Direct Positive Gums
These photographs were made somewhat similarly to the standard Gum Bichromate process, differing in two ways:
1. The gum solution utiized white gauche on black paper instead of darker pigments on white watercolor paper
2. The gum papers were exposed in-camera, rather than contact printed with a negative.
The emulsion was prepared as follows:
15mL of gum arabic solution (provided from The Photographer's Formulary)
15mL of saturated potassium dichromate solution
1g of white gauche
The papers require a double coating for the image to have a reasonable range of
tones. After applying the first coat to the paper, allow the paper to sit and "gum up"
for approximately 10 minutes before applying the second coating. Triple coating
was not tested.
Exposures ranged from about 2.5 - 4 hours in sunlight. The papers were developed
in a tray of water under incandescent lighting.
The papers seem to keep for a maximum of about two days. The longer the between
coating and development, the longer development seems to take -- the last one of
this set took nearly four hours!
In the end, I feel like this has the potential to produce interesting photographs, but the prohibitively long development time is a major issue. I believe there are two ways to remedy this:
1. Use ammonium dichromate in place of potassium dichromate. Ammonium dichromate has a higher solubility in water than potassium dichromate; a saturated solution could lead to shorter exposure times.
2. Build a custom camera with a larger aperature (potentially utilizing a large magnifying glass in place of the standard camera lens).