The Sfumato reference article from the English Wikipedia on 24-Jul-2004
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Sfumato

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Sfumato is a term coined by Leonardo da Vinci to refer to a painting technique which overlays translucent layers of colors to create perceptions of depth, volume and form. In Italian sfumato means "blended" with connotations of "smoky". Best example of a sfumato painting is the Mona Lisa[1].