I use tannic acid for the same reason Loris mentions...excessive staining of the highlights. I've been working on a process that suits me and taking lots of notes. There is a lot of conflicting information and misinformation out there. Mike Ware is a good source for correct info. So much cyanotype literature has been published by people who have no concept of the chemistry and/or take sloppy notes.
Here are my steps:
1) use original cyanotype process (I haven't yet tried Mike Ware's Cyanotype II)
2) coat Arches Platine paper two times for increased shadow density in final print
3) add stop bath to adjust pH (I have an electronic meter) of wash bath to 5.5-6
4) wash print 3x in new water from step 3
5) wash in dilute solution of hydrogen peroxide
6) wash print in fresh water from step 3
6) soak in tannic acid solution (1g/100ml) for 5-10 minutes, agitate occasionally
7) prepare sodium carbonate (washing soda) bath with pH around 8-9
8) agitate print in sodium carbonate bath until just BEFORE print reaches the desired tone (blue steel -> reddish is the transition)
9) immediately move to acidic wash bath of step 3
10) wash thoroughly
This 'split toning' gives you the best control over final color. The acid bath prevents loss of highlights and density during the wash process. The peroxide fully develops the iron blue salt so that the tones you see are the final tones. The tannic acid keeps the stock white.
I am working on integrating Gallic acid to add purple/black to the image but I am having trouble with the iron gallate bleeding and destroying sharpness. A few more months of experiments and I'll have something I am happy with.
Last edited by amuderick; 01-18-2009 at 10:18 PM. Click to view previous post history.