Mortars

Primers are primarily used for painting with oil and tempera techniques.
Tasks of gesso:

1. Isolate the paint layer from the canvas (tight canvas).
2. Creating a uniform surface and conditions for paint adhesion.
3. Influence on the uniform drying of oil (regulation of absorbency).

The method of preparation and quality of the materials used for the mortar, determines the durability of the painting.
The composition of the mortar:

• binder,
• filler,
• water.

About the specific characteristics of the mortar is determined by the binder.
About the properties - the balance between the amount of binder and the amount of filler.

Binders

Binders used for mortars can be:

• adhesives (animal or vegetable),
• drying oils,
• varnishes,
• emulsions (glue and oil or egg).

Filling materials

Chalk, gypsum, clays, whites.

Preservatives

Phenol, formalin, thymol.

Plasticizers

Glycerin, castor oil, honey.
Gesso with plasticizers requires disinfection.

Mortar
It must consist of several thin layers:

1. The first layermust bond well with the substrate.
2. The last layermust bond well with the paint layer.

In the oil technique, the first layer of mortar should not contain oil at all. The last one, on the other hand, should approach the oil technique.
As a filler, it is good to use a dye that accelerates drying (lead white, umbra).

Elasticity of the gesso

It is checked by pulling the reverse of the canvas with a finger or by rolling it.
The gesso should be dried well. After drying, a preservative should be sprayed on the back of the canvas. Formalin tannins the protein bodies and therefore it is not added to the gesso, but sprayed after drying.

Absorbency of the gesso

Should be minimal, just enough for the paints to bond well with the ground.
Purely adhesive gesso primers for oil technique are not good, as they are very absorbent.
Purely oil-based gesso primers are also not suitable, as they do not give good adhesion through their non-absorbency.
Emulsion gesso primers are best.