From: Sandy King (sanking@clemson.edu)
Date: 07/23/02-08:15:21 AM Z
I hope that some of the chemists on the list can offer some
suggestions on a problem.
Background. Sometime in January or February of this year I asked for
suggestions on getting more contrast with VDB. The classic VDB
formula as I mix it has a very long ES of about 2.45, with the result
that most of my negatives print fairly flat. As noted earlier, adding
dichromate to the sensitizer or to the water during development, does
not really increase contrast.
In response to my question Liam Lawless sent me the folling note.
>Sandy,
>
>I've done a quick experiment and think the VDB scale can be shortened by 3
>or 4 steps, without increasing printing times or diminishing D-max (though
>results appear to be somewhat variable on different papers). To compare the
>result with straight VDB, have a look at
>http://www.artphoto.pwp.blueyonder.co.uk/vdb.htm/
>
>Instead of 9 g of f.a. citrate in the "A" solution, I used 3 g f.a. citrate
>plus 6 g ferric citrate. This is a very fine brown powder that looks like
>powdered rust, and is not very soluble; a sludge forms at the bottom of the
>mixing vessel. Increasing the tartaric acid to about 4 g in "B" seems to
>help dissolve it, but some still remains. It can be filtered out, or just
>take care not to shake it up before using the sensitiser. The "C" solution
>still uses 3.8 g silver nitrate; the modified sensitiser has a paler colour
>but coating and processing are as normal.
I have found that the mixture of ferric citrate would ammonium ferric
citrate at the ratio suggested by Liam does indeed result in a
shortening of the VDB scale by about 3-4 steps as Liam suggested.
However, it is *very* difficult to get the ferric citrate to go into
solution. Liam initially mailed me a few grams of ferric citrate in a
fine powder form which took a couple of weeks to go into solution.
However, I later purchased some Ferric Citrate in granular form that
has so far resisted all of my efforts to go completely into solution.
I have heated the solution and left it standing for days on end but
there is still a significant part of the ferric citrate that settles
to the bottom of the bottle. Does anyone have a suggestion as to how
to make ferric citrate go into solution at the ratio of about 6-7g of
ferric citrate per 35ml of water?
Sandy King
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