Artists as diverse as Vincent Van Gogh, John James Audubon, Paul Cezanne, William Blake and Georgia O'Keefe all worked at least some of the time using watercolor paints. The medium, which uses plain water as the dispersant for its pigments, has great versatility in application and an almost ethereal liquidity in finished appearance. One of several basic techniques for watercolor application is the wet-on-dry method.
Watercolorists first select their paints for some basic qualities that will fit the particular paintings they want to do. Among those qualities are transparency, staining quality, tinting strength and sedimentary quality. Transparency is the ability of a paint to allow colors underneath it to show through. Staining quality refers to how durable the paint is once it is dried and re-wet. Tinting strength refers to the capacity of a paint to overpower other paints when it is mixed with them. Sedimentation refers to how quickly the paint's color thins during a brush stroke. Understanding these characteristics is important when you anticipate using the wet-on-dry method, because this method joins two different colors along a distinct line.
The Hard Edge
Wet-on-dry technique is used to place a hard edge on the painting. Watercolors are particularly good at creating blended and soft edges, but many paintings will lose coherence without some hard edges. Wet-on-dry might be placing a hard edged stroke on plain, dry paper; or it might be placing a hard edge over an existing coat of dried paint. In the case of painting over existing dry paint, it is important to choose a paint with low transparency. In either case, the brushstroke has to be authoritative, leaving a clean edge, and the paint should not be so thin that it bleeds.
Loads and Beads
For wet-on-dry, the brush needs to be "loaded;" that is, the brush should be full enough of paint that when the brush point is held down a small bead of paint forms at the tip. If the bead drips when held down, but does not drip when held horizontally, the brush is properly loaded. This bead is what will be applied to the paper to create the hard edge. You can test the bead on scrap paper to ensure that the paint is thick enough to prevent bleeding. If the test pattern bleeds, mix more pigment into the water to thicken the paint bead.
Applying the brush stroke for wet-on-dry is a timing art. Beginning the stroke and touching the surface late can cause you to miss the line and create a tail where you began; touching the surface and hesitating before you begin the stroke can cause the paint to pool, making a blob at the beginning of the stroke. The touch and stroke need to be simultaneous, with a smooth draw of the brush along the desired line. Subsequent strokes require reloading the brush and beginning the next stroke exactly where the last stroke's bead left off.
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