Adhesive technology

The glue technique is used more for decorative work. The binder is glue, usually carpenter's glue. Rarer is the use of gelatin and casein, since gelatin sets faster after cooling than other types of animal glue, and casein is more difficult to dissolve.
In the glue technique, you should pay great attention to the "strength" of the glue solution. Too weak a glue will not bind the dye sufficiently and will rub off when touched by hand. An adhesive solution that is too strong will result in a glossy painted surface and cracks in the paint layer.
A normal concentration of glue gives good adhesion to dyes and a velvety matte surface. It is recognized by the fact that after cooling down the glue does not solidify into a hard jelly. You can also try with your fingers. If the odlip is light, then the concentration of glue is sufficient. The best solution is 6% - however, it all depends on the species of glue and the quality of the dyes.
For example, chalk, umber, Cassel earth, ultramarine, sienna, ivory, soot, require a slightly stronger glue solution, so that they do not rub off after drying.
The glue solution must be thin enough to easily drip off the brush.
In the glue technique, the tone of the paints changes after drying. Wanting to avoid great differences in color intensity, the percentage of glue solution should be strengthened. However, you then need to add a plasticizer to increase the flexibility of the paint layer and prevent cracking (honey or glycerin).
Honey was often used in the glue technique on boards in the Middle Ages, but then the whole thing was covered with varnish. Honey stops the paint from drying.
Wanting a stronger glue solution, one takes 20% of it. In such a solution the paints do not brighten too much.
So that at this concentration the glue does not jelly, a 4% cake of quenched lime is added during its dissolution. This addition also has a preservative effect.
You can also stop the glue from setting after it has cooled by treating it with an acid, such as nitric acid (for 100 g of glue, 20 g of nitric acid in 100 cm3 of water). When doing so, however, acid-resistant dyes should be used.
The glue technique is not permanent and can only be used in a dry room.
From moisture, the glue decomposes and rots, the paint layer washes away.
Wanting to have more permanent results with the glue technique, it is necessary to add wax emulsified in alkalis, or fix it with preservatives after painting (formalin.
Much better results are given by the tempera technique.