Updated: Mar 5, 2020
This is an updated version of the Lumiere Brothers Cobalt Process of 1893
The light sensitive agent is Potassium Cobalt III Oxalate. The initial image is a coral colored pigment of Cobalt II Ferricyanide. It is toned to a dark brown with Selenium/Sulfide toner from Anchel’s Darkroom Cookbook.
See the Safety Chapter on this site.
PAPER: Arches Platine, Hahnemuehle Platinum Rag, or hot press watercolor. Buffered papers need acidifying by soaking 5 minutes in 1% oxalic acid solution and drying.
SENSITIZER: Part A: 20% Potassium Cobalt III Oxalate solution.
Part B: 15% Potassium Ferricyanide solution.
Mix 1 volume A + 1 volume B together as the working solution. It has a limited life after mixing.
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: yellow bulb
Coat the paper with a flat brush or coating rod. 10% Tween 20 can be used to make coating smoother. Typical coating is 2 ml working sensitizer, 1 drop Restrainer, 1 drop Tween.
Dry with a fan or a hair dryer on low. Avoid high heat as it will fog the image.
Expose to UV light through a negative in a print frame.
Wash in water with a pinch of oxalic acid to remove all green color in the paper. It takes 3 trays and up to 10 minutes to get all the excess chemicals out of the paper.
The image is coral colored Cobalt II Ferricyanide.
It can be toned to a dark brown with Selenium/Sulfide toner (outdoors or fume hood). I use the Selenium/Sulfide toner in Anchel’s Darkroom Cookbook. Wash well and dry.
Humidity and oxygen in the air degrade the image over time. An acrylic varnish such as Golden MSA can help preserve the image.