Duotones
http://www.graphic-design.com/Photoshop/duotones/index.html

Design and Publishing Center
2002

Abstract:

Duotones are a combination of two grayscale images, using two negatives and two printing plates. Sometimes we see super print quality using two blacks, to add midtones or highlights to an image, or to extend an image's overall tonal range. Back in the '80s the Time-Life Photography series of books used Black and metalic Silver inks to produce probably some of the best high-grade black and white photographic images ever printed. They're no longer in print, but you can find them in most better public or university libraries.