There are a myriad of different blue print formulae: virtually every ratio of ferric ammonium citrate to potassium ferricyanide has been tried at one time or another. Early on, the brown form of ferric ammonium
citrate was used, but today the green form is used exclusively as it is more sensitive and produces a longer tonal scale. Most of the ferric non-silver processes, other than cyanotype, have fallen out of use, but I
provide them here in case anyone should wish to experiment with them.
The two cyanotype solutions keep indefinitely, stored in brown glass bottles. Bacteria tends to grow in Solution A. Simply filter it before use. Coat with a rod or brush. Double coating is recommended for
greater depth. Air dry or use very low heat. Almost any paper can be used, but best results are had with non-buffered papers (i.e., papers that are still somewhat acidic) such as Arches Platine, Crane’s
Platinotype, or Whatman’s. Cyanotypes must have an acidic environment, or they will fade, so they are generally mounted on non-buffered mount board.
Negatives should be somewhat contrasty, in the 1.2 to 1.4
density range. Exposure is made in sunlight or under an ultraviolet lamp. Print very dark, as the image is reduced considerably in the wash. Wash in tap water.
Toning is possible. Though not necessary for permanence, toning can give interesting variations on print color. A 5% solution of lead acetate is said to give a deeper ultramarine color or sometimes a grey or violet
color (at longer toning times), though the solution is quite toxic. Tea and coffee are also used as toners. A teaspoon or two of instant coffee in a liter of water is my favorite toner. Christopher James has a
lengthy section on toning in his book.