The term grisaille is generally thought to mean gray and that is what the French word means in English.

While I was not taught to paint using a grisaille (after cast painting studies that is), I was taught that the definition of a grisaille was a fully realized underpainting done in grays. The idea was to first work out your drawing and values in the grisaille, then glaze your final colors over that. The history given for this was tied to the invention of oil painting itself, which was an outgrowth of tempera (where an artist would underpaint in a single color). Many artists from the Renaissance made the switch from tempera to oil and in some cases they even underpainted in tempera and then overpainted in oil. Of course tempera is a translucent medium and oil does not have to be. Nonetheless, some artists have continued the practice of doing a grisaille underpainting in oil paint.

As with many art terms, the definition of grisaille is a gray area, especially today. I do not believe that it was traditionally so.

In his Artist’s Handbook, Ralph Mayer defines grisaille as a technique of monochrome painting in two or three shades of gray, as in the imitation of bas-relief. . . More specifically, the method of painting in full modeling in black and white or other simple contrasting tones, and applying transparent color over this in thin layers or glazes.

Perhaps the best example of a true grisaille is Ingres’ Odalisque in Grisaille from the Met in New York.

So, is a brown sketch-like underpainting a grisaille? F.W. Fairholt, in his 1903 edition of A Dictionary of Terms in Art, defines grisaille as being: In grey. A style of painting employed to represent solid bodies in relief, such as friezes . . . by the means of grey tints. . . Many painters make the frotte, or first sketch of their pictures, in a brown tint, to which the term en grisaille in sometimes misapplied.

There are many examples of unfinished paintings and sketches which were begun in a brown wash. One of my favorites is van Dyck’s Princess Elizabeth and Princess Anne, now in the National Galleries of Scotland.

The umber or warm browns that we often see in Venetian and Flemish flesh shadows is a generalized flesh shadow color. Many of these painters had numerous workshop assistants and this convention allowed them to speed the process as well as to keep the flesh consistent. I have no doubt that a true grisaille has been used in the past, and that some schools made it their main method. I just don’t generally see it in the paintings that I am attracted to (Titian, van Dyck, Velazquez, etc.).

The catalog from the National Gallery (Washington) van Dyck show in 1990¹ has a solid chapter on van Dyck’s methods. The author quotes a manuscript, attributed to van Dyck, which outlines van Dyck’s painting stages:
-Stage 1 is the sketch. This is the brownish paint underdrawing which places and outlines the forms.
-Stage 2 is the underpainting. Describing underpainting he says, “he has to see to it, of course, that those colours used in the underpainting are basically the same as those with which he is going to do the final painting, that is, that underpainting of nudes will always be done with carnation colours.”

An unfinished Velazquez from the Prado:

When seen in person, much of the paint visible on the Velazquez is the first and second layer. There is a debate as to whether this painting is an unfinished painting or merely one that is less realized and I have seen it in person many times, on occasion for hours on end looking at this exact issue. The paint layers are quite thin with a rough reddish-brown imprimatura showing through in many areas. The more worked-up areas have slightly thicker paint and therefore the imprimatura does not show as clearly. The representation of form is in color, not grisaille.

References

¹ Anthony van Dyck, by Arthur K. Wheelock Jr. et al. Washington, DC: National Gallery of Art, 1990 -This out of print book can be viewed online and in full here.