Watercolor is really nothing more than water-soluble pigment plus a binding agent, gum arabic. Water is used to dilute the pigment and spread it -- usually across paper, on the end of a brush. The water evaporates without a trace, leaving a translucent stain through which light reflects off the paper beneath. Add opaque Chinese White to the mixture and you have what the French call gouache and the English call bodycolor. Light reflects off, not through, this mixture, and the resulting look is quite different. (Monochrome watercolors, usually in browns, blues, or grays, are referred to as wash drawings.)

The challenge comes in the application. Normally the watercolorist works from light to dark, reserving the blank paper for areas of highlight, and building up washes for areas of density or shadow. Watercolor works best on a relatively small scale. Washes seem to lose power if spread across too vast an area of paper. It takes mastery to keep a very large watercolor from looking like a patchwork of smaller ones. And that's just one aspect of this unforgiving medium. If you've ever tried it, you know how quickly a single misstep can turn an attempted masterpiece into a muddy ruin. Success requires a sure eye and a surer hand.

Success also requires stamina, because watercolor painting can be quite a physical act. Some artists vigorously work away at the paper. Highlights emerge through inventive techniques such as sponging or stopping out (lifting off pigment) or scratching out (abrading the surface). Turner sometimes used breadcrumbs to lift off still-wet pigment; and he kept a fingernail long for scratching out. When asked how he created his extraordinary effects in watercolor William Henry Hunt had a simple answer: He just "fudged it out." The development of these and other techniques -- such as dry brush and wet brush -- is fascinating, but let's leave it for now.