Whatman Watercolour paper was coated with new cyanotype sensitizer (this
uses ferric ammonium oxalate rather than the citrate).
The paper was cut into four test strips which were separately exposed for
two minutes using the same step wedge and Cosmolux UV 40W fluorescent tubes
with internal reflectors. Unfortunately owing to operator error with the
timer (I forgot to reset the counter) one strip was slightly underexposed.
The test strips were rinsed in four different solutions for 10 minutes:
1. 2% sodium chloride (iodised table salt with an anticaking agent, if that
matters)
2. 2% citric acid (I bought 500 gm at a health food shop)
3. distilled water
4. Canberra tap water (used the underexposed strip, so this isn't really
valid).
The strips were then further washed in tap water (no hydrogen peroxide) and
air dried.
I'd say there's very little difference between distilled water, tap water,
and the salt solution (salt water might be just barely better), but the
citric acid does show significantly darker blues.
The citric acid solution does need frequent changing with this paper (maybe
I overcoated or didn't use enough solution - some of the prussian blue
seemed to be floating off in the water and staining the whites, especially
the back of the sheet).
As they say, your water supply may vary, but as per Mike Ware's article on
new cyanotype (Ag+ Photographic, vol. 7 (1995) p. 79), a 1 or 2% citric acid
wash aid for cyanotype is definitely worth trying. It doesn't have to be too
exact - 1 level teaspoon weighs 5 gm which is about right for every 250 mls
of water; 2 level tablespoons are about 20 gm which is rough enough for a
litre.
Philip Jackson (pjackson@nla.gov.au)
Canberra, Australia (where they took the fluoride out of the water supply
for awhile, after an enormous public outcry, but have since snuck it back in
again, though at a reduced rate).
PS. I'll be off the list for a few days holiday shortly.