How IT WORKS
1. Mix a sensitizer from an iron salt (ferric ammonium citrate), a mild acid, and dissolved silver nitrate, and brush it onto paper under dim light.
2. Let the coated paper dry fully away from strong light. Like cyanotype, it stays usable for a short while once dry.
3. Contact-print it under a same-size negative in sunlight or UV light. The image also prints out visibly during exposure, deepening to a purplish brown well past what the finished print will look like, so progress is easy to judge.
4. Wash the print promptly once it looks sufficiently exposed. This clears most of the unreacted silver and immediately shifts the color to its true warm brown.
5. Fix briefly in a diluted sodium thiosulfate bath — Van Dyke brown is unusually sensitive to strong or prolonged fixing, which can bleach the image back out — then wash thoroughly.