I really like the blue colors of cyanotypes.
I use (and here are the german names)
- in destilled water:
- Kaliumhexacyanidoferrat(III (red)): 16g /100 ml
- Ammoniumeisen(III)-citrat (green): 25g/100 ml
The solutions, once mixed 1:1 are UV sensitive (300nm). I coat the paper under red light or weak daylight.
Under UV-Light the Eisen(III) im Ammoniumeisen(III)-citrat is reduced to Eisen(II):
Fe3+ + e– -> Fe2+
In the waterbath this goes in solution and meets the Kaliumhexacyanoferrat, building the not water solubale and nice Prussian Blue („Berliner Blau“), a deep blue.
4 Fe3+ + 3 [Fe(CN)6]4- -> Fe4[Fe(CN)6]3
I use aquarell paper to coat it with the solution, dry it in the dark.
Foto Paper (inkjet) works well, too. The „resolution“ is better. But I like the texture of the aquarell paper much more than the smooth (imho boring) perfection of the inkjet „paper“.
I prefer the weight of 300g or more for my paper.
The choose of paper is one of the most important things to get your desired outcome.
Wood is working mostly fine.
I need about 3min to 5min of sunshine exposure in the summer, 10 min in the evening.
Cloudy sky is very variable…
If you use UV light sources you have to test them. I need abaout 30 min with a 365nm 50W UV light source at 25 cm distance, depending on paper, negative and solution.
It is working better than my previous tried 395nm source.
Some guys stop the reaction with a bit of citric acid (Zitronensäure) in a short stopbath, i do not.
The blue builds with time and darkens considerably, you can quicken the reaction with a bit of H2O2 Hydrogen peroxid (Wasserstoffperoxid) in the water. I just wait while the paper is drying.
Developing in pure vinegear gives more tonal range.
The paper needs than to be rinsed or bathed in water about 5 min.
If the prints lighten up over months or years you can store them in the dark to regenerate or give them a short bath in thin Hydrogen peroxid solution to freshen the blue color up.