The recent correspondence on kallitypes, which developed into a discussion on
how to identify platinum prints, gave rise to new questions.
Although we did conclude, over our foie gras, at the al fresco lunch of list
members beside the Seine in Paris in Spring, on Saturday, that if one were to
drop TNT onto a print, and it were to explode, one could be fairly certain that
it was a platinum print.
But, to me, the most significant question that remained unanswered was the use
of chelating agents in clearing prints made using the iron processes. The
difficulty related first to the development of iron stains, sooner or later, in
certain of these processes and secondly to the need to avoid the use of acids,
such as HCl, as clearing agents given their deleterious effect on the cellulose
in the paper.
In my experience this is not a problem that arises with the two popular
processes based on the use of the iron salt ferric ammonium citrate.
Cyanotypes, when using negatives of the right density and given the right
exposure, give clear highlights and an exquisite range of tones and freedom from
fading.
Kallitypes based on ferric ammonium citrate also, when fixed and washed
properly, give a very fine range of tones without staining or fading at least
in the fifteen years that I have been making them. When mine have faded it has
usually been vey quickly, within days, and I have concluded that I had been
careless in my processing.
I mention these two first as the clearing agent for the cyanotype is water and
the the brown kallitype, hypo and water.
Incidentally, colour, pinks and greens, can be introduced into the brown
kallitype by very slight variations in the processing.
Iron staining, again in my experience, only enters the equation when the
oxalates are introduced to the chemistry. Did not Herschel use water as the
clearing agent for his chrysotypes ?
So the first question which arises is whether it is the use of oxalates that
gives rise to the staining problem. This staining has been evident in pt,
pt/pd, black kallitypes and chrysotypes.
As to the chrysotype those I have made using ferric ammonium citrate as the salt
have never stained. But staining has occurred with both my own and those of
other people when ferric oxalate has been used as the light sensitive salt and
potassium oxalate as the developer.
This problem has not arisen in my own chrysotypes when I have added tiny amounts
of pt to the sensitiser. In my own case the clearing agent used was EDTA
tri-sodium salt.
Similar problems seem to arise when silver is substituted for the gold. Not
finding the colour of kallitypes sensitised with ferric oxalate and developed
with the classic recipes, particularly pleasing, I substituted silver nitrate
for the potassium chlorplatinate in the classic process. The results were really
quite exciting. But EDTA tri sodium salt does not appear to be as effective as
one would wish as a clearing/chelating agent.
Similar problems arise with pt/pd and pt and pd prints I have made myself and
those I have seen of others
Given the need to avoid the use of strong acids, even in very weak solutions, as
clearing agents; and the need for impractical amounts of weaker acids, eg citric
acid to achieve the desired effect, apart from any possible deleterious effects
these may have on the paper, and the apparent ineffectiveness of EDTA tri-sodium
salt as the chelating agent, the question arises as to the need for a clearer
understanding, at least on my part, as to which chelating agents are the most
effective in removing contaminating iron and other salts from the print so that
it does not either stain or fall to pieces.
Do we need separate chelating agents to rid the print of both the residual iron
and potassium salts and is EDTA tri-sodium salt the most effective.
Terry King