Watercolor painting
Transparency

In the partisan debates of the 19th-century English art world, gouache was emphatically contrasted to traditional watercolors and denigrated for its high hiding power or lack of "transparency"; "transparent" watercolors were exalted. The aversion to opaque paint had its origin in the fact that well into the 19th century lead white was used to increase the covering quality. That pigment tended to soon discolor into black under the influence of sulphurous air pollution, totally ruining the artwork. The traditional claim that "transparent" watercolors gain "luminosity" because they function like a pane of stained glass laid on paper—the color intensified because the light passes through the pigment, reflects from the paper, and passes a second time through the pigment on its way to the viewer—is false. Watercolor paints typically do not form a cohesive paint layer, as do acrylic or oil paints, but simply scatter pigment particles randomly across the paper surface; the transparency is caused by the paper being visible between the particles. Watercolors may appear more vivid than acrylics or oils because the pigments are laid down in a purer form, with few or no fillers (such as kaolin) obscuring the pigment colors. Typically, most or all of the gum binder will be absorbed by the paper, preventing the binder from changing the visibility of the pigment. The gum being absorbed does not decrease but increase the adhesion of the pigment to the paper, as its particles will then penetrate the fibres more easily. In fact, an important function of the gum is to facilitate the "lifting" (removal) of color, should the artist want to create a lighter spot in a painted area. Furthermore, the gum prevents flocculation of the pigment particles.
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