Scumble

To drag a lighter, more opaque color over a darker one. Used to great effect to obtain mid-tones and transitional gradations between tones on oil and acrylic paintings. In old master work the darks are thin and transparent. Like real shadows they are insubstantial, hard for the eye to focus on. The lights on the other hand are opaque and attract the eye with texture and a sense of solidity. Scumbling was the technique that linked these quite opposite areas of paint.