TRAGACANTH is gum extracted from bruising the branches of the tragacanth bush. It provides the most important binder for watercolors and for gouache and other opaque colors generally sold as tempera paints, although in fact they do not contain an emulsion. The more exactly named "egg tempera" and "oil tempera" are emulsions.
You already know how painting techniques and their appropriate colors are named according to the binder employed. Watercolors are an exceetion to this. They are named after the water, used so copiously and noticeably in the technique, which dissolves them and thins them. It is never worthwhile to prepare watercolors oneself. We shall describe only how the colors we buy are produced. Tragacanth is the binder, but other substances are added:
1. Ox gall, or rather its salts. This faocilitates a fine dividing up of the pigment, which thus allows a very thin application.
It has the property of neutralizing the smallest traces of wease. Ox gall can be bought in every art shop for the purpose of neutralizing the surface of paper, which often becomes marked with grease on its way from the factory to the user. It may not be visible, but it will prevent the paint from taking evenly; thus, it is a worthwhile precaution to rub ox gall over otherwise untouched paper.
2. Honey, sugar syrup, or glycerine. Substances which prolong the drying time of watercolors-particularly those in porcelain pans-so that they are more quickly soluble.
3. Boracic or benzoic acid salts. They act as preservatives. Without them all glues in solution putrefy.
The characteristic smell of watercolors is often given by oil of bitter almonds, added solely to disguise the rather unpleasant smell of the glue.
Watercolor pigments are often so finely ground that they act as colloids. This is noticeable if an attempt is made to wash off the color. Some colors, although they become fainter, will stay in the paper. Genuine watercolor painting was made possible only by this fine grinding of the pigments.
The method was evolved in the seventeenth century and was developed to its full perfection by the English painters Girtin (1773-1802) and Turner (1775- 1851 ). Until then only "gouache" colors had been obtainable as finely ground pigments. Unlike true watercolors, the latter are not fully transparent and require the addition of white to make them lighter.
Gouache colors as now sold are an unnecessary halfway house between watercolors and opaque colors. The pigment of the latter is hardly finer than that of all the rest, including the finest sorts of powder color. Opaque colors can be prepared at home and should be used within a few days of mixing. They generally use tragacanth as a binder, but can also use gum arabic, cherry gum, or egg white.
CHERRY GUM is extracted from the cherry laurel. It was used in earlier centuries as a glue for cherry gum tempera.
GUM ARABIC is extracted from Arabian and Indian acacias. Both gums are very light and almost colorless in solution.
ALBUMEN is the main constituent of egg yolk, which was, before the development of fresco-buono, the most frequently used binder for mural painting. The fat content of the yolk makes it less liable to dissove after it has dried than egg white, and its color soon fades in the light. If egg white is used it should be whipped and then strained, so that it loses its coagulated, sticky consistency and mixes more easily with water.
I would caution you against mixing glue colors for yourself from the usual trade powder colors unless you are going to use large amounts in a short space of time. Too much preservative can have a deleterious effect, and it is difficult to judge the right quantity for small amounts of paint.
It is best to avoid preservatives altogether. The advantage of mixing glue colors oneself is that they are then certain to be pure pigments, for bought opaque colors, especially the socalled poster colors, are often mixed with other materials to give them body and density. Fillers are added to make the paint more uniform and solid, and also to make it cheaper. This reduces the intensity of the color, and is particularly noticeable in mixtures, which rarely have the brilliance you would expect from the brightness of the constituent colors. Complementary juxtapositions, too, are less strong in effect because all fillers have a dulling effect.
If opaque glue colors are used only occasionally it is best to buy what are sold as "artists' tempera colors," as they are most likely to be undiluted pigments, if they are made by a reputable firm. The binder used is only glue, not an emulsion. These incorrectly styled "tempera" colors give an optical effect similar to that of a genuine mat tempera or oil painting.
Since few artists today prepare their paints themselves, it is difficult to buy the right jars to keep them in. We therefore illustrate the type of jar provided for the author by a glass factory at small cost. It can also be used for mixing up the paints, and will save you the trouble of trying to fill tubes once the paint is mixed, a procedure fraught with difficulties and waste.
Once the paint is kneaded in the jar, it should be rolled together to form a smooth lump and covered with a close-fitting plastic disc. A piece of ordinary adhesive tape can be made into a handle for lifting it. The jar itself is closed with a close-fitting rubber lid. The plastic disc will keep the paint from contact with the air and is especially necessary when there is not much paint left. It can easily be lifted out with tweezers.
A stiff plastic spatula can be used to take out the paint. The jars are easily cleaned: they should first be wiped with a paper towel, and then washed out with water or white spirits, according to the binder used. Jars such as these can be obtained from firms supplying cosmetic factories, or a druggist will get them for you.
There is the danger with all glue colors you make yourself, and especially with casein, that you will put in too much binder. Even if it is thinned with water, it will make a skin over the painting. Carpenter's glue, tragacanth, gum arabic, and cherry gum should all be mixed in the proportion of 100 cc. of water to 7 gr. of dry substance. If the solution is stronger than this, because of careless measuring, the paint may scale or crack.
It is therefore advisable to make trial strokes with the brush to see if the paint is just firm enough. Better too little binder than too much! If the color does not hold firmly on the painting, it can be sprayed with a very dilute solution of glue. A glue solution can be tested by smell. If it is beginning to go bad it is useless.