Materials used in tempera and watercolor techniques

Like all protein bodies, glue belongs to the group of macromolecular bodies.
It consists mainly of either collagen or the albumin group.
Types of collagen:

• ossein,
• chondrion,
• glutin.

Animal glue (carpenter's glue) is obtained from the stock of bones and hides, and is composed of glutin (of which there is more in skin glue) and chondrin (more in bone glue).
Both of these components are proteinaceous bodies that undergo shear during cooking.
The glue includes nitrogenous and sulfur compounds, so animal glue tends to rot. Preservatives should be added to it.
Animal glue is used primarily in the glue technique or for mortar.
Before dissolving, animal glue should be soaked in water for 24 hours.
A good grade of glue swells and dissolves easily when heated in a water bath.
The better grades of glue give a clear and transparent solution.

Casein

It belongs to the group of complex protein compounds. It is the main component of milk. Cow's milk contains about 45% of it.
Casein is obtained from curd formed at 60°C from artificially acidified milk.
For painting purposes, curd yielding casein must be formed from skimmed and fresh milk, which does not contain milk sugar and bacteria.
Wanting to obtain curd that does not contain the mentioned ingredients, the milk must be artificially acidified with acetic, nitric or sulfuric acid.
Curd is washed with water and developed with alkali.
Wanting to eliminate residual fat and other extraneous components from curd, you can cut it again with acids into curd after obtaining casein glue, and then turn it into casein glue again by repeating this process several times.
Finally, the curd is washed with water and strained through a fine sieve. Then it is dried at room temperature.
Casein does not dissolve in water either cold or hot (it only swells). It dissolves only under the action of alkalis, giving a strong adhesive, capable of emulsifying oil and varnishes.
When it dries, it strongly changes in volume and is prone to cracking.
Casein solutions obtained with lime are insoluble in water after drying.
Casein solutions do not require special preservatives, as casein solvents simultaneously preserve them.
Casein paints should not contain more than 10% casein, otherwise they crack.

Vegetable adhesives

Vegetable glue does not contain nitrogen or sulfur and is therefore resistant to rotting. However, it does not have as much adhesive power as animal glue.

Gum Arabic

Is a product secreted by tropical acacia trees. The best species comes from Senegalese acacias (Kordofan Gummi).
This species is sold in the form of round colorless or lightly tinted brown pieces the size of a walnut.
It dissolves in cold water.
Gum Arabic is separated from the solution with the help of spirit.
To obtain glue for one part of gum Arabic, two parts of hot water are taken, in which it dissolves perfectly.
After one day, the glue is ready for use.
It stores for a long time without preservatives.
To add elasticity, 15 parts of glycerin are added to 100 parts of gum Arabic.

Cherry glue

Obtained from the resin of cherry trees,as well as other deciduous trees.
Darker than gum Arabic, but stronger than it and less brittle.
It dissolves with difficulty in cold water.

Dextrin

It is obtained from starch - several species ranging in color from white to brown.
Colored species dissolve better in water and are used in watercolor.
Dextrin glue is brittle.
Dark dextrin species are characterized by hygroscopicity.

Tragant

It is obtained from the leguminous plants of the Astragalus family.
In cold water it only swells, dissolves in hot water.
It gives a colorless glue of medium density (3 ÷ 4%).

Starch

The best is obtained from rice, ordinary from potatoes, as well as from chestnut and wheat.
Wanting to obtain glue (klajster) one takes for 1 part of starchmalt
8 ÷ 10 parts of water.
The mixture is put on the fire and, stirring, is heated until it boils.
For elasticity, glycerin can be added.
Starch clay is easily dissolved in water. However, it is resistant to moisture and microorganisms. It rots with difficulty and does not adversely affect dyes.
For a binder it is not suitable, as it is not very malleable. This can be improved by adding alkalis, acids and resin oil.
Such starch becomes insoluble in water after drying.

Fig tree sap

The sap of young fig shoots has the appearance of milk. It is a natural emulsion consisting of vegetable glue and resinous bodies.
It exhibits adhesive properties and an acidic reaction. It perfectly dilutes the egg while preserving it.

Honey

It consists of sugar substances:

• dextrose,
• levulose (in equal proportions),
• a certain amount of ordinary sugar,
• non-sugar compounds,
• water.

Purified honey dries very slowly turning into a transparent film, easily soluble in water.
Honey is used to give elasticity to glue, to hold drying of watercolors, and also as a binder in honey watercolor.
For painting purposes, it is better to use honey devoid of dextrose (it does not crystallize).
To obtain levulose alone, pure honey is left at room temperature until it crystallizes. Then 1 part of crystallized honey is mixed with 4 parts of spirit and shaken vigorously.
A light yellow spirit solution of honey is formed.
After evaporation of the spirit, what is left is a thick syrup - uncrystallized levulose (very hygroscopic).

Glycerin

A component of fats and vegetable oils. A polyhydric alcohol.
There is glycerin either anhydrous or with a lower or higher water content.
Invented in 1779, it has been used in painting since the mid-19th century.
At ordinary temperatures, glycerin does not volatilize. It does not combine with volatile and oily oils.
It strongly pulls moisture from the air. It does not spoil or rot, rather it is a preservative.
It is sometimes used as a plasticizer in the manufacture of adhesives.
More glycerin in glue stops it from drying out.

Ox bile

Increases the durability of emulsions. It is used as an additive to water binder paints to improve their application on paper.
The painted surface can also be smeared with this preparation.
Garlic juice has similar properties.

Borax (boron sodium salt).

On sale it comes in crystals or powder of white color.
4 parts of borax are dissolved in 100 parts of water.
Borax with shellac (1 : 5) forms a white, water-insoluble varnish when dry.
It dissolves casein and preserves it.

Ammonia water

It is used as a volatile alkali.

Soda (sodium dioxide)

Casein solutions with soda are characterized by weak alkalinity.