<< >Thanks for the correction Luis. This is a good illustration of the dangers
of
>relying on secondary sources - owners of Cassell's Cyclopaedia (1911) may
like
>to correct the entry on page 314 which gives the incorrect date!
And they may wish to enrich their library with a better Encyclopedia;-) >>
Luis
Just the first one that came to hand - by no means the only one I have.
However it is a useful source as to the state of the art in 1911 and certainly
a fascinating read. Well worth buying if you come across a copy at a
reasonable price.
I'd have had to walk another 2 metres to pick up various other volumes. One is
Clerc's 'Photography, Theory and Practice' which also has some possibly
relevant information to offer. Apart from telling me that 'Sepia' paper was
introduced by H Shawcross in 1889, and it also gives a formula produced by
mixing green ferric ammonium citrate with ammoniacal silver nitrate (E.
Valenta, 1899).
For both of these it suggests the use of alternatives to a standard fixer. The
first gives a more opaque image (for use as a paper negative) by 'fixing' in a
15% solution of hydrated sodium sulphite (or 7.5% anhydrous) and for the
second it suggests a 3 to 5% hypo solution to which 1 to 2% of anhydrous
sodium sulphite has been added.
Either could well be a better solution for use with the related VDB process to
avoid either bleaching of the highlights or deposition of iron(III) compounds
in the paper fibres.
Forgot to mention before that chemists will possibly be interested in the fact
that it was Dobereiner (better known for his triads) who first discovered the
light sensitivity of ferric oxalate around ten years before Herschel's work.
Peter
Peter Marshall
On Fixing Shadows, Dragonfire and elsewhere:
http://faraday.clas.virginia.edu/~ds8s/
Family Pictures & Gay Pride: http://www.dragonfire.net/~gallery/
and: http://www.speltlib.demon.co.uk/