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However, the effect of the refractive index depends on the pigment context. Specifically, pigment transparency depends on the ratio between the refractive indices of the pigment and the medium around it. The surface reflection or scattering of light greatly increases at the boundary between two substances with very different refractive indices. For example, both water and air appear transparent in themselves, but the surface of a lake appears reflective, and bubbles in water appear silvery, because the ratio between the refractive indices of air and water is fairly large (1.0 for air and 1.33 for water, a ratio difference of 33%). The powdered, dry watercolor pigments available in jars, even when scattered as a thin layer, appear as completely opaque powders when air is the surrounding medium (right).
Surface scattering also increases if this boundary is a rough rather than smooth surface: smooth window glass is transparent, but ground glass or sandblasted glass is opaque. Watercolor pigments are both individual particles and aggregates or agglomerations (irregular clumps) of particles, which have complex and microscopically "rough" surfaces.
Finally, surface scattering increases if we multiply the number of surfaces light must pass through. This happens as we make the particles smaller and present them as a mass. Thus, large crystals of sugar appear transparent, but powdered sugar appears opaque; we can see through large drops of rain, but tiny droplets of fog obscure the view; ice is transparent but snow is opaque; beer in the glass is transparent, but beer in the tiny bubbles of the foam is opaque, and so on. So the "transparency" of a large chunk of material doesn't tell us much about its behavior in tiny particles, such as pigment particles.
Light that is not scattered at the particle surface can be either absorbed or reflected. Pigments that on balance absorb more light than they reflect appear dark valued; pigments that efficiently absorb only some wavelengths of light within a narrow spectral band and reflect the rest will have a specific hue that is intense or saturated.
three ways that light interacts with pigment particles
(a) all light wavelengths are scattered at the pigment particle surface, producing white reflectance; (b) light wavelengths are selectively absorbed by the pigment, and the rest are reflected along with some scattered (white) light; (c) selective absorption is enhanced and scattering is reduced by a surrounding medium (brown) with a similar refractive index. (Colors symbolize different light wavelengths; light itself does not have color.)
The color we see is actually a mixture of colored and scattered light. At the rough surface of a pigment particle or pigment aggregate, none of the scattered light has been absorbed by the pigment to create color (figure a above). If white light strikes the pigment, white light is scattered back, and this white reflectance desaturates or neutralizes the color that results from any light that has actually entered the pigment particle and been selectively absorbed and reflected by the pigment (figure b above).
The Function of a Paint Layer. One of the principal functions of a paint vehicle is to increase the proportion of reflected "colored" light over scattered "white" light (figure c above). The vehicles used in modern paint media all have very similar refractive indices — 1.47 for gum arabic, 1.49 for linseed oil or polymerized acrylic resin. These are close to the refractive indices of some pigments and almost the same as the refractive indices of laking substrates used to make dyes into pigments. (For example, the RI ratio difference between linseed oil and aluminum hydroxide is only 4%, and between linseed oil and ultramarine blue is only 9%.) These small refractive differences greatly reduce the proportion of scattered white light at the vehicle/pigment boundary. They also increase the proportion of partially absorbed (colored) light, creating both a darker and a more intense (higher chroma) color. Indeed, this is why almost any light absorbing surface, such as concrete, wood or fabric, appears darker and more intensely colored when it is wet.
So paint vehicles can reduce substantially the light scattering at pigment particle surfaces, allowing more light to be selectively absorbed by the pigment — if they replace air as the boundary medium. We need to look carefully at the microscopic relationship between the air, dried vehicle, pigment particles, and support (the canvas or paper) to determine whether this occurs.
Oil and acrylic paints are usually applied to a nonabsorbent ground, so all the pigment and vehicle stays on the surface; they harden primarily through a chemical change with a relatively small loss of ingredients through evaporation, so the surface volume of paint remains roughly the same. As a result, the pigment particles remain completely surrounded by hardened vehicle, which reduces the scattering of light and creates a relatively darker, more saturated color. Much of the light scattering takes place at the single boundary between the air and the smooth paint skin, not at each of the millions of pigment particle surfaces. And the thickness of the paint layer ensures that most pigment particles are surrounded by other pigment particles, which increases the chance that light scattered by one particle will be selectively absorbed by another before it is reflected back to the viewer. These paints have a typically richer color appearance.
Watercolors Don't Form a Paint Layer. Watercolor paints are also intensely colored when wet, because the wet vehicle completely surrounds the pigment particles and reduces the surface scattering of light. But when watercolors dry, all of the water in the vehicle evaporates, signficantly reducing its total volume. As this happens a substantial amount of the dissolved gum arabic, glycerin, dextrin and glucose in the paint vehicle, and the dissolved surface sizing of the paper, is drawn by capillary action into the tiny spaces between cellulose fibers, where it hardens and dries.
At the same time, the individual cellulose fibers, which were compacted and flattened during manufacture, become somewhat untangled when wet: the paper sizing dissolves and the fibers soften and expand as they absorb water. As they dry, the fibers do not return to their original positions but form a deep, tangled mat. (The mechanism is much like what happens to a smoothly woven wool sock you've put through the wash — it comes out much thicker and fuzzier!) As a result, the painted paper surface viewed under a microscope looks strikingly like an extremely dirty, deep pile white carpet, combining the textures of cellulose fibers, pigment particles and the embedded deposits of dried vehicle and tub sizing. (Many pigment particles, along with the brightener or filler particles, have also settled into these "gum sinks.")
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 raw watercolor pigments
are not transparent |
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the microscopic reality of watercolor paint on paper
before paint is applied: the relatively flat surface of new watercolor paper consists of compacted cellulose fibers under a thin coating of gelatin tub sizing; after paint is applied: loosened and expanded cellulose fibers are strewn with pigment particles, while the gum arabic vehicle and gelatin sizing have drained into paper crevices.
The final situation is very different from an oil painting: the largest pigment particles are left "high and dry" clinging to individual cellulose fibers, and many of the smallest particles sink deep into the cellulose crevices where they are hidden from light (image at right). There is no uniform nonabsorbent ground, so much of the vehicle drains away into the paper. As a result, there is no uniform paint layer and no paint skin: the pigment particles are strewn on top of, between and underneath the cellulose tufts (image at right). Light scattered by one particle has a smaller chance of striking and being "colored" (selectively absorbed) by another particle before it is reflected back to the eye — the cellulose gets in the way.
Without a thick surrounding vehicle and smooth paint skin, light scattering greatly increases at the exposed pigment particle surfaces. This is why watercolor paints appear to whiten or fade as they dry, visible proof that "white" light surface scattering has increased and the pigment particles have become more opaque. (Lack of a surrounding vehicle is also why many pigments are less lightfast in watercolors than in oils or acrylics.)
Thus, there is no paint layer to represent the "stained glass" pane that light must pass through two times. The Victorian painters falsely reasoned that watercolor paints must behave in the same way as oil paints, when in fact watercolors create a completely different pigment environment.
Transparency Occurs Between Particles. So why does cerulean blue, with a refractive index of 1.8, appear much more opaque than the quinacridones, with a refractive index of 2.0? Because there is a sparser coating of smaller pigment particles in the more transparent paint. Pigments in a smaller particle size hide less of the paper (or paint) underneath, making the color appear more transparent. Transparency happens between pigment particles and not through them. Common examples are the "transparent" appearance of dust on a table, or a scribble of graphite pencil on a printed page. Even with a fairly heavy application, some of the text is still visible underneath, though tiny carbon pigments absorb almost all the light that strikes them!
The reason there is a sparser coating is that the color intensity of "transparent" pigment particles is usually very high: less pigment is sufficient to provide the surface color. This is probably the main reason for what appears to be watercolor paint transparency. The individual pigment particles in most opaque pigments — venetian red, chromium oxide green, cadmium red or cerulean blue — have a relatively dull color, so a greater bulk of pigment must be slathered on the paper surface to get a characteristic color appearance. These paint colors are usually formulated with a much higher concentration of pigment. Thus, chinese white appears relatively opaque, while quinacridone red appears transparent, even though both have the same refractive index (about 2.0) and the same particle size (around 0.1µm) and aggregate size. Chinese white (zinc oxide) is formulated with over three times the volume of pigment to gum vehicle as the quinacridones, because the quinacridones have a very high chroma and good tinting strength. As a result, a much smaller quantity of quinacridone pigment is applied to the same surface area of paper ... just like making a much lighter scribble with your graphite pencil.
 techniques for glowing color |
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 Why is it useful to dispel these longstanding myths? Because a factual understanding of your painting materials ultimately gives you greater artistic skill. Believing in a nonexistent dimension of color, or a false mechanism of "luminosity" or transparency, hides from you the many ways that color intensity (chroma) can be controlled.
Of course, colors change their apparent brightness, transparency or hue depending on the context in which they appear. This is really a color design rather than a color mixing problem, and is discussed elsewhere.
Context aside, control depends primarily on the chroma of the pigment itself and how much the paint is diluted with water when applied to the paper. The next sections of this page explore that single issue in great detail, but it is not the only factor under your control.
The average pigment particle size and the particle size distribution can also affect finished color. In many watercolor paints, the smaller particles of a pigment tend to be less saturated and lighter valued than the larger particles. These duller, paler and smaller particles also remain in liquid suspension longer than the more intense, darker and heavier particles, which sink first to the paper surface and into the paper crevices. The lighter particles form a thin sediment on top, obscuring and scattering the brighter color underneath.
You control these paint attributes through your choice of watercolor brand, and your choice should be made after carefully testing your watercolors for tinting strength and particle size. I find a simple yet revealing test is to mix moderately diluted solutions of different paints (roughly 1 part paint to 5 parts water), pour each paint into a shallow mixing pan or palette, and let the solution completely evaporate. Variations in pigment color (especially those caused by added fillers or brighteners) will become quite visible. By gently wiping away the upper layers with a moist paper towel, you can also compare the color of the smallest and largest particles in the paint. By testing in this way you will reliably identify the paint with the richest and most consistent color.
The simple, very effective but rarely mentioned way to achieve bright, pristine color is to apply the paint in one brushstroke and let it dry. Repeatedly brushing the wet paint seems so necessary or irresistible in watercolors — to smooth out irregularities in the paint texture, or combat blossoms or backruns, to adjust a color that has gone wrong, or to darken a color that has lightened too much. Yet repeated brushing of wet paper increases the unraveling of cellulose fibers at the paper surface, which increases the fuzzy canopy of microscopic fibers over the pigment particles and forces a larger number of pigment particles into the paper crevices. Both will dull the color.
What confuses many painters is that the effects of repeated brushing depend on several attributes of the paper, not the paint! These include:
• the wetness of the paper — if the paper is still damp or wet when rebrushed or repainted, the surface sizing and some of the internal sizing is still dissolved, which loosens the mat of cellulose fibers (making them more sensitive to brush abrasion) and increases the capillary action that pulls pigment particles deeper into the paper crevices
• the pulp content of the paper — papers made with cotton linters have much shorter cellulose fibers than papers made with cotton or linen cloth or raw fiber, and these shorter fibers are more likely to loosen and fuzz up when repeatedly brushed
• the surface or finish of the paper — hot pressed papers are more heavily compacted and tub sized, while rough papers are loosely compacted and generally have less sizing
• the amount of tub sizing — whatever the surface finish, a thicker layer of sizing binds the surface fibers longer, and fills in many of the deeper surface crevices, holding more of the paint on the paper surface.
All these attributes vary across different brands of paper, so you may have to experiment with different papers and paper finishes to find the surfaces that work best for you. It may be useful to use a sheet of Yupo paper as a "control" or standard of comparison. The Yupo is actually a sheet of satin textured plastic, with no surface sizing and no crevices for the paint to sink into.
To evaluate the color effects due to your paper, simply make up your usual solution of a few different paints — a dark paint (ultramarine blue, PB29), an intense paint (cadmium scarlet, PR108), and a dull paint (chromium oxide green, PG17) — and paint them two different ways: as a single stroke of paint that is left alone to dry, and as a single stroke of paint you repeatedly stroke or blend with a clean moist brush until it dries. This "fussing" with the paint will produce a noticeably duller finished color.
You can also compare the single stroke with a stroke that you add water to as it dries (deepening the wetting and prolonging the capillary action of the paper), or compare a stroke made on completely dry paper and a stroke made on paper that has been thoroughly soaked and is still quite damp. By comparing these different paint application methods across different brands and surface finishes of paper, you will develop a clear understanding of the impact of paper and brush techniques on the finished color.
Whatever the cause, many problems with highly scattering paint or roughened paper surface can be remedied with one or more coatings of gum arabic: either applied as a foundation treatment before painting begins, or on top of the finished color as a final layer of one or more glazes. The foundation coating seals the paper surface to keep the pigment from sinking into the paper; the finish coating reduces surface scattering.
You should experiment to find the best dilution, but I find that most liquid preparations sold by art retailers (from manufacturers such as Daniel Smith, DaVinci or Winsor & Newton), diluted to approximately 1 part gum arabic solution to 2 parts distilled water, will give an easily brushable coating that darkens and intensifies the apparent color (reduces white light scattering) without creating a disagreeably glossy surface.
Usually two or three coatings of gum arabic are required to produce a satisfactory result, but this allows you to vary the amount of gum arabic you apply over different parts of the painting. Be careful, however, as the gum arabic darkens the color by reducing the surface scattering — this means increasing the gloss — so unless gloss is what you want, aim for a "matte" finish that is dark but not shiny. This means the benefit depends on how much of the problem is caused by surface scattering: a gum glaze does wonderful things for a dull passage of carbon black pigment, but has a much less dramatic effect on many synthetic organic pigments. As with any glazing application, be sure the outside and inside of the paper, as well as the surface layers of paint, have completely dried before you add a new layer of gum wash.
The English Victorian watercolorists innovated the trick of glazing the paper with one or more coats of zinc oxide (Chinese white) to seal the paper and provide a more reflective coating for the painting. Titanium white will serve just as well. This works best on a rough rather than a hot pressed finish, as the white paint has a tendency to lift if brushed too aggressively with subsequent paint layers.
| not black, not light |
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Jim Kosvanec suggests a simple rule to obtain the "maximum luminosity" (he apparently means maximum chroma) in watercolors. His rule: use paint mixtures that are diluted just enough to "bead up" like water on the palette.
Unfortunately this rule is wrong for many paints, and doesn't really work with large puddles of mixture or well used (scratched up) plastic palette surfaces. But focusing on mixture viscosity (dilution) is the right answer.
You're forced to use the viscosity of the paint mixture as a guide to the right mixture of paint and water because you must choose the right paint mixture before you apply the paint. But you are actually aiming for a specific goal in the dried paint appearance ...
"not black, not light"
... which means that the most intense finished color always appears as a happy medium between two extremes.
If the paint is too thick, the color will appear blackened, darkened or bronzed (bronzing makes the surface look shiny, leathery, dull or blotchy). The paint will also be harder to apply smoothly, and may appear streaked or uneven.
If the paint is too thin, the color will be lightened by the white paper showing through, and this lightening lowers the color chroma.
By avoiding both extremes, you'll find the perfect mixture for each type of paint. The examples below show what I mean.
"not black, not light" rule applied to four pigments
paints shown (top to bottom): yellow ochre (PY42), quinacridone maroon (PR206), green gold (PY129), gold ochre (PY43)
The optimal mixture in each row (as measured by a spectrophotometer, and confirmed by my eye) is the one in the middle. Notice how the two mixtures on the left show some darkening or blackening of the color, and at the extreme left show considerable darkening or graying, with patches of bronzing. The two mixtures on the right are "light," with the whiteness of the paper punching through. The middle mixture avoids both, and is the most intense color possible with that pigment in watercolors.
Now an important announcement: optimal paint dilution is different for different pigments and different brands of paint. Paint brands differ in the quality of pigment they use, in the amount of pigment they pack into the paint, in the thickness of the vehicle, and in the use of additives or fillers. The methods I describe will help you find the optimal dilution for any paint brand, but if you switch brands, you may find your previous rules don't apply. This is why I specify paint brands for all results reported here.
| exploring dilution |
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You want to hit a visual goal, but you have to work with the paints while they are wet. This means you have to learn the signs of mixture viscosity or dilution, and use these to guide your paint mixing.
The most convenient way to do this is to start with raw paint, add water to it in measured steps, paint a color sample at each step, and observe how the dried color changes. The method of measured steps is important, because you want to learn the specific ratio of paint to water that produces the best results.
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 microscope image of an
ultramarine blue wash on
cold pressed watercolor paper |
| Materials. You will need the following materials: (1) a large flat mixing surface, such as a flat plastic palette, an enameled butcher's tray or large porcelain dinner plate, (2) a durable, high quality watercolor paper notebook, the spiral bound kind with high quality, heavy weight paper (see examples here and here), (3) a #6 or #8 kolinsky or synthetic round brush or (better for the job) a 1/2" kolinsky or synthetic flat brush, and (4) a set of metal kitchen measuring spoons.
You will never regret buying and using a watercolor notebook for this task! It preserves your results for future reference, and it provides you a place to paint the color swatches and to write your comments about the mixing process and the sensual qualities of the wet color mixture.
Use a flat, white mixing palette if you can, as you want to see the wet color appearance and the thickness or viscosity of the paint mixtures on a flat surface in the same way you normally see the paints you mix for painting.
Finally, test the accuracy of the spoons by measuring 1/2 teaspoon of water 6 times into a plastic or wax cup: this should exactly equal 1 tablespoon and 3 teaspoons. If not, get a better set.
Procedure. Now choose any dark, intense tube paint in your paint box (such as cadmium red, quinacridone violet, ultramarine blue, cobalt blue, or phthalo green), and follow these steps to learn the differences in mixture concentration:
• Raw paint. Squeeze out enough raw paint directly from the tube to fill the 1/2 teaspoon measuring spoon; make sure the tube is completely filled, then level off any excess paint with the paint tube nozzle or a palette knife. (I know this seems like a lot of paint at first, but you want enough surplus to make all your test swatches ... and you have to let go of stingy feelings about your paints if you want to get rich color.) Scrape as much of the paint as you can out of the spoon and onto your flat mixing surface, using only a premoistened (damp) #6 or #8 round brush. Then, with the paint left stuck to the brush, gently paint a 1" square patch of color at the top left of a blank page of your watercolor notebook; try to brush the paint to a thin, even coat. Label this patch "raw 1:0."
• Syrupy concentration. Fill the 1/2 teaspoon with water and add the water to the pile of raw paint. First, use the clear water and your brush to clean as much as you can of the remaining raw paint out of the mixing spoon, at the same time dissolving the raw paint embedded in the brush tuft. Then completely dissolve the paint on the palette by gently stroking and daubing it against the mixing surface with brush. This will take a few minutes, but don't rush. You will have to repeatedly scrape off raw paint clinging the brush on the side of the palette, push this paint back into the water, and mix with the brush. When you're done, there will be no clumps of raw paint on the brush tuft or ferrule or on the mixing surface, and the paint mixture will have a thick, even consistency like corn syrup or molasses. If daubed on a bare area of palette, the mark will retain its shape rather than contract into a round puddle; if you drag your brush through the mixture, you will create a furrow that only very slowly closes; and no liquid falls from the brush when you lift it. With this mixture paint a second 1" square patch of color next to the first, and label it "syrupy 1:1" — 1 part paint to 1 part water. Add any observations you want about the mixture quality or paint texture.
• Creamy concentration. Add another 1 teaspoon of pure water, which is double the amount of water you just used to dissolve the paint. Stir with the brush until the mixture is completely smooth, and check for any undissolved lumps of paint lurking in puddle. The mixture should now have a viscous but loose consistency like cream or olive oil. If daubed on a bare area of palette, the mark will slowly and partially contract its shape, but will not make a round puddle; if you drag your brush through the mixture, you will just see bare palette underneath, and paint will close over the trace in a few seconds; no drops fall from a saturated brush when you lift it. Paint a third 1" square patch with this mixture, and label it "creamy 1:3".
• Fluid concentration. Add 3 more 1/2 teaspoons (a total of 1-1/2 teaspoons) of pure water to the mixture, and stir well. The mixture almost has the fluid consistency of pure water, but it does not yet behave exactly like water — you have only loosened up the creamy or oily texture. If this mixture is daubed on a bare area of the palette, the mark will slowly contract into a round puddle; if you drag your brush through the mixture, paint will quickly close in over the trace; ripples dance over the surface as you stir it; the reflection of a beaded or curved edge will appear around the sides of the puddle; drops will fall from a saturated brush when you lift it. Paint another 1" square sample with this mixture, and label it "fluid 1:6".
• Watery concentration. Add another 1-1/2 teaspoons of water, mix well, and paint a 1" patch of color. Observe this mixture carefully, and decide whether (1) it has thinned so much that it has the same consistency as pure water, and (2) you can clearly see the white of the paper showing through the painted color swatch once it has dried. If so, label it "watery 1:9"; if not, label it "fluid, 1:9". Then add another 1-1/2 teaspoons of water, mix again and paint another test patch, which should now behave like pure water and show definite paper whiteness through the color patch. Label this patch "watery 1:12".
• More watery concentration. Continue by adding 2 tablespoons of water to the mixture to produce sample dilutions at 1:24, 1:36, 1:48 and 1:60, each time mixing thoroughly and then painting a 1" patch of color. These mixtures also have the consistency of pure water, but you will discover that the pigment texture and paint color change in subtle and interesting ways. Label these patches "watery" with the dilution you used for each one.
You now have a series of 10 color patches, arranged left to right in decreasing concentrations of paint, similar to the examples shown above. Look at all the samples under good lighting, both straight on and from the side, and identify (1) the last patch to show any blackness or dullness, and (2) the first patch where the white of the paper begins to show through. The optimal color dilution is usually either one of these two dilutions or a dilution in between. This optimal mixture will appear to be a clean, radiant color, the most intense and characteristic color for that pigment, without any streaking, blotching, discoloration, fading or weakness.
This assumes you applied the color evenly and without retouching as it dried. Remember, how you brush out the color will affect its appearance. Always apply the paint evenly, quickly and without fussing. Don't charge the brush with so much paint that the test swatches are puddles of mixture. Don't apply a second layer of paint to a swatch, or wick more color into a wet swatch with the brush, or brush the swatch after it starts to dry. Paint all swatches in exactly the same way.
I urge you repeat this exercise for other paints in your palette, both different types of pigment (organic and inorganic, dark and light, sedimentary and transparent) and different hues, and record the results in your watercolor notebook, to see how much color variety each type of pigment can create, to find the optimal dilution for each type of pigment, and to familiarize yourself with the sensual qualities of the paint mixtures that signal the different proportions of paint and water. Your goal is to learn to recognize the proportions of water in paints just by observing how the mixture stirs, flows, and clings to the brush.
When you're finished, you will have your test samples and mixing recipes in the notebook where you can refer to them as needed. You will not want to do these tests over again, and you still have several blank pages if you decide to repeat your observations on new paint colors or new brands of paint.
| the range of paint appearance |
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Let's now look at the test swatches and the variations in the finished paint appearance. The swatches pictured below, made with a mixture of ultramarine blue and burnt sienna, illustrate the qualities of paint at each concentration.
To preview the main conclusion: paints are most saturated at a creamy to almost fluid concentration. When the concentration is thicker than creamy, almost all paints exhibit a dull or blackish bronzing. When the concentration is fluid or watery, the white of the paper shows through.
paint qualities at different concentrations of mixture
The raw paint patch has a flat, lifeless appearance. The textures of both paint and paper are entirely obliterated, and brushstrokes have left streaking or botches in the matte surface. However the tonal value is as dark as possible, and for small areas of dark values, or for drybrush textures (prepared by flicking a damp brush across a pile of raw pigment), this concentration is perfectly usable.
Paint at syrupy concentration dries to essentially the same pigment concentration as raw paint, with a slightly lighter value. It is not much easier to apply with a brush. The paint flows thickly and reluctantly, and is awkward to spread in an even layer. The main problem is that paint at this concentration will almost always bronze (acquire a sticky, leathery or shiny surface texture) in areas where the paint puddles slightly — as it has done in the lower left corner of the illustration swatch. In fact, bronzing is usually more obvious and uncontrollable at this concentration than it is with raw paint! You may also notice that paint at this consistency takes a long time to dry completely, sometimes up to an hour. And the color quality is also often poor — cadmiums in particular appear grayish and dull, and phthalocyanines, quinacridones or earth pigments turn blackish. Only some of the unsaturated cobalt pigments look bright at this density. This is a good paint consistency for drybrush or other textural effects that do not require pure color as much as dark or strong color, but for most painting needs more water is desirable.
The best ratio of paint to water is often the creamy concentration (usually around 1:3 to 1:5 for many types of pigment and brands of paint). This is the point where many paints reach their maximum chroma — greatest brilliance of color — and characteristic color value (dark for blues, light for yellows). The paint flows from the brush evenly and easily enough to be completely workable. This mixture will not bronze when it dries, even in areas where it has puddled, and although the color is very dark it is also luminous and pleasing. Textural variations caused by pigment and paper just begin to appear. The color has a quality of "not black, not light." There are no spots of blackened, grayed or darkened color caused by overthick application, but there is also no apparent lightness from the paper showing through the paint.
For many dark or intensely tinting paints with a small particle size, such as the phthalocyanines, transparent iron oxides, metal azomethines and dioxazine violet, the creamy concentration will still be too thick. These paints look best at a fluid concentration (around 1:6 to 1:8 for most brands of paint). For the mixture of ultramarine and burnt sienna, however, we can see signs that the mixture has been diluted beyond the optimal point. The paint is showing obvious textural variations — granulation, flocculation — and the color has noticeably lightened. Because the white paper is starting to add to the total reflected light, the color is effectively mixed with some white light, lowering its chroma.
At a watery concentration (usually 1:10 up to 1:60 and higher) all paints, regardless of pigment type, are less saturated than a richer mixture. This is the mixture density most often used for washes, foundation tints, and the like; it is also the mixture density most often used by novice painters who want to "save paint" or "make colors transparent".
These paint:water ratios are suggestions only. The mixing ratios and liquid texture depend on each brand's paint formulation. Some paints have a clayey consistency out of the tube, while others are syrupy or runny; some are made with a lot of gum arabic, others are loosened up with water; some contain honey, others do not. And even if the vehicle were exactly the same for all paints, pigment quality and particle size affects the optimal dilution — higher quality paints can tolerate more water. You must learn these variations for your preferred brand of paint, by actually working through the dilution steps described above, and noting the paint:water ratios that give you the best results for each pigment. Look at the finished color with your eyes, and learn the best way to handle the paints you use.
Weaker paint mixtures are highly desirable. They shift the paint's expressive potential away from a strong color statement toward a strong textural statement. A whole new range of flocculation and granulation effects begins to appear, and these liquid and pigment textures are unique to watercolors and visually pleasing. Because of random pooling and water flow, the paint can also produce a much greater variety of backruns and color gradations. (In fact, the paint must be applied carefully to achieve an evenly textured area.)
Many types of pigment combinations — phthalos with quinacridones, quinacridones with cobalts, cadmiums with earth colors — tend to separate more quickly and more visibly in these weak washes, producing beautiful mottled variations of contrasting color. (In the examples above, you can see separation of the brown burnt sienna and blue ultramarine in the two lefthand bottom samples.) Use each dilution to best effect: bright and flat color from creamy concentrations, subdued but textured color in watery concentrations.
Notice how — just by adding more water — the color quality in these four dilute swatches changes from the colors of a stormy sky to the pearly grays of dawn. Always explore the full range of a paint's color qualities, not just the color in heavy mixtures.
| dilution and lightness |
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It's worthwhile to illustrate here the relationship of paint lightness to the ratio of paint to water in a paint solution.
quantity of water required to lighten watercolor paints
paints shown: phthalocyanine green (PG7), phthalocyanine blue (PG15), quinacridone maroon (PR206), cadmium orange (PO20), cobalt teal blue (PG50)
This chart (and all those that follow) shows the relative dilution of a paint in terms of its measured lightness on the value scale at the bottom, which ranges from pure black (a value darker than any paint can achieve) at left, to a value equivalent to white (blank watercolor paper, or white paint) at right, in half steps of an 11 step value scale.
Different paints start out at different values: phthalocyanine blue is very dark, while cobalt teal blue is rather light. This means that added water has a greater effect on darker (more concentrated) paints.
Starting at the syrupy concentration (1:1, or 1 part paint to 1 part water), and adding 9 parts water (to make a 1:10 or fluid mixture) raises the value of phthalocyanine green (PG7) about 2.5 value steps. For a lighter valued paint, such as cadmium orange (PO20), the same 1:10 dilution raises the value by only 0.5 step. Other paints fall in between.
Both the phthalos and cadmiums have relatively high tinting strength. The cobalt teal blue (PG50) has lower tinting strength, so the 1:10 dilution raises its lightness by more than 1.5 steps, even though it is a much lighter paint than the phthalos.
At around a dilution of 1:200 (which takes all steps to a value of 8 or above), the dilution curves for most paints seem to get steeper, which again depends on their tinting strength. Below this point, however, there is a roughly a straight line relation between value and added water.
A few convenient guidelines come out of these dilution explorations:
(1) If you start with a paint at syrupy concentration (1:1), diluting it to 1:10 with water will decrease the value range of the highest tinting paints by about 25%. (To understand value range and how to convert it to value steps, see this page.)
(2) For most paints diluted above a value step of about 8, the effect of added water is determined primarily by the paint's tinting strength and opacity.
(3) The dried color appearance and wet handling characteristics of paints are more sensitive to added water at higher paint concentrations and at darker values.
If you're used to working with paints in watery mixtures, you will find the thicker mixtures involve some stumbling around before you get your bearings. They will be unfamiliar. This is the side of paint mixing that artists require practice to master. When they do, their paintings achieve a new level of transparency and brilliance.
| mixing shortcuts |
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This last section discusses some aids for mixing everyday in the studio and in the field, and addresses some alternative paint mixing methods.
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| A Mixing Card. It is valuable to keep your mixing notes and paint swatches in a notebook for future reference. I actually went through my customary selection of paints, worked out the best dilution ratios for each paint, then made a small mixing card with a color swatch for each paint and the optimal mixing ratio noted next to it. I also added the paint's value and chroma at this optimal mixture, as indicated on the artist's value wheel and artist's color wheel, to assist in color design. This card reminds me of the optimal paint to water mixture, and shows me what that mixture looks like — especially useful when mixing with pan colors, since I can't "squeeze a bead" from a dry pan.
Mixing Recipes for Chroma. In the studio, I sometimes measure water with an eyedropper or (for larger quantities) a set of kitchen measuring spoons. In the field, I either use a brush as a measuring tool, or use a paint well as a measuring cup. I typically use an Eldajon palette or Pike palette in the studio; each mixing well in the Eldajon, filled to 1/4" from the slanted front edge, contains roughly 46 drops of water — about 6 of my measuring units for a 1/2" bead of paint. So if I squeeze a 1/2" bead (which I already know is equal to about 8 drops of water) in the well and fill it to the standard level, I've added roughly 38 drops of water, making an 8:38 or approximately a 1:5 mixture of paint and water.
This is a little more water than desirable for many paints, so I squeeze a little more paint in the paint well to make a more viscous mixture around 1:4. I squeeze out less paint for a fluid mixture closer to 1:6 or 1:8. You can work out similar recipes for the specific brands of paint, tube sizes and palette styles that you use.
Where did I get these recipes? I wrote them down. The following table shows my results for the optimal mixing ratios in some paints I use frequently:
| optimal paint dilutions |
| pigment name | C.I. name | mfg* | paint:water |
| dilute less | |
| quinacridone violet | PV19 | MG | 1 : 3.0 |
| naphthol red | PR112 | MG | 1 : 3.5 |
| nickel dioxine yellow | PY153 | DS | 1 : 4.0 |
| cadmium scarlet | PR108 | WN | 1 : 4.0 |
| cadmium red medium | PR108 | Hb | 1 : 4.0 |
| cobalt teal blue | PG50 | DS | 1 : 4.0 |
| burnt sienna | PBr7 | WN | 1 : 4.0 |
| raw umber | PBr7 | WN | 1 : 4.0 |
| burnt umber | PBr7 | WN | 1 : 4.0 |
| benzimidazolone orange | PO62 | WN | 1 : 4.5 |
| benzimidazolone yellow | PY154 | Hb | 1 : 4.5 |
| hansa yellow | PY97 | DS | 1 : 5.0 |
| cadmium yellow deep | PY35 | WN | 1 : 5.0 |
| cadmium orange | PO20 | MG | 1 : 5.0 |
| quinacridone carmine | PR N/A | WN | 1 : 5.0 |
| quinacridone rose | PV19 | WN | 1 : 5.0 |
| ultramarine blue | PB29 | MG | 1 : 5.0 |
| cobalt blue | PB28 | MG | 1 : 5.0 |
| nickel azomethine yellow | PY150 | DS | 1 : 5.5 |
| cadmium lemon yellow | PY37 | Hb | 1 : 5.5 |
| cerulean blue | PB36 | MG | 1 : 5.5 |
| sap green | . | WN | 1 : 5.5 |
| green gold | PY129 | WN | 1 : 5.5 |
| cadmium yellow | PY35 | MG | 1 : 6.0 |
| quinacridone red | PR209 | MG | 1 : 6.0 |
| phthalo green YS | PG36 | WN | 1 : 6.0 |
| quinacridone gold | PO49 | WN | 1 : 6.0 |
| dioxazine violet | PV23 | WN | 1 : 6.5 |
| phthalo blue GS | PB15:3 | WN | 1 : 6.5 |
| yellow ochre | PY43 | WN | 1 : 6.5 |
| gold ochre | PY42 | WN | 1 : 6.5 |
| cobalt turquoise | PB36 | DS | 1 : 7.5 |
| venetian red | PR101 | WN | 1 : 7.5 |
| phthalo green BS | PG7 | WN | 1 : 8.5 |
| dilute more | |
| *manufacturers: DS = Daniel Smith; Hb = Holbein; MG = M.Graham; WN = Winsor & Newton or Maimeri |
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Straight from the paint well, and applied evenly with a #8 round brush, these mixtures are all very close to peak saturation. (Note that these ratios are brand specific; other brands will require other ratios.)
Mixing Recipes for Lightness. The relationship between dilution and lightness can also be handled systematically, although this may require you to go through a lot of trial and error.
Again, this is a technical issue that you can clarify by careful measurement and study, although careful measurement probably should not be your standard working procedure. What you learn will become intuitive through frequent application.
The general problem is this: how much do you have to dilute a paint in order to reach a specific value (lightness)? In other words, if you have made a value design, and you want to paint an area with a specific color, how do you determine how much to dilute that paint to reach the desired lightness when the paint dries?
The high and low limits of the value range are known: pure white paper has a lightness of around 97, and the darkest value possible with a given paint is equal to its value range, listed in the guide to watercolor pigments, subtracted from 97. The trick is to find the amount of dilution that will give us any value between those two extremes.
The second issue is: do you want to hit the right value with one coat of paint, or with several? If you are building layers of glazes, for example to model figures, then the dried value of each layer you add has to take into account the value of the layers underneath. So there are two kinds of dilution, exact and layering, to work out.
| dilution recipe for burnt sienna |
| Value (L) | %
VR | Exact Dilution | Recipe* | Layering Dilution | Recipe* |
| 97 | 0 | 0.000 | 0/684 | 0.000 | 0/684 |
| 94 | 5 | 0.001 | 1/684 | 0.001 | 1/684 |
| 92 | 10 | 0.005 | 3/608 | 0.003 | 2/608 |
| 86 | 20 | 0.010 | 5/494 | 0.005 | 3/570 |
| 81 | 30 | 0.023 | 7/304 | 0.013 | 6/456 |
| 76 | 40 | 0.044 | 10/228 | 0.021 | 8/380 |
| 71 | 50 | 0.089 | 15/152 | 0.046 | 11/228 |
| 66 | 60 | 0.140 | 12/76 | 0.050 | 12/228 |
| 60 | 70 | 0.190 | 18/76 | 0.050 | 12/228 |
| 55 | 80 | 0.240 | 12/38 | 0.050 | 12/228 |
| 50 | 90 | 0.290 | 15/38 | 0.050 | 12/228 |
| 45 | 100 | 0.343 | 20/38 | 0.054 | 13/228 |
| *Recipes are in drops of paint/drops of water Equivalent measures (T = tablespoon, t = teaspoon, 3 teaspoons in a tablespoon, 1 teaspoon = 76 drops): 684 drops = 3 T, 608 drops = 2 T + 2 t, 570 = 2T + 1.5 t, 494 = 2 T + 1/2 t, 456 = 2 T, 380 = 1 T + 2 t, 304 = 1 T + 1 t, 228 = 1 T, 152 = 2 t, 76 = 1 t, 38 = 1/2 t. |
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The table shows dilution recipes to achieve either an exact dilution or a layering dilution with a burnt sienna paint. I use the Robert Doak or Winsor & Newton burnt siennas, as these are made with a transparent red iron oxide that can withstand fairly heavy layering without turning a scabby brown. The table shows 11 value steps from full strength to white, so that the lightest step can be split in half for better resolution.
This table can be used for any paint. First squeeze out a healthy amount of raw paint (burnt sienna or whatever pigment you want to use), then dilute it to a creamy concentration. This is usually the most concentrated paint mixture that will not blotch or bronze when applied with a brush, and is also close to the maximum value you can reach with that paint, so it also represents 100% of the value range (VR), in the bottom row of the table.
This row has a lightness that is 45 for burnt sienna. For a darker paint, such as phthalo green, the 100% would equal a lightness (L) of 40; for a lighter paint, such as cadmium orange, 100% would equal a lightness (L) of 60. No matter: the starting lightness is always 100% of the value range, so the lightness is always increased by the same percentage value. If you want to cut the value of the phthalo green or cadmium orange by 50%, use the dilution recipe on that line of the table. The resulting lightness will be about 70 for phthalo green and 80 for cadmium orange.
I usually don't use a single paint to cover the full value range. In figure painting, for example, I use burnt sienna from the lightest flesh tone down to a dark middle tone, then a darker pigment (such as benzimida brown or ultramarine blue) to take the modeling into the darkest values.
Shots From the Floor. Well, that's all I have, so I'll open the floor to questions. Yes, you in the back.
Does a person need to be insane, or merely obsessive compulsive, to grind through all these procedures?
Well, mixing paints is one of the most obscure aspects of watercolor painting: it's rarely explained to artists in the detail required. Oils and acrylics are almost always perfectly usable straight from the tube, but with watercolors this is rarely true, the artist must dilute them somewhat to use them to best effect.
So judgment and skill in mixing up paints is a prerequisite to painting effectively. The same thing happens when you learn etching. Students go into etching with the idea that it's all about skill in etching metal plates and printing papers, and they come out with the realization that much of it is skill in preparing, mixing and applying inks. Watercolorists have to come to the same realization about their paints.
To get this skill, I think you have to approach the problem in a way that gives you an unambiguous and accurate insight into mixing effects. Then, by simple repetition of this unambiguous painting routine, you transfer that knowledge to your intuitive habits with brush and palette.
Take the time to do it right, write down what you learn, follow explicit mixing procedures at the start, and it will all become second nature in a matter of a few weeks. (I'm assuming you paint almost every day.)
OK, but can't I just mix paint really thick, and brush it out in a thin layer?
Of course. But the thickness of the paint applied to the paper has a significant impact on the finished effect. When you brush on thick paint in a thin layer, the paper finish (surface) shows through more clearly (often the paint "pinholes" over the small depressions in the paper), and the brushstroke quickly runs out of paint, creating a scratchy, streaked and dull finish. That's fine, if you want a scratchy, streaked and dull finish. Do you want it?
Can't I just mix up colors that look best to me?
Sure, if you also know the optimal mixture for maximum color chroma. The question is: do you know the optimal mixture for each paint you use? After all, you think your television picture looks good, until you see an LCD widescreen TV — and then you realize what you've been missing. If you simply mix paints in your habitual way, because you've gotten accustomed to the results, then exploring the full range of paint appearance may show you a new range in your paints — and show you how to get that range whenever you want it.
Can't I just use thinner mixtures applied in several glazed layers to get the required pigment density?
Yep, you can. Many artists do. But — despite the many artists and watercolor books who claim the contrary — my own tests show conclusively that:
paint glazed in several thin layers is not darker, richer or more saturated than a single layer of the same paint applied at the optimal consistency.
See for yourself. Mix 1 level teaspoon of paint with water to the optimal dilution suggested in the table above. (For example, the optimal proportion for cadmium red is 1:4, so you'd add 4 teaspoons of water; phthalo green BS, at 1:8.5, requires 8-1/2 teaspoons of water.) Completely dissolve the paint, then paint a single 3" square of the color on a sheet of watercolor paper. Now add the same amount of water to your mixture (4 teaspoons for cadmium red, etc.), and paint a second 3" square using two layers of paint — be sure to let the first layer dry completely before added the second. Now, double the quantity of water (8 teaspoons for cadmium red, 17 teaspoons for the phthalo, etc.), mix well, and paint a third 3" square, applying four layers in the same way.
Because you've doubled the number of paint layers each time you doubled the quantity of water, you've applied the same quantity of pigment to paper using both procedures. So this is a test of whether glazing by itself can produce a darker, richer, "more luminous" color. In my experience it can't.
Paint applied in several layers does have a different finished appearance, and you will see this in your paint samples. It tends to be flatter and more homogenous, because minor imperfections in one layer are disguised by the others. Pigment texture — flocculation in ultramarine blue, or that sweet powdery texture of cadmiums or naphthol reds — is obliterated. The paper fibers tend to fluff up under repeated wetting and drying — run your fingers over the different glazing samples — which gives the paper a subtle velvet texture that adds to the flat effect. It is also easier to paint perfectly controlled color gradations in this way, since imperfections in the value transition from light to dark can be evened out nicely. But the claim that you get a darker or richer color by this method is simply false.
There are drawbacks to multiple paint layers. Paints contain gum arabic, and when a thick layer of paint dries on the paper, the dried vehicle largely seals the surface — the paint is effectively another coat of external sizing. The first layer of color is applied over absorbent paper, but the second and subsequent layers are applied over the accumulating layers of gum arabic. The common result is that the later layers of paint dry more slowly — they're not helped along by the absorbency of the paper — which gives mixed pigments (and granulating pigments with a significant color difference between large and small particles) more time to separate. The result is a less consistent, more blotchy surface. To avoid this problem, painters who "paint in glazes" often use a separate layer to paint each pigment, to avoid pigment separation in a single layer made with mixed pigments; or they start first with a very diluted mixture, and work up to higher concentrations, so that the last, very dark layer hides the imperfections underneath.
These problems with paint appearance are reduced if you use papers with a rough finish (R), because this breaks up the paint surface and provides greater texture for the pigment to lay on. But this rougher surface makes the multiple glazes harder to do, because the edges of color areas are randomly broken up by the paper texture, and these edges are harder to match with each new coat of paint.
Even if you use glazing, your work is easier if you can get close to the desired color on the first pass, because each new application of paint requires you to shade forms and match edges all over again. This gets tedious really quickly.
When using the "not black, not light" rule on hot pressed (HP) papers, you may find that you need to add slightly less water than usual because these papers are in general the least absorbent. Pigments are more likely to pool and dry slowly on a hot pressed surface, but the paint can also be spread out more thinly. If you need to use a juicy wash, you will have to wrestle with blotching and backruns caused by the puddling water. The solution then is often to use a smaller brush, which applies smaller quantities of paint.
In summary: paint concentration, brush capacity and release, and paper color, absorbency and surface texture all play a role in finished color appearance. Vary the concentration of the paint mixture with a brush and paper you are familiar with. When you've mastered the mixing side of things, introduce variations in the paper surface and the kind of brushstrokes or size of brush you use to expand your control of color across a range of painting situations.
| technical data |
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The information above is really all you need to work out the optimal paint mixtures for your needs. This last section, though technical, shows how paint chroma varies with paint dilution in some major pigment groups. This may help you visualize what actually happens to color as paints are diluted. It also identifies some major groups of paints that behave in similar ways as they are diluted, which can assist you in memorizing the best mixtures to use for each type.
I made these studies by sampling a representative group of 40 single pigment paints, proceeding systematically through the mixing steps described above (using an eyedropper to measure the amount of added water), making test swatches at each step (12 swatches for each paint, from raw paint to watery concentrations), and finally measuring the color attributes with my Spectrolino spectrophotometer and graphing them with an Excel spreadsheet. (I only describe pure pigments, not convenience mixtures, which typically behave as the average of the pigments in them, weighted by tinting strength and proportion of mixture.)
My exploration suggests that dilution affects color differently in five types of paints: (1) saturated warm pigments, (2) cobalt (granulating) blue and green pigments, (3) very dark blue or blue violet pigments, (4) phthalocyanine pigments, (5) unsaturated warm pigments, and (6) iron oxide ("earth") pigments. All other paints or pigments I've looked at behave in ways resembling one of these six groups.
Saturated Warm Pigments. To get saturated color, let's start with saturated pigments. Happily, these show a simple and consistent dilution change: they are most saturated at the highest concentration that shows no bronzing. From that point, any added water steadily pulls the chroma down to zero (at white).
dilution curves for saturated warm pigments
dilution curves begin at the syrupy (1:1) concentration, and end at white
The warm pigments shown in the chart are diverse in color and chemistry — cadmiums (red PR108, orange PO20 and yellow PY35), pyrrole red (PR254), benzimidazolone orange (PO62) and nickel dioxine yellow (PY153). And there are many others not shown here — benzimidazolone yellows (PY151 and PY154), hansa yellows (PY3, PY97 and PY65), perinone orange (PO43), pyrrole scarlet (PR255) naphthol scarlet (PR188) and reds (PR112, PR170). The dilution curves for all these saturated red, orange or yellow pigments have the same shape.
The "walking cane" bend at the very top happens because the first drops of water loosen up the chroma without much affecting the tonal value. So the curve rises vertically. Add a little more water, and the saturation does not increase, but the paint begins to lighten — the curve turns sideways. Add still more water, and the curve begins to drop quickly — the whiteness of the paper is now contributing to the color, making the overall appearance lighter valued but also duller (desaturated).
Because these colors are already light valued and highly saturated, the decline in saturation is significant as the color is lightened, even slightly. But the optimal concentration is not hard to find. As long as the paint appears dull or gray, or bronzes when it dries, you need to add a little more water or apply the color more thinly.
Once you find this optimal mixture, study the fluid texture carefully so you can mix it up again to the same consistency — by eye, rather than from a recipe. Most of these paints reach maximum saturation at a creamy concentration.
Again, the viscosity (thickness) will be different for different paint manufacturers, because manufacturers mix up their paints with different proportions of pigment, gum arabic, glycerin, fillers and water. But you're helped by the fact that these synthetic inorganic and organic pigments are often among the most expensive used in artists' materials, and can appear undesirably dull if too much pigment is put in the paint. As a result, paint manufacturers use only as much pigment as necessary to get a competitive color. This is why "out of the tube" is close to the optimal mixture. (Incidentally, a tinting test on the cadmium red in a watercolor, oil or acrylic paint line is a revealing test of paint brand quality.)
Cobalt Pigments. The saturated synthetic inorganic blue pigments have much the same dilution curves. However most of the cobalt pigments are actually rather dull, with a maximum chroma of 50 or less. These have a different dilution trace.
dilution curves for synthetic inorganic blues
The saturated reddish blue pigments — ultramarine blue (PB29), cobalt blue (PB28) and cobalt blue deep (PB73) — just stretch the "walking cane" shape out to a very low value. These behave in the same way as the saturated warm pigments: they are most saturated at the highest concentration that shows no bronzing. Ultramarine in particular almost seems to glow if painted at a syrupy dilution.
The greener (less saturated) blue and green cobalt pigments are different. The first small doses of water do not raise the chroma much, if at all, which means saturation is slightly reduced as the paint is lightened. For the dull cobalt turquoises (dark PB36 and teal PG50) and cerulean blues (PB35, Winsor & Newton, and PB36, M. Graham) there is almost no "walking cane" hook in the dilution curve: saturation changes very little across syrupy to fluid dilutions. The same pattern appears in the cobalt and chromium green pigments (PG17, PG18, PG19, PG29 and PG50).
With these paints, the second major type of pigment, the optimal concentration is again the minimal dilution necessary to eliminate bronzing. Because of the lack of a peak in the dilution curve, however, you have much more room to vary the paint consistency (the redder cerulean blues are the exception). It's more common to use the cobalt and chromium paints in heavily diluted mixtures anyway, as sky or ocean washes; because they are relatively unsaturated, heavily diluting the paint gives the color a delicate pastel glow.
Dark Blue/Violet Pigments. While we're in the blues, there are a few dark, finely divided and relatively unsaturated pigments, such as prussian (iron) blue (PB27) and the synthetic organics indanthrone blue (PB60) and dioxazine violet (PV23), that are important to painters for the very dark hues and smooth mixtures they add to a palette. These are the third major type of pigment.
dilution curves for dark, unsaturated blue violet pigments
In these very dark paints the dilution curves rise sharply in chroma as the first drops of water are added, then flatten out quickly as the color starts to lighten. This gives the curve a characteristic domed or wedge shape. The peak chroma is reached after perceptible lightening of the color occurs.
The reason: you can't have chroma without hue, and the deep dark value and pigment density of the raw paint actually obscures the pigment hue — the paints look almost black. Lightening is required to make the color show clearly. For these pigments, a weak creamy concentration is necessary to produce the most characteristic color appearance. Both dioxazine violet and prussian blue make excellent alternatives to carbon black paints, when applied in thicker concentrations.
Some artists use chinese white (PW4) or titanium dioxide (PW6) to lighten these dark pigments and give them a creamier texture. The graph shows what happens when titanium white is added to indanthrone blue. A small amount of added white is especially useful to kick up the saturation of the very dark values, producing glowing dark blue violets; dilution with water gets a slightly brighter color in the higher values. Be aware, however, that adding a white pigment tends to degrade the lightfastness of any pigment, especially dark or stongly tinting paints.
Phthalocyanine Pigments. The kinked curve typical of dioxazine violet resembles the dilution curves for phthalocyanine pigments, both blues and greens, the fourth major category of pigment. Like dioxazine violet, these are typically very dark valued paints out of the tube: in some brands of paint at maximum concentration, phthalo green BS (PG7) can obtain darks that exceed those of carbon blacks. The difference is that they gain substantially in brightness, to a chroma of 60 or more, as they are diluted, creating a domed curved with a higher, more pronounced peak that is near the midpoint of the value scale (around 5).
dilution curves for phthalocyanine pigments
Chroma rises steadily as these paints are diluted, reaches maximum at around a CIELAB luminosity value of 40 (blues) to 60 (greens), and then declines to zero saturation (at white) as more water is added. For the phthalos, the optimal color is a fluid concentration that produces a lightness around 50 in the dried paint. (Use the artist's value wheel to learn this judgment.) This point is easy to judge visually: let the "not black, not light" rule take you to a greater dilution of the paint than for any other pigment.
An important class of pigments — the saturated (bluish) quinacridones — are difficult to categorize, because each hue presents a different dilution curve. As the chart below shows, the inflection in these colors is variable — walking cane for the redder colors, whiplashed golf club for the bluer (magenta or red violet) colors.
dilution curves for saturated bluish quinacridones
However, the same mixing rule applies to the quinacridones and the phthalos: a fluid concentration that produces a lightness around 50 in the dried paint. That is, you should dilute more with the darker paints. Because the change in chroma is rather steep on both sides of the inflection point, it is easy to find by visual comparisons of dried color samples — the color when brushed out does not have splotches of darker hue within it. Different brands of quinacridone paint also vary in the amount of dilution required. This is an important pigment group and it is worth the time to learn the quirks of the paints brand you use.
Iron Oxide Pigments. The sixth and last major group of pigments is the earth colors made with yellow, red or black iron oxides.
dilution curves for earth (iron oxide) pigments
These curves show a very subtle change in chroma as water is added. Usually a fluid concentration is required, and as the pigments are quite fine and relatively heavy, they always need to be stirred before charging the brush. But the "peak" or top of the dilution curve is often flat, which gives you freedom to move the mixture around. The optimal saturation is spread over a wider span of values, making these earth hues very flexible in mixtures and forgiving in color design.
All these pigments, when laid down at maximum concentration, present a very distinct grayish or dull appearance when dried. You need to add enough water to break up this matte surface with the texture of the paper finish, and let the smaller (lighter) pigment particles shine through.
Last revised 11.12.2007 • © 2007 Bruce MacEvoy |
 a mixing card for visual color
matching with dilution recipes
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