As mentioned earlier in connection with ready-made colors, "artists' tempera," as it is called, will prove fairly safe; but it is better to obtain pigment in powder form and make it up for yourself, using an appropriate glue solution. In this way, you will be sure of painting with pure and unadulterated colors.
Deciding whether to use a white or a tinted ground will be determined by your particular aim; and whether white or tinted, it should playa part in the picture as a whole. A delightful effect is achieved when a tinted ground is left to show in places. Using glue colors, and carrying out the underpainting in watercolors, the color can be made to darken very gradually so that the tinted ground is distinctly preserved.
On a white ground, the underpainting has a naturalness and delicacy which looks wonderfully fresh against the overpainting. The final coat may also be applied more thickly, if this is in keeping with your aims and methods. From a technical viewpoint, this is quite safe, provided that the layer of paint is less thick and cakey than it is with alia prima oils. The technical possibilities of glue painting are increased if casein is used in place of nonreversible glue. This can hardly be done, however, without restricting color choice, since it means that no coloring matter sensitive to either chalk or lye may be used.
Even when casein has been left to set for a full week and is completely insoluble in water and the water used for thinning has evaporated - it will be all but impervious to simple overpainting. Prolonged washing is the only method. Hence, the best use for glue colors is for really extensive and time-consuming work. For instance, pictures requiring glazing are difficult to do well because the layer beneath the nonreversible glue dissolves so rapidly. With casein, too, the colors lose rather more of their brightness than with the glues previously mentioned.
When a mat painting is completely dry, it can be given a sheen by brushing or by rubbing with a woolen cloth. This makes the colors appear somewhat deeper and is a procedure which thoroughly suits the medium. If the paint is laid thinly onto a white ground it will look like a genuine fresco. Naturally, it is a prerequisite that either the untreated sheet of paper or the glue ground should be prepared with casein-bound pigment before painting begins so that, even when the ground is untreated, it is given a layer of casein.
If you try applying this undercoat on cardboard not treated with glue, you will find at once that the cardboard will roll up with great force. Fresh casein is a powerful binding agent, and it is often insufficiently thinned. When this happens, it rolls off in broad flat cakes, taking even the primer with it. For this reason, the paint layers should barely go beyond the point of indelibility.
If this point is not reached, a very strong dilution of casein can be used afterwards as a fixer. The most risky procedure is the impasto use of a casein color containing too much glue. It detaches itself stubbornly from the ground. Abstain altogether from impasto work with casein colors. However, if this is the very effect you want, but at the same time you wish to avoid the yellowing and browning which often accompany oil painting, then you are better advised to use true tempera.
The same bristle brushes should be used for casein colors as for oils. For large-scale work, such as mural painting, it is better to use correspondingly larger round brushes and to reserve the usual flat brushes for fine detail. Round brushes always take up more color, and naturally induce a different brush technique from flat ones, which are best suited to the juxtaposition of rectangular patches of color.
When using both types of brushes in the same picture, you will find the best way is to start with the larger round brushes and then paint in the details with flat brushes, so that ultimately the entire work is painted over. This is the only way to achieve any unity. Otherwise, if you leave one part of the picture as painted with round brushes, while another exhibits the cubist style which comes of using a flat brush, this unity will be lost.
You can also follow this method with glue colors, for which both round and flat hair brushes are indicated. As a rule, flat bristle brushes are too stiff for malleable glue colors, even in relatively thick applications. They are also apt to scrape off the water-soluble underpainting.