Painting techniques for plaster

Oil technique

Least suitable for plaster. Oil decomposes from the lime and alkaline salts contained in the plaster. In addition, it deprives the walls of breathability.
Oil paints darken and yellow quickly on poorly lit architectural parts. The shiny surface of oil painting reflects light and is poorly visible.
The oil technique in wall painting was still used in the 12th century, but quite rarely.
In the Renaissance it was used more often (Leonardo, Ghirlandaio, Poloindo).
And to protect the oil painting layer from wall moisture, some of the Renaissance masters added resins. However, these were experiments that paid dearly, as works executed in the Renaissance using this technique have been very badly preserved ("Last Supper" by Leonardo in the monastery of Santa Maria della Garcia in Milan).

Glue technique

The most popular, especially in Italy due to its dry climate.
Animal glue is used as a binder. So that the carpenter's glue does not set, it is developed with a certain amount of quenched lime: the glue solution is boiled with a 4% solution of quenched lime.
The least concentrated glue solution (6%) is suitable for painting ornaments.
The most concentrated glue solution (20%) for painting figures.
Before painting, the plaster is primed as above. You can also prime with skimmed milk.
A gesso of glue, chalk and water is also used. However, the glue primer is not resistant to moisture in the air, as the glue loses its binding force from moisture.

Tempera

For painting walls, egg tempera and casein tempera are the most suitable in terms of durability.
Cennino Cennini puts tempera above the glue and oil technique. Tempera was very popular in the Middle Ages and Renaissance, on par with fresco.
Often, due to the difficulty of painting frescoes, a mixed technique was used: fresco and tempera.
Depending on the binder set, tempera has different strengths and therefore variously covers the pores of the plaster. However, even very strong tempera covers the pores much more poorly than oil.
The tempera binder should always be used fresh and therefore it is not advisable to use ready-made paints for wall painting. It is best to prepare everything with your own hands.
The following dyes can be used in tempera:

• zinc white,
• ochre of all shades,
• Neapolitan yellow,
• cadmiums,chrome red,
• iron reds,
• green earth,
• chrome green,
• cobalt green,
• sienna,
• umber,
• mars,
• ultramatte,
• cobalt blue,
• wine black.

(Zinc white behaves better in tempera than in oil. Umbra needs more binder and takes a long time to dissolve with water. It often breaks down casein emulsion.)

Egg tempera

The Renaissance tempera described by Cennini belongs here.
It consists of:

• a whole egg,
• a small amount of fig juice
• and diluted wine in an amount equal to the volume of the egg.

Protein bodies: egg white, casein, animal glue and others, have the ability to form solid chemical compounds with lime (lime albuminates) and are among the compounds insoluble in water. Therefore, egg tempera laid on dry but not solid plaster, still containing lime compounds, does not spoil, but further fixes by entering into a compound with lime. On the other hand, on damp walls that do not contain calcium compounds, the egg rots and spoils.
The tempera from the whole egg is not as strong as from the yolk alone or from the artificial emulsion. It has a lightness of tone and is similar to fresco. It does not cover the pores of the plaster, and after drying it brightens.

Egg tempera with artificial emulsion.

This is a stronger tempera. It consists of:

• a whole egg,
• boiled oil (½ the volume of the egg),
• vinegar or spirit (½ the volume of the egg).

If spirits are added instead of vinegar, 1/4 of the volume of the egg is taken, diluted with water and 1 g of soda is added.

Casein tempera

It is great for plaster. You can use different sets and types of this tempera, but avoid using it too strong. You use the same sets as for easel painting:

• 50 g casein,
• 20 g of borax (or 5 g of soda or ammonia),
• 280 cm3 of water.

Such a casein solution can emulsify an equal amount of oil. It is better to add slightly less oil (1 : 2/3).
This tempera dries faster than egg tempera (the egg oil contained in the yolk takes a long time to dry) and becomes insoluble in water.