From: Jeffrey D. Mathias (jeffrey.d.mathias@worldnet.att.net)
Date: 02/05/02-07:28:43 AM Z
Don Bryant wrote:
> In Sullivan and Weese's book, The New Paltinum Print, on page 78 David
> Kennedy and the late John Rudiak report this variation for Ferric
> Oxalate: For each 100 ml of liquid ferric oxalate solution add 18 gm
> oxalic acid and 6 gm EDTA tetra sodium. This is reputed to increase the
> speed of the ferric oxalte without any degradation of print quality.
As Carl has mentioned, this is likely far too much OA and EDTA. Much
information on this can be found in my guide (from link below), remember
to check the studies.
EDTA: the only benefit I have noticed is use as an additive to the
sensitizer to aid in subsequent clearing. Too much can result in
"bleeding" as Carl has mentioned. EDTA can allow the mixing of greater
solution strengths of the sensitizer which could be a benefit if the FO
powder used is very weak (see below).
Oxalic Acid (OA): this has several benefits as an additive to the
sensitizer. I have noticed it to provide better sharpness, and to
"clean up"-"crisp up" the look of a print, and to even increase the
speed a bit (also dependent on other factors). However, the use of too
much will degrade print quality and too much can even harm some papers
also reducing print quality. One should be very cautious about just
adding a bunch of OA to the sensitizer or pre-treating the paper with
OA. Each paper should be studied to find the optimum usage of OA. Also
one should realize that some OA may already be present as an impurity in
the Ferric Oxalate.
Ferric Oxalate (FO):
Don Bryant wrote:
> ...
> Is there a difference in the B&S F.O. powder and that which is obtained
> elsewhere? ...
>
> Is manufacturing F.O. difficult? ...
It is not difficult to make, but one should have some basic chemistry
lab knowledge and skill. One manufacturing method by Viscay is included
in my guide. There can be a substantial difference in various FO
powders. However, the main difference I have found from B&S, Artcraft,
Viscay, and my own is the percent of "active" FO in the FO powder, the
most common impurity being Oxalic Acid. Once the purity percentages of
FO are known, solutions of the same strength based on these percentages
and with the adjustment of OA will provide identical results.
Also, the strength or purity of the FO cannot be determined by the
specific gravity such as suggested by Stevens (unless perhaps the FO is
100% pure) because the impurities such as Oxalic Acid (and others) will
influence the measurement. It is highly unlikely that powered FO can be
produced at higher than 98% purity without very expensive equipment.
An advantage of a more pure FO powder is that a lower solution
concentration can be used. However, allowing the sensitizer solution 24
hours to fully dissolve is still recommended. Using a greater strength
sensitizer will typically not make a difference in the print, however a
weak sensitizer may produce a weak print. The metal solutions should be
balanced to the weakest FO solution that provides the best print; more
will be wasted and less will result in a weak print.
-- Jeffrey D. Mathias http://home.att.net/~jeffrey.d.mathias/
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