Wash, in painting, thinning and spreading an applied colour with water or other volatile paint media (also called softening); wash drawing, as much as drawing au lavis (s. lavis). Washing, or wash, a painting technique, in which paint of a very thin consistency is applied over a previously underpainted area, which shines through. The same technique in oil, tempera and other film-forming media is called a glaze.
The Confederate cavalryman pictured above has been washed in various places with heavily diluted brown or black artists’ acrylic paint. The wash on the rider’s right gauntlet was not distributed evenly over the entire surface of the underpainting and has left an unsightly edge after drying. The feed bag in front of the rider’s right knee was not painted prior to the wash. This mistake could be remedied by carefully setting highlights, but this would be more complex than painting the feed bag in a beige colour and repeating the wash. The horse shows selective washes, so-called pin washes, between muscle parts and on the bridle in order to emphasize these areas.
A paint medium suitable for washes must offer an even application of paint pigment even when it is highly diluted, and it must not become blotchy. This requirement severely limits the range of water-soluble paint mediums, which is why fine India inks are often used for washes. Pigments suspended in oil are generally well suited for washes, and they are also suitable for glazes on surfaces underpainted with water-based paints.
Source: Meyers Großes Konversations-Lexikon, 6. Auflage 1905–1909