Preparation of plaster for fresco

The wall is prepared depending on the type of fresco.
For "buon fresco", the plaster (first layer - "aricciato") is applied directly to the brick or stone.
The wall must be saturated with water beforehand.
The joints between the bricks are deepened for better adhesion of the plaster.
The wall is saturated with water before plastering (the night before), and the next day - half an hour before imposing the first layer of plaster - also.
A damp wall does not sap the water from the plaster, allowing it to mature better and gain strength. Plaster laid on dry masonry becomes brittle.
The first layer of plaster (a rare diluted lime batter with coarse sand) is laid in what is called a "splash". The thickness of this layer is approximately 2 cm.
Dry sand is poured into the lime dough.
The first layer must thoroughly fill the gaps between the bricks and level the entire wall well.
After some time, when this layer has solidified enough that no trace remains when you touch it with your finger, a second layer of the same plaster is applied, only thinner (about 1.5 cm).
In the second layer, the amount of lime dough is slightly larger.
The third layer of plaster already belongs to the so-called "intonaco". Fine sand is added to the lime dough, with the thickness of the entire intonaco to be between 0.5 cm and 1 cm.
In intonaco, the sand can be replaced with marble flour, grated brick or other thin filler.
The intonaco is leveled with a board by sprinkling the surface of the plaster with water to make the surface more even.
Intonaco is put on such a surface to be painted in one day.
The number of layers of intonaco is basically arbitrary (in practice from 2 to 8).
The more layers, the longer you can paint (it does not dry as quickly).
The thickness of all layers (aricciato and intonaco) can reach 4 cm.
Several layers of plaster is stronger than one thick layer and keeps moisture longer.
A very important thing is to maintain the right ratio between lime and filler. It largely depends on the quality of the lime - the fatter, the more filler.
Here are two tried-and-true proportions:
According to Vitruvius:
3 volumes of coarse and 2 volumes of medium sand are taken for 1 volume of lime.
According to Cennini:
for 1 volume of lime take 2 volumes of sand (coarse or fine).
You can also take 1 part of lime to 2 parts of sand by weight.
When mixing the lime dough with sand, do not add too much water (so that it mixes better), because excess water in the mortar will cause the plaster to crack later.
If the aricciato is made of fine sand and lime, it is necessary to make incisions on its surface with a sharp tool to create better adhesion for the next layers.
It is better to use coarse sand for aricciato, as such mortar is more durable.
As for the time interval between applying each layer of mortar, it should be laid before the bottom mortar dries completely, as fresh layers bond better.
Lime mortars should be used fresh, as prolonged storage in the air causes the formation of a coating of calcium carbonate, which later harms the durability of the plaster.
For fresco work always take river water, because tap water is sometimes contaminated with iron salts - very harmful.