Thanks Elizabeth! This looks like a good possibility, though perhaps there are other candidates for this word? At any rate, here's what the Oxford English Dictionary (online thru my local library) has to say about sfumato:
See quot. 1869.) Also as n., the technique of softening outlines and allowing tones and colours to shade gradually into one another; a softened outline or hazy form produced in this way. 1847 Manual of Oil-Painting 126 Sfumato. Painted with a light, vapoury touch. 1869 EASTLAKE Mater. Hist. Oil Painting II. 206 Another quality which was adopted from Leonardo..was the ‘sfumato’ system the imperceptible softening of the transitions in half-tints and shadows.