A photographic print of reddish brown Copper II Ferrocyanide (Hatchett’s Brown) pigment on paper.
C J Burnett coined the term in the 1850s for a process that used a uranium salt as the light sensitive agent, with the image of Hatchett’s Brown. In the 1860s J B Obernetter used Ferric Chloride as the light sensitive agent. I updated the process below.
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PAPER: Arches Platine, Hahnemuehle Platinum Rag, or hot press watercolor. Buffered papers interfere and need acidifying by soaking in 2% citric acid, washing and drying. SENSITIZER: 12% Copper Sulfate, pentahydrate and 12% Ferric Ammonium Citrate, green. Store in a brown glass bottle. Clear emerald green solution. RESTRAINER/CONTRAST AGENT: 25% Ammonium Dichromate solution. Other agents: 10% Ammonium Persulfate, 20% Sodium Chlorate, or 40 volume hydrogen peroxide (beauty supply store). This reduces fog and increases contrast. SURFACTANT: 10% Tween 20 solution. SAFELIGHT: low wattage tungsten or yellow bulb
Steps:
Coat paper with a flat brush or coating rod. A typical coating is 2 ml Sensitizer, 1 drop 10% Tween 20, and 1 drop Restrainer. Dry with a fan or hair dryer on low. Expose through a negative to UV light or sunlight in a print frame. Develop in 1% Ammonium Thiocyanate, 0.5 % Citric Acid solution for 3 minutes. The image now is pale gray Copper I Thiocyanate. Wash in 3 trays of tap water with a 1/4 teaspoonful Citric Acid or a slash of vinegar per liter until all orange color is cleared from the print. The image is barely visible as a faint gray area. If any iron is left in the paper, blue cyanotype stains occur. Tone in 2% Potassium Ferricyanide solution to the density you want. Wash in water with a splash of vinegar or pinch or Citric Acid if your tap water is alkaline to remove all residual chemicals. Dry and marvel. The image can be toned dark brown by soaking in Selenium/Sulfide toner from Anchel’s Darkroom Cookbook. Do this outdoors or in a fume hood. This toning can be done after development and wash, without using the Potassium Ferricyanide toner, or a finished Cuprotype can be toned in the toner. It can be toned black by soaking in a solution of 0.5% para-phenylenediamine, 1.5% Catechol (pyrocatechol), 0.5% citric acid for 2 minutes, then washing very well and hung to dry. The color develops as it dries.