Aquatinting produces prints with many delightful variations of shading.
Aquatinting is related to engraving and etching and uses acid to make marks in metal printing plates.
An aquatint begins with a copper or zinc plate covered in powdered resin. The artist heats the plate gently to melt the resin, which forms a fine very slightly bumpy coating. Next the artist dips the the plate in acid. The acid eats through the resin most where the resin is thinnest and then erodes the printing plate. This produces an even, speckled appearance to the plate giving aquatint prints their characteristic look.
The artist then etches the outline of his picture, and may also cover some parts of the plate with wax so they do not hold ink (and leave the paper white). The artist then puts the plate in the acid bath again, progressively stopping out (protecting from acid) any areas that have achieved the designed tonality. These tones, combined with the limited line elements, give aquatints a distinctive, watery look - hence the name "aquatint". The artist the washes and dries the plate and is ready to use it for printing.
Aquatint prints are relatively rare compared to prints made by engravings,etchings or lithography, as the process was relatively difficult and time consuming.
Here is an aquatints from 1807 by William Daniell of a betel nut palm tree:
Here is a very high quality aquatint from the 1850s by by Henry Alken:
www.engraving-review.com has lots of information about antique print engraving, Japanese Ukiyo-e engraving and the latest engraving techniques such as laser engraving.