I've been playing around with gum bichromate a lot, with most of the time directed to what will and what wont work as a pigment. I've got 2 recipes ready for sharing.
All of my gum bichromates use this formula (which covered two 4x5):
- 3ml Gum Arabic
- 3ml 10% Potassium Dichromate
- 3/16th inch cube pigment
Powdered graphite gives great tonality and a very neutral black. It responds well to brush development. The brand I am using is General's Pure Powdered Graphite and costs about 9 USD for 6oz. That works out to about five to ten cents a mix.
5 by alexfredhill, on Flickr
4 by alexfredhill, on Flickr
Mull 2 grams of powdered graphite with a minimal amount of Gum Arabic (about 1ml) to make a stiff paste. I mull for about 5 minutes, to get a fairly even distribution of pigment. (I've mulled for 15 without any apparent change in draw down quality. I'm still a noobie with mulling). This should give you enough for one or two 3/16th pigment cubes.
Jacquard Pearl Ex
3 by alexfredhill, on Flickr
2 by alexfredhill, on Flickr
1 by alexfredhill, on Flickr
There are so many different Pearl Ex pigments. I have played with most of the duo colors, as well as the macropearl and micropearl powders. Because the particle size of the pigment, mulling doesn't seem to be necessary or have much of an effect. The duo colors are most impressive over a dark color. I have found doing a cyanotype with a duo color (or macropearl) over it to be very pretty.
Use a pallet knife (or muller) to mix a minimal ammount of gum arabic to .5 gram of Jacquard Peal Ex pigment. Once it's well mixed take a 3/16th inch cube and combine with above formula.