scumble Art [ 'skʌmb(ə)l ]
■ verb give a softer or duller effect to (a picture or colour) by applying a very thin coat of opaque paint or a layer of light pencil or charcoal shading.
■ noun a coat of paint or layer of shading applied in this way, or the effect thus produced.
Origin
C17: perh. a frequentative of the verb scum .