Monday Morning Art School: the opacity of paints

To understand refraction, just remember that hideous invention of the 1970s, the wet t-shirt contest.

The Logging Truck, 16X20, oil on linen, on exhibit this month at Camden Public Library.

Opacity (or ‘hiding strength’, if you prefer) is a simple way of describing a paint’s refractive index. Opaque pigments refract, or bend, more light. Transparent pigments (which really ought to be called ‘translucent’) allow light to pass through to bounce off the substrate before it returns to you. Understanding the opacity or transparency of your paints gives you more control in color mixing and glazing. This is obviously important for watercolors, but it matters in oil paints and acrylics as well.

The hiding power of a paint is also dependent on the ability of a pigment to absorb light. That’s why black is opaque—it’s bouncing no light back at us. For most pigments, it’s a combination of these two properties—the ability to absorb and scatter light—that give us opacity.

Mountain Path (the Susurration of Dried Leaves), 11X14, on exhibit this month at Camden Public Library.

Most 20th century pigments, like ultramarine, are milled very small. They have a particle size of less than 1 micrometer (something akin to white flour). Milled mineral pigments can have particle sizes of over 100 micrometers (more like sand). Moreover, the size of these mineral pigments isn’t consistent; they are, after all, basically ground-up rocks. Some of these mineral pigments can cause an effect called granulation, which watercolor painters prize.

In watercolor, smaller particle size gives you higher tinting strength, more transparency, and more staining, because the pigment particles more easily penetrate the paper. In oils and acrylics, smaller particle sizes make the pigments more transparent and saturated. In watercolors, there’s just less pigment covering the paper, which allows the paper to show through. Even opaque pigments look more transparent when diluted, although they do not usually excel at being treated like transparent pigments.

Spring Allee, 14X18, on exhibit this month at Camden Public Library.

That brings us to the question of paint quality. Students are often instructed to ‘buy good paints’ without any idea why that is important. Pigment load is the primary consideration. Manufacturers make paint more cheaply by adding less of the good stuff. Compensating for inferior pigment load can build bad habits in the beginning painter. Buy a good student-grade paint from a good manufacturer, like Gamblin, Winsor & Newton, or Grumbacher.

The boiled linseed oil you buy at the hardware store is never appropriate for oil painting. It darkens and turns yellow with age.

A pigment’s natural refractiveness is only one consideration. The binder it’s suspended in also affects what’s refracted. You have only to think of that hideous invention of the 1970s, the wet t-shirt contest, to understand this. (And then ask yourself: what the #@$ were those young women thinking?) A t-shirt that appears opaque when dry will suddenly become transparent when wet. Air does not have the same refractive index as water. That’s why watercolor shifts in color as it dries.

And of course, acrylic and linseed oil binders also play a role in refraction. They never disappear on drying, so their refractive index, if close to that of the pigment, can render some paints permanently transparent.

Evening in the Garden, 9X12, is on exhibit this month at Camden Public Library.

We know that, as linseed oil ages, the refractive index increases. This can cause oil paint to lose its hiding strength, which is why we see pentimentiappearing hundreds of years after masterworks were painted. To avoid this, painters need to learn to use sufficient quantities of paint. And, of course, acrylics, alkyds, and water-miscible oils have not been around long enough to have any track record on the subject.

Most paints fall somewhere in the middle of the continuum of opacity and transparency. The most opaque are titanium white, carbon black, raw sienna, burnt umber and yellow ochre. We typically use white to create opacity, but there are times when its lightening properties make that inappropriate. In those instances, one of the other opaque pigments is appropriate.

Zinc white, sold as China white to watercolor painters, is not as opaque as titanium white. (It’s also more brittle.) That’s why its application in oil painting is limited to glazing, but is also why it’s so useful in watercolor.

(I have two more openings in my Tuesday AM online class and one in my Monday night class, starting January 3-4. If you’re interested, the information is here.)

Cheap paint is a false economy

Don’t skimp on paint quality, or you’ll defeat yourself from the outset.

Ogunquit, by Carol L. Douglas. If the pigment isn’t in the paint to start with, you can’t magically enhance it. 

When I send supply lists, I suggest brands. These are Golden for acrylics, QoR for watercolor, and RGH or Gamblin for oils. In pastels, there is too much variation in hardness for a blanket recommendation, but I like Unison myself. Of course, nobody’s paying me for these endorsements; they’re just my preferences.

That doesn’t mean these are the only good art supplies out there. They have a combination of pigment load and handling characteristics that I like. There are many excellent makers of paint out there. They come in a variety of price points, but price is not the sole indicator of quality.

Late October, Beauchamp Point, by Carol L. Douglas

There are an equal number of horrible paints on the market. You might think you’ve saved a few bucks, but they’re an expensive mistake, one that will cost you time in learning. Don’t skimp on paint quality, or you’ll defeat yourself from the outset. Instead, cut down on the number of colors you buy.

All paints (and pastels) consist of pigment and a binder. There are differences in the quality of binders, in the amount of pigment the manufacturer uses, and how the pigments are stabilized. There may be filler added, or drying agents.

Most major paint brands in the US subscribe to voluntary associations of quality control. (RGH is an exception; that’s too bad, because their paint is excellent.) The most well-known is Colour Index International (CII), a database dating back to 1925. It contains over 27,000 individual products sold under 13,000 different product names. This standard classification system gives you the facts about the pigments in your tube.

Autumn Farm, by Carol L. Douglas

Just as Benjamin Moore uses names like Yukon Sky to peddle grey paint, art paints are often marketed with evocative names. These names appeal to our sense of tradition, even when the old paint has no relationship to its namesake. If you buy Naples Yellow thinking you’re buying an historic pigment, think again: the modern paint is a convenience mix replacing the historic (and toxic) lead antimonate.

Expect to find, at minimum, the following information on the label of your paint tube:

  • Manufacturer’s name or common name for the color.
  • The CII number and, sometimes, the name of the pigment(s).
  • The manufacturer’s lightfastness or permanence rating.

The CII code consists of two letters and some numbers. Most paints start with a “P” which means it’s a pigment, not a dye. The next letter is the color family:  PR is red, PY is yellow, etc. The number is the specific pigment included in the tube.

Save this link somewhere accessible from your phone:

You’ll need it when you shop. This pigment guide was built for watercolors but is generally true across all media. (Watercolor is the canary in the coalmine of pigments). All painters should understand lightfastness, transparency, and color shift. Granulation, bloom and diffusion, however, are watercolor-specific issues. 

Winch, by Carol L. Douglas

When you compare paints with the same names, check their CIIs. Are they the same or different pigments? A “hue,” is made of a blend of less-expensive pigments. There is nothing inherently wrong with hues, but they don’t behave the same as the pigments they’re named after. For example, “cadmium yellow hue” may look like cadmium yellow coming out of the tube, but it makes insipid greens.

There’s little to be gained by buying a hue mimicking a more expensive pigment. If you are comfortable painting with a hue, then learn what’s in it and mix it yourself. You always have the greatest flexibility by working with pure pigments (rather than mixes) out of the tube.

Most manufacturers include their own lightfastness ratings on the tube. This is a measure of how quickly the color fades. If it’s not listed, look it up.

The series number tells you the price. Are pricier pigments better? Not by a long shot. Twentieth-century manufacturing gave us a new world of inexpensive pigments, which tend to be less toxic, higher in chroma and lightfast.

I’m thinking about supply lists because it’s time to send them out for Find Your Authentic Voice in Plein Air in Tallahassee, Florida, in early November. There are enough students to go, but there are still openings, so I’d be excited if you signed up. s

From there on in, it’s all Zoom, Zoom, Zoom until the snow stops flying. The Tuesday morning class is sold out; there are still openings for Monday night Zoom classes.