Mezzotint was invented in the 17th Century by Ludwig von Siegen, an amateur artist of German origin. To appreciate the significance of this event, picture a time before photography or the publication of sumptuously illustrated art books, when the only way to get an art education was as an apprentice. To learn about the techniques of great painters, you might try studying engravings or etchings, but these stylized linear replications could only depict form and design and impart nothing on the handling of paint, brush strokes, surface texture, and little on chiaroscuro and other lighting treatments.

Now imagine it is Amsterdam, 1642 -- the year Rembrandt painted The Night Watch -- and a graphic technique is introduced which can reproduce every textural nuance and smooth tonal gradations ranging from blackest black to stark white. Finally portraits could be printed with supple skin tones instead of mask-like cross hatchings, with light that shimmered over satins and steely armor, melted over voluptuous velvets and dove into deep, dark shadows. At last one could study prints that conveyed information about the artist's handling of paint. It would be as exciting and revolutionary as the invention of photography or digitized imagery. To grasp the artistic and commercial implications of such an invention is to understand the magnitude of the impact of mezzotint.

Mezzotints are printed in much the same way as etchings and engravings; the image-making process differs in that one works from black to white, or deductively. In essence, it's similar to the manner of drawing by which one covers a sheet of white paper with charcoal until it appears black, then draws the image using an eraser that removes the charcoal. However, in mezzotint, one works on a thin sheet of metal, usually copper. First, a tool called a rocker is used to create a field of burrs over the surface of a copper plate. If inked, the roughened plate surface, called a ground, would print as a dense black tone. Scraping away or burnishing down the burrs reduces the depth of the ground. The shallower the ground, the less ink it holds; the thinner the ink film the more transparent the black and the lighter the shade of grey on the printed image. Where the burrs are removed entirely, the smoothed plate prints white. Variations in the process imbue the printed image with characteristics that are often unique to a given print or artist.

Carol Wax
Art of Darkness, 1996

Information about other techniques can be found here:
http://www.printdealers.com/learn.cfm