I agree with you 100% regarding a 1% solution, but was thinking more of the
health and safety problems that might occur in handling the concentrated
acid while diluting it. I've had rubber gloves leak, even when I'm doing the
right thing, to say nothing of the times I've absent mindedly done something
stupid. Apologies to Pete Frederick if this seems like "nannifying", but if
there is a safer (preferably slightly alkaline) alternative that performs
just as well, it would be preferable.
Maybe it's worth asking about the pH of 5% sodium metabisulfite and how it
compares to 1% sulphuric acid? Maybe a slightly stronger sodium
metabisulphite (or even sodium sulphite or HCA) solution could match the
acid. On the other hand, if as you say, the weak sulphuric acid clears
readily, and at this concentration is indeed totally innocuous to paper, I'm
probably over-reacting.
I don't recall any awful warnings regarding sodium dithionite, but if you're
planning to get some from a major chemical company, Pete, it might be worth
asking for a material safety data sheet first. Apparently it's been
previously recommended by Clerc and others. Forget it if it's nasty, but
it's probably all right, since dithionite (Na2S2O6 * 2H2O) appears to be
related to hypo (Na2S2O3 * 5H2O). Hypo might be worth trying too, unless
I've completely misunderstood their chemical relationship.
Mungo Ponton's notes (and presumably his images) probably didn't survive, or
so the Photographic News (3 Sept. 1880, p. 428) noted shortly after his
death. Incidentally, the PN (20 Aug. 1880, p. 402) noted in in its obituary
for Ponton that his process was popular with schoolboys for making
photograms: "a sheet of paper was furtively washed over with the yellow
liquid under the shadow of the school desk, and exposed to light in the
sunny playground. Fern leaves, lace, copy-book covers, nay, even the human
hand[!], were employed in those days as negatives, the washed and dried
prints being duly put away among other boyish treasures." A few years
later, however, after one of the many flurries in the photographic press on
bichromate poisoning, a small packet labelled the "Boy's Photographer", sold
as a toy for 1d, and consisting of a small amount of potassium bichromate,
was referred to Scotland Yard (PN 18 Sept. 1885, p. 606).
My faded dichromate image was probably produced in the late 1970s (I was
still a schoolboy, albeit in high school), and if I recall correctly was
prepared by coating cartridge paper with dichromated gelatine. The print was
given a layer of wax, a la Alfred Stieglitz, "for added gloss and
brilliance"; and has been stored in the dark ("among other boyish
treasures"), though not in an "acid free" environment. Klaus is undoubtedly
right about the colour being due to chrome green. We'll all be interested to
hear how your more systematic tests go, Pete. Its probably just a matter of
keeping notes of your reflection density readings, labeling and dividing
each test strip, putting half in a sunny spot, and keeping the other half in
the dark.
Philip Jackson
PS Argyrotype instructions are in the archive, and are also available
(perhaps more easily) through the Photoforum web site. Basically, it's an
iron-silver process, developed by Mike Ware, that uses ferric ammonium
citrate (the same sensitizer as most cyanotype processes) and silver
sulphamate (rather than the nitrate generally used for kallitype or van dyke
brown processes).
PPS. Incidentally, Derek, can you confirm that the new kit instructions
suggest washing for 30 min., up 10 min. from Mike's original instructions?
This seems excessive for a plain paper process with no gelatine to retain
the thiosulfate in the paper fibres, but maybe the kit makers have
undertaken extensive tests to justify this new recommendation? Do the kit
instructions make any reference to a hypo clear (it could just be a 1 or 2%
sodium sulphite solution), which should save both time and water?