In summary, the four methods of approach enumerated below should make it easier for the reader to grasp the countless ways in which thick glue paint can be used:
1. Simply cover a white ground on prepared or unprepared paper, using flat hair brushes and opaque color.
The paint is laid on in the same way as with alia prima oil colors, contriving an immediate and complete opaqueness. An underpainting may be executed using opaque color thinned in water.
2. Begin with a light coat of watercolor on unprepared white paper using a round hair brush, and then finish it off by painting the picture over a second time with opaque color alone, using a flat hair brush. Your first attempt will show, however, that you must be more familiar with this working method before you can extract from it all the lovely effects of which it is capable: the thin layer of watercolor shines a good deal brighter than the opaque color which follows it.
Since underpainting is justified only when its effect is still discernible in the finished picture, leave the lightest and brightest parts untouched, and apply the opaque colors only to those of medium, dull, and dark hues. The same is true of the two types of. paints as of the two types of brushes: you will obtain a satisfactory effect only when, even leaving a few spaces, every part and every object in the picture is painted over with opaque color.
To take just one concrete example: if one were painting a landscape with a watercolor undercoat, it would be a big mistake to leave the sky untouched, solely because in nature it stands out so brightly against the colors of the
earth and vegetation.
To give just one possibility: the picture would gain greatly in depth and breadth if at its zenith the darker colors of the sky were overpainted in opaque'color, the foreground in the same way, while on the horizon the opaque color were most sparingly used for both sky and earth.
3. Proceed as with No.1, but on a tinted ground. This is undoubtedly the easiest and most direct way of using opaque color.
4. Proceed as with No.2, but on a tinted ground. In this way, the tinted ground takes on some of the luminosity of the underpaintings, so that the colors look restrained and at the same time related to one another. Using this method, complementary values of gray can be obtained very nicely: for example, by using predominantly red tones over a green ground; or by laying ultramarine blue dark patches over yellow ochre.
The most luminous colors and the high lights can, of course, be brought out only by applying completely opaque paint. Reference has been made so far to watersoluble glue colors, but the same remarks apply to casein colors. When a watercolor is intended for glazing, underpaintings will be carried out in well-diluted casein color, and lightcolored plaster may be substituted for white paper. Using casein, a possible variation on procedures 2. and 4. is to glaze the opaque layer, a process which seldom succeeds if watercolors are used over a layer of opaque paint, as the watercolor all too easily causes the opaque layer to run and smudge.
For the beginner, glue and casein colors are the least problematical materials with which to obtain experience of practically the entire range of painting techniques. Their range includes the use of watercolor for glazing, as well as the relatively primitive alia prima oil technique, the different types of fresco, as well as the balanced build-up of a multi-layer painting in tempera, using a resinous solution or a mixed or alternative technique.
An attempt may even be made to achieve the characteristic shades of pastel work, without doing violence to the materials, for the chalky tone of opaque glue painting comes very close to that of the ij>astels. And all this can be done without the long waits for the paint to dry out and without having to come to terms with the catalystic peculiarities of the pigments. As soon as the water has evaporated, both with watersoluble glue and with casein, the paint layer is ready to be worked over again.
Lastly, glue colors are less trouble for the artist to prepare himself than, say, the preparatory ground, and require less in the way of study and experiment. Carefully executed, glue color painting holds its own, from a technical standpoint, with any other kind of painting; but we must remember what it is intended for. A picture painted in permanently watersoluble glue color should be placed behind glass and framed; and casein colors are the right medium for wall painting in a room or for a large-scale panel with a dead mat surface.
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