Egg tempera paintings were executed using a paint consisting of a pigment in a egg-based binder or medium. The oldest examples of egg temerpa painting date from as early as the first century AD. Wooden panel is the only period support for this medium because a fully dried and cured egg tempera painting is inflexible. These paints dry very fast. They remain soluble in water for a time (about a week). After a week, the paint film more completely cures and is less readily soluble with water.
Egg tempera painting was the forerunner to oil painting techniques. The yolk of the egg, used alone in many techniques, is a fatty (oily) substance that contains an emulsifier that thickens the consistency of the yolk. When using egg tempera, the paints are applied thinly with values and colors being built up in layers. Compared to period oil painting techniques, in which a pigment is ground into a drying oil and applied in thin layers, it is easy to see the relationship between the two mediums. Also, the preparation of substrates and grounds for oil painting is very similar to the preparations developed for egg tempera.
The only appropriate period support for egg tempera painting is wooden panel. Poplar was the wood of choice for Italians with walnut being favored in northern Europe. Wooden panels that were aged, free of oils and knots were chosen.
Period size and grounds
Wooden supports were first prepared by sealing their surfaces with size, a glue usually calf hide-based or fish-based. A gesso ground (consisting of slaked plaster and glue) was then applied to wooden supports to provide a receptive working surface for egg temera paints. Because gesso dries to an inflexible and rigid state, it is unsuitable for use on cloth supports.
Egg tempera paint
Egg tempera paint was created by mixing pigment with egg (either the egg yolk, the yolk and white together, or just the white} in a small dish. Water was added to made the paint flow easily. A modern method for making a simple egg tempera is:
- Separate the white from the yolk. Put the white aside.
- Dry the yolk by rolling it from hand to hand, drying your hands constantly. Carefully rolling the yolk on a paper towel also works.
- When the yolk is dry, gently pick up the yolk with the tips of the fingers of one hand. Hold the yolk over a clean dish.
- With your free hand, prick the bottom of the yolk sac to release the yolk into the dish. Discard the yolk sac.
- Mix water into the yolk at a 1:1 ratio. Set aside.
- Put a small amount of pigment into a clean dish. Use only a small amount as egg tempera dries very quickly.
- Using an eyedropper, slowly add the yolk-water mixture into the dried pigment and mix until a smooth, workable consistency is reached.
Gold leaf gilding was common on the religious paintings of the time. The gold leaf was sometimes applied in the early stages of a painting, but sometimes near the end. The glue used to adhere gold leaf to the painting was called mordant and has a reddish tone. After gold was applied to the mordant, the gold would then be burnished to a high sheen with a semi-precious hard stone or a hound's tooth.
For painting on wooden panel:
- A charcoal drawing was completed. This was either on parchment to be transferred to the wooden panel, or drawn directly on the panel itself.
- An ink wash was applied to fix the drawing to the panel. The ink wash was used to reinforce the drawing and acted as a guide to the painting moving forward. A stiff brush or feather was then used to remove all charcoal from the panel.
- A full value underpainting was applied over the ink drawing. The colours in this underpainting were very limited or monochromatic. This is called grisalle. In parts of Italy a verdaccio underpainting was used which uses the colours of black, green, and pink.
- Full color is then applied over the underpainting, adding colour to the previously muted or monochromatic underpainting. There is no need to augment the tempera paint to fulfill the "fat over lean" rule as it generally applies to oil painting. Though egg tempera dries to the touch very quicky, the paint film does not fully cure until a week has passed. During the time, fresh egg tempera paint may be added on top of dry egg tempera, and the two paint films will bind strongly.