VERNIS MOU or
This is a particular variety of etching; instead of covering the surface with the usual resin, the artist spreads a composition of warmed resin and tallow using a brush or roller, creating a softer ground.
He than places a thin sheet of paper on the plate and
draws on it with a sharp pencil (fig. A).
Under the pressure the soft wax sticks to the back of the paper, and comes off with it when the sheet of paper is removed (fig. B).
This technique is also called crayon or color-engraving because it produces an image similar to drawing. Different kinds of pencils and paper yield a variety of effects.
This technique dates back to the eighteenth century (J.- Charles Franšoise 1717 - 1759) and was invented to imitate the rough line of the pencil as well as the softness and gradation in shade of pastel-colours. Today it is almost always combined with other techniques.
(from "tecniche dell'incisione originale" INClub Firenze)