Panel Painting 101:
Basic Optics of Panel Painting
Let's start with an experiment: the next time you're in a museum, find a gallery displaying paintings on both panels and canvases. Stand in the middle of the gallery and try to pick from a distance – without reading the labels – which paintings are made on panels, and which are made on canvas. Chances are good that you'll be able to distinguish between them with great accuracy. Try it and you'll see! The panel paintings will usually have a higher gloss; their colors will be richer, clearer, more vibrant, and more brilliant. By comparison, the paintings on canvas will look duller, grayer, and show a smaller range of contrast. The blacks won't look as black and the whites won't sparkle like they do on the panel paintings.
Since the panel paintings show a greater range of tonal values and higher contrast, edges and details in the panel paintings look sharper and crisper. The shadows and highlights contain more detail. What differences between the panels and the canvases account for this?
The answer involves both painting materials, and optics. Canvases absorb paint and reflect light differently than panels. Even if the cloth fibers in a canvas are well "sized" to cut down their absorbancy, a canvas is simply more porous than a panel. Cloth fibers are much more loosely packed than the materials that make up a panel, so grounds and colors soak in to the canvas to a greater extent:
This leaves a more textured surface which, as we will see, reflects light differently than the smoother surface of a panel. It's typically very difficult to smooth all of the texture out of a canvas. To some extent, you can "fill" the valleys and just glaze over the peaks of the canvas surface:
But evaporative shrinkage of the paint makes that a very difficult job, with the paint re-conforming to the surface as it dries. (Acrylic and other water-based paints are most notable for shrinking when they dry. Pure oil paint doesn't shrink, but when it is mixed with resins or solvents, the evaporation of the solvents results in shrinkage as well.)
When light hits a smooth surface, it reflects off of it cleanly and without scattering. But light hitting a textured surface scatters, and is jumbled by the time it reaches your eye, reducing it's intensity.
Controling whether a surface is smooth or textured allows us to effect the way light interacts with it. For example, to make a painting medium have a "matte" finish, paint makers add in large-sized translucent particles so that it will not dry with a smooth surface:
Just as texture gives paint a matte appearance, the texture of a painting effects the light that hits its surface. So the next time you're in that museum gallery, try to visualize the surfaces of the panels and the canvases, and imagine how light from them is reaching your eye. It's just possible that you'll always see – and make – paintings a little differently from then on.
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