MATERIALS & METHODS
Thompson, Daniel V., Jr., Research and Technical Adviser, The Courtauld Institute of Art, University of London. The Practice of Tempera Painting. New Haven: Yale University Press. 1936. Fourth Printing, 1946.
It is a good plan to add to this egg mixture two or three drops of vinegar, or of 3 per cent acetic acid. This has the effect of making the mixture less greasy, and a little easier to use. Some yolks are very oily indeed, and they are made more liquid by adding this weak acid. The acid does no harm, and more may be used; but if there were any great excess of it, it might injure ultramarine blue, and it is therefore just as well to use only a little. This acid has a slight preservative action on the yolk of egg; and no other preservative should be used. Oil of cloves or eugenol is often recommended, [p. 96] but they are objectionable. Sodium benzoate is used in some commercial temperas, and perhaps does no positive harm; but the egg mixture will keep for several days in a cool room without any preservative at all, and it is better to replace it every two or three days than to try to keep it going longer.
It is a good plan to have two tempera bottles, and to transfer the unused tempera from one to the other at the end of a day's work. The used bottle can then be washed with soap and hot water, and rinsed with boiling water, to destroy the organisms of putrefaction. The use of two bottles in this way prevents the formation of hard crusts of egg on the inside of the bottle and around the neck. These crusts are very hard to remove; and when they are allowed to remain, the tempera usually tends to spoil quickly. Tempera was once called "la pittura al putrido," but this nickname need not be deserved. A little care and forethought will prevent any trace of unpleasant odor about the studio.
The mixture of egg yolk and water is your tempera, the binding medium which holds your colors in place on the panel. It is added to the colors which have been ground with water, and the process is called tempering. The composition of the tempera may be varied in innumerable ways, and some very valuable properties may be given to it which the egg yolk alone does not possess; but there is no better single all-purpose tempera than the plain egg, and it is advisable for any tempera painter to master the discipline of this basic medium before he experiments with others. [pp. 96-97]
Tempering the Colors
Tempering the colors is often the hardest part of this technique to learn at first. There is no formula for it; because every pigment and pigment mixture needs a different amount of tempera. With care and practice, a skillful painter can produce a beautifully even, velvety, matte surface without fear that any part of it may be insufficiently tempered. But this surface beauty is lost when the painting is varnished, and it is therefore hardly worth the trouble and risk that it means to a beginner to achieve it. His safest rule in tempering colors is to add enough egg to every mixture to make it a [p. 97] little bit shiny when it is first painted out and dry. In this way, he may be quite sure that he has enough tempera in his colors. It is better not to have any great excess; but any excess is better than the least deficiency. If there is not enough tempera, the colors will dry out unevenly, usually lighter than expected, the paint will pile up unpleasantly, and the painting will have a chalky appearance. Furthermore, when it is varnished, any parts which have been insufficiently tempered will turn dark and spotty. If care is taken that each mixture dries with a slight gloss, the values do not change in drying, the paint goes on smoothly and easily, the gloss will disappear in a day or two; and when the painting is varnished there will not be any unpleasant surprises.
The process of tempering is extremely simple. A little of the paste of ground color is placed in a color cup, and about an equal bulk of the egg yolk medium added to it. It is stirred with the brush, thoroughly; and the tempering is then tested by making a stroke or two on a gessoed panel, a little block of plaster of Paris, or a lump of natural raw umber. Let the trial strokes dry for a moment or two. If they are dull and chalky, add a little more egg to the mixture, and try it again If they dry very slowly, and come out very shiny, try adding a little more of the ground color to the mixture; for the proportion of egg is probably unnecessarily high. The perfection of tempering is to produce a mixture which just does not shine when dry; but to achieve this, one must run a grave risk of insufficient tempering, and it is not worth while. Too much tempera will make the painting greasy; but even that is better than the chalky look of colors too little tempered. Aim at a slight gloss; add more tempera if you find a color drying dead; and you will soon find what sort of mixture gives the best results. [pp. 97-98]
Reserves of Color
It is a good plan always to mix your colors first without tempera, and to keep a reserve of the untempered color, in case you add too much egg to the rest. The Color cannot be kept long after it has been tempered, hardly, indeed, from one day to the next. But any mixture can be kept on hand indefinitely before the egg is added as long as it [p. 98] is not allowed to dry up. Since it is not easy to match the color mixtures exactly, it is a help to keep a little of each important mixed tone that you use, in case you want to return to it later on to make some change. Do not try to temper too much color at a time. Keep the untempered mixture by you, and add the egg to portions of it as you need them. The tempered color should be stirred with the brush every time the brush is filled, so that it will keep the same constitution until it is used up. [pp. 98-99]
Liquidity of the Mixtures
It is easier to test the correctness of the tempering while the tempered mixture is fairly thick and creamy; but when you come to paint with it, the tempered color should be thinned out well with water. It is very important that the color should be thin enough for the brush to move perfectly freely over the gesso. There should be no feeling at all of pushing or dragging about it. This is hard for the beginner to realize, particularly if he has been used to painting in oil. It is actually quicker and easier to build up an even tone with thin coats. Thick color dries slowly, and if it is touched again with the brush while it is still wet, makes unsightly spots, uneven in color and surface; while with thin, well-diluted paint, the brush skips quickly back and forth over an area, building up any desired amount of solidity, a little at a time, but without delays, and very easily. [p. 99]
Handling the Color
The secret of ease and expedition in tempera painting may be summed up in one simple formula--Get your tempering right, keep your color liquid, and have your brush squeezed almost dry. In a word, make haste slowly. Tempera painting is as far from the technique of the water color on the one hand as it is from the plastic paint-brushing of oil handling on the other. It is much closer to the technique of pencil rendering; and the tempera painter will do well to bear that comparison in mind, and handle his brush as if it were a pencil. When you want to lay a tone in tempera, do not try to  float a wash, and do not try to spread the paint out with the brush; but run over the area to be covered with quick, easy strokes, like pencil marks. Follow this first coat with a second, applied in the same way, running the strokes perhaps at a little different angle, according to he form; and continue in this way until you have built up as deep and even a body of color as you want.
If he color is thick, the brush will not work like a pencil. It will not move freely, or make an even stroke. You must add water to the color until it offers no resistance to the free movement of the brush. If the bush is simply dipped into this liquid color, it will make very wet strokes at first, until the excess liquid is used up. The ends of these strokes will show as little blobs, and it will be impossible to keep the tone even. A good brush has a certain natural capacity, depending on its size, and within the limits of that capacity it will feed liquid color down to the point as evenly as a good fountain pen. If you will dip your brush into the liquid color, and stir it thoroughly, and then wipe the excess color off the brush, first on the edge of the color cup and then either on a cloth or on the back of your left hand, you will find that it will give a whole long series of uniform strokes; and you can paint with it with as much control as you can draw with a pencil. This perfect control of the stroke is, of course, much more important in modeling than in laying flat tones; but it should be practiced from the beginning. A careless painter, to get an even tone, is obliged to put on coat after coat, and to blot out the ground and drawing with a thick mass of paint; while a skillful painter can establish an even tone with less labor and more luminosity by laying the color neatly as he goes. Ay the end, for accents and strong modeling, you may want to apply some drawing strokes with quite thick paint; but the preliminary work is best done thinly. [pp. 99-100]
Basic Principles of Tempera Painting
In the application of the tempered color, it is necessary to understand and apply the optical principles which govern the behavior of colors in tempera. Regardless of the pigments it contains, no well-tempered mixture is absolutely opaque; and, as each mixture ordinarily [p. 100] contains some white, it is not absolutely transparent. Its opacity and transparency are relative, and are conditioned by the values over which the mixture is applied. If it lies over a lighter ground, it appears transparent. Light strikes the paint, passes through it, strikes the lighter ground, and is reflected back to the eye through the layer of paint. Some light has been lost in surface reflection, some has been absorbed in passing through the paint film in each direction, and some has been absorbed by the ground, But as long as the ground is lighter than the paint, it will reflect some light back through it, and produce an effect of transparency. Any color in tempera laid thinly over a lighter ground will act as a glaze.
When the coat of paint lies on a ground of equal value with itself, or on two or three coats of similar color and value superposed, the surface underneath it is no more reflecting than the paint itself. Light passes into the paint layer and is reflected according to its color and value, and the effect produced is of opacity. Any color in tempera can be made to look opaque by putting on repeated coats of it. If, however, the color lies upon a ground darker than itself, the ground, instead of reflecting light, or having no effect upon it, absorbs part of the light that the layer of paint transmits. The color appears darker; and, being composed of a suspension of finely divided particles, it takes on a cloudy, smoky quality which may be called "opalescence." This quality, to which, in nature, the sky owes its color, and mist or smoke against dark objects, their blueness, stands half-way between opacity and transparency. [pp. 100-101]
Control of Form
The basic principle of tempera painting is control, control particularly of this relation between color values and transparency. By developing [p. 101] the design in intermediate tones, intermediate both between the extremes of light and shade, and between the extremes of opacity and transparency, the tempera painter acquires perfect control over the expression of his forms. Keeping in reserve his strongest lights and darks, and establishing first over the whole region of half tone, shadow, and reflected light a set of opalescent tones, tones neither transparent nor opaque, but convertible toward either at his will, the tempera painter is able to govern the modeling of form with as much precision as he wishes. [pp. 101-102]
The Function of the Monochrome Drawing
The whole composition is worked out in monochrome on the gesso surface of the panel, the modeling indicated ordinarily by lines or washes of ink on the pure white of the gesso. By painting over this shading with tones always somewhat lighter than the parts of the shading that they cover, the half tones and shadows and reflected lights in the drawing are automatically converted into opalescent tones of paint. In practice, three values of color over the monochrome drawing usually suffice for this purpose. The ink shading is divisible roughly into dark, middle, and light; and three corresponding divisions of slightly lighter color laid over the drawing will produce a graded opalescent tone, or scumble.
If the ink drawing is executed in washes, this graded scumble will be continuous; for the darker ground runs evenly beneath it. If, however, the graded values of the drawing have been suggested by lines or hatchings, averaging the effect of black lines and white gesso, the effect of the painting will alternate correspondingly between dark [p. 102] lines of opalescence and less dark, transparent spaces of color between them. The value contrast will, of course, be much less than in the drawing. The opalescent effect of the painted tone will in any case be directly proportionate to the depth of value secured in the drawing, whether the drawing be done in wash or in line. Each method has its uses and its adherents. [pp. 102-103]
The Order of Painting
Turning now to the actual painting of the panel, consider where you shall begin. If there is any flesh painting, it is best done last; for it is easier to key a comparatively small area of flesh to a large area of drapery and background than the other way about. With this exception, however, it is advisable to begin with the most distant planes and to work forward. The advantage of this order of work is that the edge of a forward plane can thus be painted over anything that lies behind it. If two areas are simply butted, both painted up to a line, the edge between them can never be made quite so clean and clear as when one overlaps the other slightly. When you are bringing a tone up to an outline, draw the edge with the heel of the brush, not with the point. The heel gives a firmer, truer line; for it can be controlled better than the point. [p. 103]
A Hypothetical Case
As nothing can be more distant than a sky, we may begin the discussion of painting procedure with a hypothetical sky, and assume, for the sake of argument, that what is wanted is an even gradation from a dark blue zenith to a white horizon. This will have been indicated in the ink drawing on the panel by a gradation in line or wash from the black of the ink to the white of the [p. 103] gesso. We shall want a corresponding gradation in transparency, from almost transparent dark blue at the top to an almost opaque white at the bottom. [pp. 103-104]
Let us choose cobalt blue as our basic pigment. If necessary, we may make up a special mixture, turning it a little green with oxide of chromium, or a little violet, say with Indian red; but in this case, we must put aside [p. 104] some of the mixed color as a separate pigment, and keep it until the painting is finished; for it may be needed again. We will then take three color cups, and into the first put a fairly large amount of the cobalt blue [pure or compounded] and into the third, a very small amount. We will add a small amount of white to the first cup, and a large amount to the third. When each of these mixtures is stirred, we shall have one cup containing a dark blue, and one containing a light blue. Let us put some of each of these into the middle color cup, and mix them together to produce an intermediate blue. Each cup should have its own brush, and each mixture must be tempered with egg for use. [pp. 104-105]
Applying the Tones
Now our darkest mixture of blue is not so dark as the ink modeling at the top of the sky; but it is darker than the drawing father down. If, therefore, we start painting at the top of the panel, and work gradually down with this dark blue, passing from the darker part of the under drawing to the lighter, we shall see the change in effect discussed above. Where the color lies upon a ground darker than itself, it appears opalescent; where it matches the value of the ground, it begins to look opaque; and if we went on to the still lighter part of the drawing, it would look transparent. Stopping gradually where the blue matches the value of the ground, let us apply two or three coats of it, thinly and evenly, just enough to make the drawing underneath disappear as a drawing, and tell only as modeling in the blue.
We then take the intermediate mixture of blue, and begin again in the same way from the lower edge of the dark blue portion, painting thinly over the dark blue edge and the whole middle part of the ink modeling of the sky, and losing the middle blue in the lightest part of the ink drawing, down toward the horizon. The [p. 105] drawing again tells just as modeling in the blue. And then we take the lightest of our three mixtures, and in the same way paint thinly over the lower edge of the middle blue, over the last of the lightest part of the ink drawing, and all the way down to the horizon. Then finally we temper some pure white, and paint thinly over the bottom part of this lightest blue until we have made the necessary transition to white at the horizon. [pp. 105-106]
Glazing over the Scumble
Now we may take a little pure blue, adding perhaps still a trace of white, but making it in any case darker than the darkest of the original three mixtures; and, thinning it out with a great deal of water and using the brush almost dry, we may play it over the top part of the sky very thinly, gradually making the tone as deep and transparent as we wish. And in the same way, the other tones may be played thinly over the next tones lighter, each acting as a very delicate glaze. We could easily give the whole sky a uniformly transparent appearance, or, of course, equally easily, a uniform opacity; but what we want is a graded transparency, from the opaque or semi-opaque white of the horizon to the almost transparent dark blue of the zenith. This we can easily secure by breaking a little transparent color over the opalescent tone that we have already established, grading it as we please by simple manipulation of the brush. "Transparent" simply means darker over lighter.
If the modeling of the sky is not perfect, the tones may be applied again, in the same order, and the form corrected. When all this has been done skilfully, we shall have a very even gradation from dark blue to white, with no suggestion of bands of separate colors. And the blue will have a luminosity, appropriate to a sky, which would be hard to secure in a water medium by any other means. We shall have accomplished this by taking advantage of the semi-transparency of the coats of tempera color. [p. 106]
Tempera vs. Gouache and Water Color
This lengthy account of a simple piece of painting may well strike the reader as tedious; but the principle that it is intended to illustrate [p. 106] is fundamental to an understanding of the essential character of tempera technique. Tempera painting is often confused with gouache, on the one hand, and with water color on the other. The basic differences among these media boil down to questions of transparency. Tempera stands midway between transparent water color and opaque gouache, and possesses a flexibility, in consequence, which neither of the others shares. Tempera is not to be thought of as a material: it is a discipline. It is possible to temper pigments with much yolk of egg and little or no white, and to paint water color with them; or, with much white and little egg, to use the same materials for gauche. But tempera painting proper means capitalizing this special character of translucency that sufficient tempering gives to thin coats of pigments in themselves opaque. It means distinguishing the effects of opacity, produced by repeated coats of a single tone, opalescence, produced by painting a lighter tone over a darker, and transparency, produced by painting a darker tone over a lighter.
This wide range of effects is easily, almost automatically controlled, and it gives the tempera painter an instrument of great power and adaptability. If he makes use of its special properties, it will reward the inconvenience to which it puts him. There is no sense in turning a studio upside down to gesso panels, grind colors, and then paint gouache. Good drawing paper stretched on a frame, or a good illustrators' board, and any of the excellent ready-made gouache preparations, will give just as good results. It is only to take advantage of the powers and beauties peculiar to tempera that the painter is justified in giving himself the trouble of practicing it. If he will be content with a graded wash of blue, it is folly for him to look farther than water color; if he will be content to blend his graded blue from dark to light out of opaque mixtures by scrubbing with a brush, his needs can be satisfied by good commercial gouache. But if he wants to control the painting to perfection, to shape the sky as he paints it, to give it some special quality of luminosity, to establish some deliberate relation between it and the rest of his painting, it may be worth his while to grind colors and break eggs instead of buying tubes and bottles in a shop. [p. 107]
Nothing is a harsher test of a tempera painter's skill in his medium than this very problem that we have been discussing, a graded blue. Even competent painters often make their broad, light gradations chalky or streaky, for want of thoughtful handling. The secret is to preserve the opalescent half tone; for it can be turned toward transparent or opaque, and shaped and modeled as easily and subtly as clay under a sculptor's thumb. [pp. 106-108]
Painting Drapery and Other Objects
Exactly the same principle applies to the painting of solid forms, such as drapery. The values are established on the white panel with ink shadings, and three main values of color are mixed, corresponding to light, half tone, and shadow, or half tone, reflected light, and shadow. For the simplest manipulation, these mixtures may be made, as before, simply a color with a little white, the same with more white, and finally with more white still. There should ordinarily be some white even in the darkest of these values, partly to insure its opalescence over the darks of the drawing and partly so that the pure color will be in reserve for final glazing. The darkest value is laid on once or twice, thinly, over the darkest parts of the drawing; then the middle value is laid in; then the lightest. They are blended nicely at their junction lines, the darker being carried over the lighter at the edge, or the lighter over the darker, according to the form. Where these tones are laid, and in what relation to each other, depends, of course, upon the system of light and shade which is being used. [p. 108]
Development of the Forms
In this stage, the form is incompletely rendered. The half tones are finished, but the lights are not light enough, the shadows are not dark enough. The drawing underneath gives some shape to the reflected lights and the shadows; but they are pearly, translucent, [p. 108] unconvincing. To give the shadows the necessary depth and transparency, it is only necessary to make a value slightly darker than the darkest [perhaps pure color, without any whites], and break it thinly, well diluted, and with the brush almost dry, over the opalescent tone which resulted from the first laying in. The color deepens and becomes transparent almost as if by magic. The transition to the pearly half tone may easily be kept; and the form will be found exquisitely plastic, shaping up under the brush with the utmost ease and accuracy. Any degree of precision is possible at this point. The painter may shape the form as delicately or as boldly as he wishes; for it will do exactly what he makes it. It is here that the individual brush strokes begin to tell; and from this point on the description of the forms and the stylishness of the handling depend very largely on the understanding and dexterity with which the brush is handled. [See "Form Drawing" in the Appendix, p. 132.]
The modeling is now complete from half tone to shadow, and it remains only to build up the lights. This is readily done with as many lighter values as the case requires. Each lighter mixture is made to produce as much effect as it will before the next is taken. The final highest lights may be touched in with quite thick paint. In general, the lights should be modeled up, with rising values; for so consistent is the tendency of tempera to exhibit its half-transparent nature that even in the lightest values a thin painting of a tone over a lighter tone will give a glaze effect. In practice, one often models the lights up, and down, and then up again, to get them strong and solid. At the very end, you may accent with extremes of [p. 109] light and dark. If a painting is pitched to the key in which tempera is most effective, there will be some part, large or small, somewhere in it, painted with pure white. It may or may not go down to pure black at the other end of the value scale; but it is almost axiomatic that the lightest stroke in the painting should be made with pure white paint. [pp. 108-110]
[Thompson, Daniel V., Jr., Research and Technical Adviser, The Courtauld Institute of Art, University of London. The Practice of Tempera Painting. New Haven: Yale University Press. 1936. Fourth Printing, 1946.]
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