As an ex surface coating technologist I have always regarded the
gum/pigment system as a simple paint, and that for best results the
pigment must be thoroughly dispersed in the gum. Could I suggest if you
can get hold of a copy, having a look at 'Surfacing Coating Technology'
by Henry Fleming Payne. It's probably out of print by now, but if my
aged memory serves me correctly in Vol. 1 there is quite a lot on the
mechanics of pigment dispersion. It may be of help in your research.
>4. I am using Cranes Platinotype and don't even know if it is a good gum
>paper.
>
I've been thinking about casting gum films (using a meyer bar) onto
Mylar which has been treated so that it will accept an aqueous coating.
This would eliminate any variables introduced by differing papers.
What's your view on this?
>6. This is a 8:1 ratio of dichromate to gum where many recipes call for a
>2:1, or 1:1 and this is ammonium dichromate!
That is most interesting! Kosar says that although the relative
sensitivity is almost proportional to dichromate concentration, the
limiting factor is determined by the dichromate concentration that
causes formation of dichromate crystals in the dried film, with
dichromate crystals on the surface hardening no longer takes place.
Now he's talking about unpigmented colloids. Could it be that the
presence of pigment prevents or reduces this effect? Whatever it is, It
certainly seems as though you are on to something!
(He also goes on to say that recrystallisation of the dichromate can be
prevented by incorporating Ferric Oxalate into the film!)
>7. These are very preliminary observations and for some of you I may be
>just re-inventing the wheel, but I have seen none of this in current gum
>literature, so I figured it is worth persuing.
>
>Comments welcome.
>
>
BRAVO! Keep up the good work!
Alex.
-- Alex Nanson alec@norlex.demon.co.uk Gloucester. UK