A medium which is handled in much the same way as impasto alia prima oils is opaque glue paint. Glue colors produce a dead mat effect, however, and can never rival the depth or luminosity of oils.
The difficulty here is that glue colors look distinctly darker wet than dry. This aspect is most pronounced in the medium hues, and means that the artist while still at work on a picture cannot know with certainty what the final effect will be. It requires some experience to use glue colors without spoiling a picture by constant retouching.
There are, moreover, technical limits, even where use can be made of impasto overpainting; glue paint applied too thickly is liable to break away from the ground, develop cracks, and crumble. Properly bound glue paint should be indelible when dry (but on no account contain too much glue). Assuming that such paint is used, it will adhere well, and hide the ground without caking.
Glue colors have greater hiding power than oils applied equally thinly. The reason for this is that oil holds the color particles permanently congealed, as it were, while glue solutions shrink noticeably in volume in the drying; when this happens on a big scale, the particles cling to the under layer like fine pebbles. Only when they are wet, therefore, can glue colors equal oils in depth.
Once the water contained in them has evaporated, leaving only the glue, the pebbled surface is left considerably enlarged and capable of reflecting more light. The color is al most as vivid as pigment in powder form, and its hiding power is correspondingly enhanced.
A study of seventeenth and eighteenth century gouache still lifes with flowers, painted on a dark ground, demonstrates the truth of this. For oil colors to have the same hiding power, they must be applied much thicker. If you apply a little glue color to a glossy paper and then draw a hard pencil line across both the paper and the patch of color, you will find that whereas the pencil only leaves a faint mark on the shiny paper, it shows up clearly on the color patch, because the rougher surface of the pigment acts like sandpaper. Conversely, a patch of oil paint would become even glossier than the paper, and the pencil would give off hardly any particles of graphite at all.
To learn to judge how much glue colors will lighten in drying, you should try making a number of brush strokes in glue color, then half cover them with a thick coating of a varnish composed of pyroxylin and amyl acetate.
This experiment should not lead you to suppose that you can impart a darker hue to a finished glue painting by var. nishing it over. What occurs in this case cannot be gauged in advance, nor can it by any means be regarded as a comprehensive painting technique. The picture is robbed entirely of its essential charm, and with it a large part of the artist's inspiration. The same thing happens to watercolors when they are varnished. Conversely, the chalky tone of an opaque glue painting can render effects which are virtually unobtainable in any other medium, although not altogether unlike a certain type of pastel.
Glue painting, then, has its own characteristics; but quite apart from this, it is well suited to sketching and to preliminary sketches for oil and true tempera painting-far more so than watercolor, for example. It should be noted that while watercolors must be applied on a wet ground, glue color, which is equally direct in its application, has the advantage of always being painted on a dry ground, so that, despite rapid execution, the paint underneath never runs or smudges.
The most suitable support for this medium is cardboard. Paper that is less than 14 ply buckles when painted and remains buckled even after the paintwhich varies in thickness-has dried out. This may occur with cardboard, too, and depends on the quality of the board and the way the paint is used. For this reason,
pasteboard, or a synthetic fiber board with good quality paper of the required thickness glued to it, provides a better support. In addition, any support which is good for oil painting will serve the purpose, as long as it is properly prepared to receive glue color. The final choice of a support is determined by the format and by the artist's aims.
Similar considerations determine whether to leave the ground white or to tint it. The best and easiest way to tint paper is to apply a very runny watercolor with a flat brush from two to four inches wide, known as the blending brush, moving it back and forth over the paper. To do this, the support must lie flat to avoid collecting paint at the bottom edge, thus ensuring that the surface is kept fairly even.
It is also possible to tint the glue ground with pastel chalk. When this is done, however, shellac should not be used for fixing, as glue colors do not adhere well to this fixative and flake off with repeated overpainting. A weak glue solution, such as gum arabic, tragacanth, or skim milk, makes the most satisfactory fixing agent; carpenter's glue is not good, as it cools down too much in the spraying and sets prematurely. The glue must be thin enough to travel up the blowpipe.
The spraying calls for particular care, since water stays on the paint surface longer than the alcohol in shellac, which evaporates more speedily. As soon as the medium has been rendered indelible, work can proceed as with alia prima oil painting. The same palette and brushes will do, and water may be used for thinning and washing.