"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. A drop or two of 1% gold chloride can be added to the sensitizer just before coating to move the image color towards purplish-brown. My main supplier of chemicals is Artcraft Chemicals (http://www.artcraftchemicals.com/) and I highly recommend them."