Etching - A zinc metal plate is covered with a soft or hard ground(an acid resistent wax) and drawn onto using a variety of sharp instruments. When the plate is immersed in acid, the bare metal, exposed by the lines of the drawing is eroded. The longer in the acid, the deeper the line and the darker it will print. Varying depths of line can be achieved by 'stopping out' - applying a varnish over the finer detail work and then placing in the acid again for a second 'bite'. In order to obtain a print, a viscous greasy ink is pushed into the etched grooves, then the surface is wiped clean with muslin, leaving only the etched areas retaining ink.
Aquatint - A powdered resin is dusted onto a metal plate and gently heated to form small globules. This acts as a resist to the acid which burns around the small particles giving an irregular texture and tone to the plate.
Drypoint - Using hardened metal objects or a diamond tipped needle, marks are created directly onto the zinc metal plate. Ink is held in these grooves but also in the burrs of metal created by the incisions as they are cut. These produce a softer, fuzzy edge to the line and are often subtler than an etched line.
Carborundum - To gain a tonal value on a drypoint plate, I have mixed a fine carbon powder with glue and painted it directly onto the plate. This when inked, absorbs the ink and depending on the density of powder used in relation to the glue, produces tones of varying depths.