From: Jeffrey D. Mathias (jeffrey.d.mathias@worldnet.att.net)
Date: 12/24/00-09:43:26 AM Z
Sandy King wrote:
> ... I mixed
> a Ferric Oxalate solution this evening, 100ml of water at room
> temperature to 20g of Ferric Oxalate powder, 20%. A lot of the Ferric
> Oxalate did not go into solutions. Is this normal?
Yes, It can typically take 24 hours to completely dissolve the FO with
intermittent shaking. It is recommended to NOT heat the solution. I
have also found as Joe Portal mentioned that heat may convert the ferric
to ferrous.
A couple people mentioned 20 minutes to dissolve (whether with heat or
stiring). I would caution that this may not be enough time to actually
completely dissolve the FO into solution; go the 24 hours to be sure.
Joe Portale wrote:
> Yes, there is a simple test. You need a 5% solution of potassium
> ferricyanide (sp?). Take a piece of filter paper or a white coffee filter
> and place a couple of drops of your ferric solution on it. Wait a couple of
> seconds for the solution to soak in. Then add a drop or two of the cyanide
> solution. If your oxalate is good, you will have no or only very pale blue
> coloring. If the more ferric present, the darker the blue. If you are
> getting anything more than a pale blue, you are in trouble. ...
The FO is bad even if a very small amount of pale blue color is
present. Prints will have fogged highlights, likely because they (due
to the FO) are fogged. Use an appropriate safelight when mixing and
working with FO.
Much information as to testing, mixing, and working with FO can be found
in the guide on my web site (from link at end of post).
> ... You may be able
> to salvage you ferric oxalate solution by adding, drop by drop, small
> amounts of hydrogen peroxide. ...
I have not seen any evidence or a paper on the salvage of FO, even
though this claim has been made before by others. If done, test
carefully.
> ... I believe the saturation point of ferric oxalate in around
> 55 to 60%.
That sounds more like the saturation point for Ammonium Ferric Oxalate.
FO will be hard to get dissolved at a concentration greater than 27%
without the addition of additives. The addition of oxalic acid and
especially EDTA can be used to achieve higher solution strengths, but I
doubt if the solution strength of FO will get to 55%.
Carl Weese wrote:
> Small amounts of oxalic acid and EDTA (I use .5 gram each when making
> 50ml of ferric oxalate) are helpful and actually beneficial to the
> solution when printing in Pt/Pd.
My tests of observing resultant prints when varying materials showed
that benefits are maximized when the amount of oxalic acid in the
sensitizer solution is from 3% to 5% and the amount of EDTA
(CAS:60-00-4) at 0.4% (Refer to studies in my guide for reasons.)
When making 50 ml FO these amounts would be:
1.5 to 2.5 grams oxalic acid (assuming pure FO)
0.2 grams EDTA (more can promote a bleeding effect of the dark areas of
the print with some papers)
Note that the oxalic acid may be reduced if the FO already contains
some. Solutions with greater than 5% FO showed a loss of quality and
definition. The oxalic acid promoted better definition, sharpness, and
crispness at levels over 3%. Most FO powders contain some extra oxalic
acid, so that amount should be considered to keep the total oxalic acid
below 5%.
-- Jeffrey D. Mathias http://home.att.net/~jeffrey.d.mathias/
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