I wish to share with you what I have learned. The Vandyke brown print is based on the first iron-silver process, the argentotype, invented in 1842 by the English astronomer, Sir John Herschel. Both processes utilize the action of light on ferric salts and their chemistry is very similar. The Vandyke process gets its name from its similarity in color to the deep brown pigment used by the Flemish painter Van Dyck. Vandyke brown prints are very simple and economical to make, with the sensitizer consisting of three readily available chemicals.
Clearing is carried out in water and fixing is done in a weak solution of hypo.
The following formula was taken from Bob Schramm's article in Post-Factory Photography . I have tried varying the amounts of each of the three ingredients but have found the basic formula to give the best results. Adding more tartaric acid seemed to increase contrast slightly and move the image color to a more neutral gray but then graininess became a problem. Adding more silver nitrate didn't have much effect, as was the case with more ferric ammonium citrate. I doubled the amount of all the chemicals in the formula in an attempt to make a single coat solution and got excellent contrast with rich blacks but grain was again a problem.