There are two classic styles of painting which we associate with the Old Masters, sfumato and chiaroscuro, and they are as alike as cheese and chalk. But we still manage to confuse them, and which artists made use of which styles.
Sfmuato and Leonardo da Vinci
Sfumato refers to the subtle gradation of tone which was used to obscure sharp edges and create a synergy between lights and shadows in a painting. As Ernst Gombrich2, one of the twentieth-centuries most famous art historians, explains: "[t]his is Leonardo's famous invention … the blurred outline and mellowed colours that allow one form to merge with another and always leave something to our imagination."
Leonardo da Vinci used the technique with great mastery; in his painting the Mona Lisa those enigmatic aspects of her smile have been achieved precisely by this method, and we are left to fill in the detail.
How, exactly, did Leonardo achieve this effect? For the painting as a whole he selected a range of unifying midtones, especially the blues, greens, and earths, which had similar levels of saturation. By avoiding the most luminous of colors for his brights, which could break the unity, the midtones thus created a subdued flavor to the picture. Leonardo da Vinci is quoted as saying "[w]hen you want to make a portrait, do it in dull weather, or as evening falls.3".
Sfumato takes us one stage further though, away from the focal point of the picture, the midtones blend into shadow, color dissipates into monochromatic darks, much the same as you get on a photographic image with a tight focal range. Sfumato makes an choice if your portrait sitter is embarrassed by wrinkles!
Chiaroscuro and Rembrandt
In comparison the paintings of Caravaggio, Correggio, and, of course, Rembrandt, have a heavy-handed approach to light and shadow. The focus of the painting is illuminated, as if in a spotlight, while the surrounding field is dark and somber – heavy, burnt browns melding to black. This is chiaroscuro, literally "light-dark", a technique which was used to great effect to create dramatic contrasts.
The effect was created using successive glazes of transparent brown. Notorious now, since the brown paints used by the later Renaissance artists tended to be tar based (bitumen) or burnt beechwood (bistre) causes problems in old master paintings by residue seeping through the canvas.
You can create the chiaroscuro effect using glazes of umber (or burnt umber if you want a warmer picture). Remember that if you need to touch up highlights near to the darkened shadow areas, you should warm your colors; add a little red to the mix to make up for the cooling effect of the surrounding darks.
1. Collins English Dictionary.
2. The Story of Art by EM Gombrich, first published in 1950.
3. Bright Earth by Philip Ball (page 123).