From: Jeffrey D. Mathias (jeffrey.d.mathias@worldnet.att.net)
Date: 02/03/00-10:20:54 PM Z
Optimizing For Maximum Quality in a Pt/Pd Print
Partial results of a study in progress.
The single most important factor affecting the quality of a platinum
palladium print seems to be the amount of metal actively placed per unit
area. This factor is dependent on the materials used, the coating
technique, the coating efficiency, the ambient humidity during coating,
substrate preparation, and likely other parameters. There are other
important factors critical to producing a quality Pt/Pd print. However,
this study centers on determining the threshold of metal actively placed
per area at which point no additional metal will noticeably improve the
quality of the print.
This study has some interesting history which may help in understanding
the Pt/Pd process. After some preliminary threshold studies an interest
emerged to study the ferric oxalate sensitizing agent. It was found
that ferric oxalate powders from various manufactures were not identical
and were not 100% pure. A study was initiated to compare the various
ferric oxalate (FO) powders. It was found that measurements such as
specific gravity and pH were useless in analyzing and comparing the FO
powders. These measurements were too sensitive to the various
impurities that were present or added to the FO. A decision was made to
compare the FO powders by way of the prints that they produced.
Comparison was made by finding the threshold for each of the FO
powders. With all other things kept constant, prints were made from an
assortment of varying strength solutions of sensitizer and the
corresponding metal solution. Previous study had established and
verified that the metal and sensitizer worked in a one-to-one ratio.
After some preliminary work, it was found that some critical parameters
had to be carefully controlled. These parameters included the coating
technique, the coating efficiency, the area coated, and the measurement
of solutions.
And then a paradox emerged, what to control and when and where to start
when everything was interconnected. The decision was to start with
coating technique. Since I have a consistent coating technique mastered
by practice and refinement over 14 years and thousands of prints, it was
decided to start with this coating technique as a base to which all
other parameters would be adjusted. A most critical relationship is
that coating technique directly influences the coating efficiency and
the coating coverage.
Coating efficiency is the percentage of chemistry that goes from the
mixture into the paper. Coating efficiency is described in my "Guide to
Platinum Palladium Photographic Printmaking" (Guide). The coating
efficiency will provide one with the total volume of coating mixture
that must be used to coat a given area on a particular substrate.
Coating coverage is the amount of mixture needed to coat a given area.
The volume of coating mixture typically consists of half sensitizer
solution and half metal solution. Optionally a contrast agent may be
added, however for this study contrast agents were not added. It was
decided to use a metal solution typical of much of my past work, so a
solution of 5 parts K2PdCl4 and 2 parts K2PtCl4 was used throughout.
The strengths of the metal solutions were that appropriate for the
strength of the sensitizer (at full potential). These can be calculated
from the metal solution calculator (or tables) in the Guide. The
sensitizer was mixed at several strengths which were previously found to
bracket the threshold.
But now a critical finding. Drops were used to make the coating
mixture, however drops were not the same size. Even when using the same
dropper, the drop size of various materials differed. It was decided to
not worry about where differences came from, whether the droppers or the
materials. Each solution, using its dropper, had the number of drops
counted to fill a 10.0 ml graduate accurate to 0.1 ml. The ml per drop
was calculated. Solutions were mixed by calculating the ml needed and
using the closest amount of individualized drops (this is likely
accurate enough for this process). More coating mixture would be made
than necessary and then another calibrated dropper was used to deliver a
consistent volume of coating mixture to the substrate, that volume
having been determined by coating efficiency and technique. All prints
had an identical volume of a known coating mixture coated into a defined
area. (Understand that this is beyond the accuracy for routinely making
prints, but necessary for calibration type work.)
The area was chosen to be 5 inches by 5 inches to fit a 4x5 negative and
a 21-step.
The substrate chosen was Crane's lot# 5302 paper.
Four FO powders were used as sensitizer. They included the following:
+ ultra pure FO powder made by Vicente Vizcay Castro (Vizcay);
+ FO powder made by me following the Vizcay instructions in my Guide
(JDM);
+ FO powder made by Bostic & Sullivan (B&S);
+ FO powder from Artcraft (via Eric Neilson) (Artcraft).
Several sets of the sensitizer was or will be prepared as follows.
+ 2% EDTA was added to get the FO into solution at 30% for the first
set.
Note: EDTA used is ethylene diamine tetra-acetic acid
Synonyms: EDTA; Complexone II
Molecular Formula: C10H16N2O8
Molecular Weight: 292.25
CAS: 60-00-4
Purity Grade: pure
+ nothing added, if completely in solution at threshold strengths.
+ 3% oxalic acid added, previous study indicated some sharpness increase
with 3% to 5% oxalic acid.
+ 2% EDTA and 3% oxalic acid added.
NOTE: Only the first set has been accomplished at this time with
solution strengths from 30% to 14% at 1% intervals.
Another consideration affecting the solution strengths was discovered.
This was that some types of the sensitizer required a different amount
of water to be added to bring them to an identical final volume. This
work is rough and will be repeated and verified, but preliminary results
showed that Vicente & JDM sensitizer required 5% more water than Bostic
& Sullivan or Artcraft sensitizer to produce the same final volume. The
reason for this is not known.
The following are preliminary results (rough) of drop comparisons (using
the same dropper).
Sensitizer (30% with 2% EDTA, 63F) volumes per drop are 83% to 89% that
of water (63F).
K2PdCl4 (20.2%, warm) volumes per drop are 96% that of water (63F).
K2PtCl4 (25.7%, warm) volumes per drop are 80% that of water (63F).
Preliminary results show that the ultra pure FO powder by Vizcay does
contain the most active FO and can reach a threshold at a lower solution
strength than the other powders. The threshold can be a viable
measurement for evaluating a FO powder. Preliminary results also show
that each of the FO powders tested can produce the highest quality
print, identical to the others, however different solution strengths
must be employed (different thresholds).
The indicator to describe the threshold has been chosen as the solution
concentration of the FO in the sensitizer, assuming that the FO powder
is 100% pure. The reason for this is that there has been no method or
measurement to accurately determine the actual active amount of ferric
oxalate present. However, it is justifiable to assume that in an actual
print what is seen is in direct relative relationship to any active
ferric oxalate present. It is assumed that any active sensitizer is
ferric oxalate, and really does not matter if it is not as the results
in the print either occur or they do not.
Understanding that no absolute determination of a sensitizer can be
made, a relative comparison of different resulting thresholds can
provide a relative comparison of the active portions of various FO
powders. Likewise a relative comparison can be made of sensitizer
additives or variations in other parameters.
The threshold data thus far for sensitizer solutions made with FO powder
with the addition of 2% EDTA are as follows.
FO powder threshold
Vizcay 22-23
JDM 24
B&S 25
Artcraft 24-25
This means that the maximum quality print (for all conditions above)
would be produced by a solution of 22%-23% Vizcay FO powder, which would
be identical to a print produced by a solution of 25% B&S FO powder.
And indeed all prints above the threshold are identical independent of
the FO powder used. Now for relative comparison, each FO powder can be
assessed relative to the Vizcay FO powder which is claimed to be of 98.0
- 99.5 % purity by Vizcay. If a linear relationship between purity and
threshold is assumed, purity for the other powders can be calculated.
FO powder purity
Vizcay 98%
JDM 90-94%
B&S 86-90%
Artcraft 86-94%
Note that previous study ("Comparisons of Ferric Oxalate Powders" in my
Guide) had suggested a Vizcay purity of 98% and a B&S purity of 89% when
studying the amount of oxalic acid needed to promote complete
solubility. The data is coarse, but results in-line and realistic.
It is not known yet how the addition of material to the sensitizer will
alter the threshold.
Some other observations indicate that the threshold has been lowered
significantly by coating at a lower relative humidity (see Note below).
A recommendation is to mix solutions one point above the threshold.
This will allow a margin of assurance. However, keep in mind that
stronger solutions will not only waste material, but the metals may
require warming, and prints may be more difficult to clear (see below).
It is expected that the metal solutions may only have to be mixed to
that of a 100% pure FO and only the strength of the sensitizer solutions
increased. However this has yet to be studied and determined. For now,
it is recommended to match the metal solution strength to the sensitizer
as this is what has been demonstrated. WARNING: Use of the threshold is
only valid for a given coating technique, coating efficiency, and
coverage area as well as other parameters. If one has a different
coating efficiency, then their threshold and sensitizer solution
strength will be different. (See my Guide for how to determine various
coating parameters.) If one coats more mixture into the area, then the
threshold will be lowered (requiring weaker solutions). If one coats
less mixture into the area (same as coating into a larger area), then
the threshold will be raised (requiring stronger solutions).
NOTE: Relative humidity may be an important parameter. All of the above
prints were made at 20% to 25% relative humidity. Previous prints made
to determine thresholds were made at 34% RH and resulted in thresholds a
point or two higher. Although volumes were not measured as accurately,
certainly this is a concern to be investigated. It is interesting to
note that prints made from sensitizer solution strengths of 18% or less
exhibited the "solarazation" effect. There may be a possible connection
between this and that of "solarization" effect related to high RH. (The
effect does not seem to be true solarization.)
All of the procedures, data, and results will be written up in my Guide
once the sets of this study indicated above and repeat and follow up
studies are finished.
Some further issues follow.
As study has also been progressing on clearing. It is interesting to
note that lower strengths of coating mixtures clear better & quicker.
Much more is to come as various sensitizer additives and clearing agents
are being investigated.
Also a thought on the talk that some put forward that ferric oxalate is
an undefined material and may have questionable results. Such talk and
speculation is likely a moot point as this study has thus far
demonstrated highly repeatable, consistent, and predictable results of
the ferric oxalate used (all 4 of them). The proof is in the prints.
More to come later.
-- Jeffrey D. Mathias http://home.att.net/~jeffrey.d.mathias/
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