Reflections on the Art and Science of Gum Bichromate

Katharine Thayer

Apricots. Tricolor Gum Bichromate. Copyright Katharine Thayer. Private Collection.

Introduction to Gum Bichromate

Myth: Gum can't express fine detail
Myth: Gum prints are coarse and lacking in subtle tonal gradation

Outline of method: How I Print Gum

The Dance of Interacting Variables

Influences on exposure time

Myth: pigments in gum emulsion act as color filters that differentially block UV

Influences on contrast
Influences on "DMax" and print tonal range

Troubleshooting

Uneven coating due to insufficient moisture
Problems that show up in development: underexposure, flaking, overexposure
Pigment Stain

Myth: Pigment stain is a function of underexposure

"Tonal Inversion"

Dichromate Stain

Materials and Equipment

Gum Arabic
Dichromate-- issues in selecting dichromate type and concentration
Pigments (in the form of watercolor paints)

Issues in determining pigment concentration
Transparency and Opacity
Myth: Chromium pigments are incompatible with the gum process

Paper
Negative

Paper negatives

Light Source
Miscellaneous supplies and materials

Tricolor Gum

Myth: you can't produce black using three colors; you need black pigment too
How RGB channels become CMY separations when inverted
Why the default Photoshop CMYK option may not be the best choice for separations

Mechanism of the process

Simple explanation of the process
Technical discussion of the chemical mechanism
Myth: The chemistry of gum is the same as the chemistry of chrome tanning of leather

Note to Mac Users: Images on this site will appear significantly lighter than originals unless monitor gamma is set to "uncorrected."

Copyright Katharine Thayer; all rights reserved.

kthayer@pacifier.com