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Written by Jay Jensen

One of the joys of painting with oils is the ability to create optical mixtures. An optical mixture is achieved by layering a color on top of a different color, or by dabbing small swatches of different colors side by side. In either case, the colors that you have applied retain some of their identity and, at the same time, are blended together by the eye to form a new color. The effect is invariably more vibrant and luminous than if you were to try to make the same mixture on the palette, and, after all, isn’t capturing the luminosity of our world the real work of a painter?

Anyone who has experimented with paint knows that mixing red and blue on the palette will make purple, red and yellow orange, blue and yellow green. These are examples of physical mixtures; no doubt that every artist needs to develop this skill in order to progress as a painter. Problems arise when the artist desires to create more complex, nuanced colors, such as for shadow areas and flesh tones. Mixing these colors physically frequently results in dull, lifeless passages on the canvas. In other words, colors that lack that essential ingredient: luminosity.

An optical mixture allows the artist to create a complex color without losing luminosity. Optical mixtures glow, optical mixtures vibrate, and they add depth to your work. We stand in awe before the work of Rembrandt and Vermeer, to name just two glorious examples from the history of art, in part for their mastery of creating optical mixtures. I would argue that Mark Rothko, an abstract painter from the modern period, was no less preoccupied.

There are three basic methods of creating optical mixtures: juxtaposition, scumbling, and glazing. Below is a brief description of each technique. Any good book on oil painting technique will contain more detailed information. Of course, feel free to experiment. Be open and alive to the “accident” that gives life and energy to your work. Also note that all three techniques may be employed in one painting. All of these effects can be achieved using acrylics, too.

The last quarter of the 19th century saw the emergence of the alla prima technique as the method of choice for applying paint to canvas. It is still true today. From that period, the work of the Impressionists is an example of how subtle, luminous color can be achieved by the juxtaposition of tiny blobs of relatively pure color. From a distance, the eye cannot see the individual blobs and, instead, mixes them together in your brain. The paintings of Claude Monet, perhaps the quintessential Impressionist work, literally hum because every square inch of the canvas has been deftly modulated by the juxtaposition of myriad dabs of relatively pure color.

Scumbling is the technique of brushing a lighter, more opaque color over a darker, more transparent one. I find this technique particularly useful when I am painting a sky; it is a great way of recreating the way clouds partially dissolve and emerge from the sea of blue. Scumbling allows you to easily control the shape and the intensity of the cloud form. Of course, this is just one example from my personal experience as a landscape painter, but there many others for this versatile technique. Experimentation will surely open many new possibilities for different applications.

Scumbling can be done when the underlying coat of paint is dry or still tacky. The consistency of the paint you are scumbling with is best when it is fairly stiff. Round, flat or fan brushes may be used to apply the scumble.

Glazing is the technique of the Old Masters. Before the invention of acrylic paint, glazing was the technique that distinguished the oil painting medium from the other two major painting mediums in Europe: egg tempera and fresco. Glazing is less popular today because the technique requires that you paint in steps and, consequently, is more time consuming than alla prima. Personally, I like building my paintings up; it gives the work a greater sense of depth, a variety of surface, and, you guessed it, luminosity.

The technique of glazing is the opposite of scumbling: a transparent coat of extended pigment is brushed over a more opaque underpainting. It is an ideal method of creating luminous shadows; for the artist can exaggerate the value and the hue of the undercoat and then tone down and darken the shadow with a glaze or multiple glazes.

If you have ever shopped for oil paint, you have surely noticed that pigments are classified by their transparent or opaque qualities. The cadmium colors are naturally opaque and are therefore unsuitable for glazing, as are titanium white and cerulean blue. Colors such as ultramarine blue, Prussian blue, alizarin crimson, Payne’s Grey, and raw sienna are naturally transparent. Their transparency can be extended by adding a glazing medium – several recipes exist, as well as some ready-made products – so that you can control the value, hue and intensity of the glaze.

For those of you who are concerned with posterity, glazing is a technically sound technique because it respects the basic principal of oil painting of fat over lean. The underpainting, especially, when it is mixed with titanium white, or diluted with turpentine, is a lean base, which means it is relatively oiless and therefore more stable. Since transparent colors are naturally fatter, i.e., oilier, plus the fact that most recipes for glazing mediums contain linseed oil as an ingredient, the glaze is fatter than the base and consequently, more flexible. The layering of the two in this order allows the surface to resist cracking and flaking over time because the glaze can contract and expand over the more stable, leaner base.

As mentioned earlier, more than one recipe exists for making a glazing medium. Here is one which you can mix yourself and that dries relatively quickly: 1 oz. Gum turpentine, 1 oz. Dammar varnish, 1 oz. Stand oil, 15 drops cobalt drier. Of the ready-made products, Liquin is an alkyd gel that dries usually within a couple of hours, a possible compromise for the impatient alla prima painter!

Regardless of whether you work figuratively or as an abstractionist, learning to create optical mixtures is a way of taking your paintings to another level of technical and compositional sophistication. That’s not to say that the alla prima technique is inferior to glazing; most of the great paintings of the last century are all examples of alla prima. But alla prima is just one oil painting technique. Acquiring some mastery of these older techniques could add some dimension to your work, and that is always a worthwhile pursuit.

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