I took a course in Mezzotint led by Ray Dennis at Brighton Independent Printmakers. It's the reverse of etching...you burnish the light back from the dark on the rocked copper plate.
Here is a brief description: Mezzotint achieves tonality by roughening the plate with thousands of little dots made by a metal tool with small teeth, called a "rocker." In printing, the tiny pits in the plate hold the ink when the face of the plate is wiped clean.
I made this print on the course.