I had some samples from various types of prints subjected to an accelerated
ageing test a year or two ago, and the cyanotypes did fade very slightly (and
permanently) but it was not really significant.
Anna Atkin's work that I've seen from over 150 years ago is still looking
good.
Of course cyanotypes need to be kept away from alkalis. They are I think
stable to acidic substances that are more often a problem (for example in the
breakdown of some papers and in atmoshjperic pollution.)
We have had considerable discussion on Kallitype and Van Dyke stability in the
past. I suspect that some easily overlooked factors such as the pH of the wash
water used may have a noticeable effect on the Van Dyke.
With Kallitype I would also expect those which have used alkaline developers
to be significantly less stable than those made using acidic developers; there
are so many variations on the process that generalisation is probably unsafe.
Also the toning used will make a difference.
Gold toned (or platinum toned) salt and albumen prints may well have similar
life expectancy to other toned silver processes, but the storage conditions
are important here. The main problem I've had personally with silver gelatin
prints is mould growth on the gelatin. I'm told this may be a sign that I've
washed out the fixer too well, as well as of the dampness of my home.
Platinum and carbon are generally though to be pretty permanent - though I
suppose you could do carbon with impermanent pigments if you wanted to. Some
prints are of course made on paper that won't last well. Gum again depends on
the pigments used. Gravure also is a good bet for permanence.
Your comments on style reinforce what I said about there being differences in
language trans-atlantic - and agree with my assessment of these.
I've always lived with assertive women (and worked with various of them as my
boss) and have few problems with them, though I do occasionally lay down my
naturally submissive nature and assert a little back.
Peter Marshall
On Fixing Shadows and elsewhere:
http://faraday.clas.virginia.edu/~ds8s/
Family Pictures, German Indications, London demonstrations &
The Buildings of London etc: http://www.spelthorne.ac.uk/pm/