For a little background to these overviews, see here.
History: Introduced in 1889 by Arndt and Troos, the Van Dyke print is part of a group of iron based processes which draw on Sir John Herschel's work on the Argentotype silver-iron process developed in 1842.
Negatives: A density range of about 1.5 is suitable.
Sensitizer: Part A: 9gms ferric ammonium citrate (green) + 35ml water
Part B: 1.5gms of tartaric acid + 35ml water
Part C: 12gms silver nitrate + 35ml water
In moderate light mix A, B and C (in that order) and age for a few days before use. The solution keeps well for about a year if kept in a cool, dark place. At times, owing to trace chemicals in various supplies of ferric ammonium citrate, the sensitizer may develop a muddy precipitate after about 2/3 of solution C is mixed. This can be allowed to settle and the clear sensitizer on top used, or adding about 2.5 gms more of tartaric acid can also dissolve the precipitate. The additional tartaric acid results in a fainter or no print-out image, but the developed image looks fine.
Contrast control: Contrast control can be achieved by (a) using ferric citrate in varying proportions in Part A of the sensitizer, (b) by adding about 10 drops of a 10% solution of potassium or ammonium dichromate to 500ml of the second bath during washing. (c) printing in diffused light (with perhaps a burst of direct light at the end) also subtly affects contrast. Refer to Christopher James' book for details.
Coating the paper: Use a glass rod or hake brush to coat the paper. Once the sensitizer sets, further coats can show up as streaks, so quick coating is helpful.
Printing: Print until the image appears about a stop darker than you want the final result.
Processing: (1) Wash in distilled water or tap water with a pinch of citric acid added for 5 mins. (2)Wash in running water for two mins. (Can be toned after this point). (3) Fix in 3% sodium thiosulfate solution with .2% sodium carbonate (washing soda) for about a minute, (4) followed by a final wash in running water for 30 minutes. Hang to dry. Ironing the print with a hot iron can increase the contrast slightly and change the color.