A scumble is a thin, broken layer of opaque or semi-opaque paint dragged over a dried under-layer, leaving the colour beneath partly visible through the gaps in the upper stroke.
Scumbling is mostly identified by texture and edge: the upper layer reads as discontinuous — chalky, scratched, or feathered — rather than as a smooth coat. The dragged stroke catches on the tooth of the canvas or the ridges of dried paint below; the effect is a softening of colour transitions and a slight luminosity, since the underlying tone shows through the openings. Painters reach for scumbling when they want a colour to read as both the upper and the lower at once — the look of bleached, weathered, or atmospheric surfaces. Distinct from a glaze (which is transparent), and from a wash (which is fluid and even).