslike and smooth sheen.