In the late 1960s, Penn’s printing experiments included the gum bichromate process, a nineteenth-century technique in which paper coated with a light-sensitive solution containing a watercolor pigment is contact-printed to a film negative using a UV light source. Rather than use paper as a substrate, Penn made his multi-colored gum prints on porcelainized steel plates which ensured perfect image registration during numerous sensitization and processing sessions.
The plates are very heavy, but the surface is characterized by a delicate layering of pigments, with an effect similar to a pointillist painting.
Penn kept meticulous records about which pigments he used on the verso of the plates. One of the many treasures in the Irving Penn Archives at The Art Institute of Chicago is a notebook with 75 pages of Penn’s technical notes on his gum bichromate experiments. The digitized images can be found here.