From: Jeffrey D. Mathias (jeffrey.d.mathias@worldnet.att.net)
Date: 04/05/00-10:55:20 AM Z
Study of the clearing of Pt/Pd prints
This study has documented some very significant findings concerning the
clearing of Pt/Pd prints. This study makes use of the revised clearing
test (in my Guide to Platinum Palladium Photographic Printmaking)
involving the use of Potassium Ferricyanide as an indicator. Thanks to
John Melanson for suggesting the use of this indicator.
Several past observations have been made and verified and can be
considered general trends concerning clearing.
Most thin papers clear better and/or faster than thicker papers.
POP type prints using Ammonium Ferric Oxalate sensitizer may clear in
about half the time of DOP type prints using Ferric Oxalate sensitizer.
This from previous work and not investigated further in this study.
Prints made with a lower solution concentration of the coating
ingredients will clear slightly faster. However, it must be remembered
that a certain threshold concentration must be used to avoid weak
prints. This from the Threshold Study.
The most significant finding with this study is that an addition of
Oxalic Acid and EDTA to the sensitizer solution dramatically reduces
clearing times. (See the procedure and results below for details.)
Only single agents were used for this study. It is suspected that a
series of multiple agents might provide improved clearing, however the
possibilities of combinations are numerous and may become a mute point
at the conclusion of this study.
CRITICAL CONSIDERATIONS:
It is most important that the following be considered carefully as they
can have a dramatic influence on clearing investigation. All relate to
fogging.
+ A safe light MUST be used. Work lights must be tested to assure that
they will not fog the coating. Sodium vapor lights are an excellent
choice.
+ Coating brushes should be kept clean. Old sensitizer material can get
into the print causing a fogging effect. Brushes should be rinsed well
in distilled water and squeegeed dry with a paper towel after each use.
Brushes should then be stored in a light tight box or bag.
+ Fresh developer should be used. Developer that has been used for many
prints may contain exposed material that will react with coatings
causing a fogged appearance. This was a culprit in this study resulting
in a lack of clearing in a reasonable time. When new developer was
used, the repeated tests showed dramatically improved clearing. It
should be routine practice to replace the developer rather than
continuously replenish.
+ Cross contamination can be a problem. The clearing indicator of a 10%
solution of Potassium Ferricyanide proved to be far too sensitive.
Uncoated samples were processed along with the other samples and
although nothing could be detected without the indicator, the indicator
would show some blue. Cross contamination is suspected to occur from
used developer, rinse, clearing baths, and especially wash water. When
samples were processed individually with fresh solutions and wash water,
the false indications did not occur. However, it was not practical to
process hundreds of samples with individual fresh baths and washes.
Thus, a 1% solution of Potassium Ferricyanide was prepared and applied
by smearing a little on with a clean, fresh Q-tip. (If a Q-tip with
indicator solution touches an uncleared sample, it can produce false
blue indications on a cleared sample. Also it is recommended to use a
dropper to drop indicator solution onto the Q-tip, without touching. Do
not dip the Q-tip into the solution. Also any trace of metal in the
Potassium Ferricyanide solution may give a false blue color.)
Evaluation was made relative to any indication on uncoated samples.
This cross contamination is not critical for individual prints, but
should be a consideration for producing archival prints. The final
clearing bath and wash water should always be fresh. Also it must be
remembered that the clearing indicator can be so sensitive as to give
false indications and should be checked and referenced with a uncoated
control.
MATERIALS:
One paper was used, Crane's paper (lot# 5302). This is a thick and
traditionally long to clear paper (typically 40 to 60 minutes). Other
papers may have different clearing times and/or respond to various
clearing agents differently.
Several clearing agents were tested. (water was 0.5 um filtered tap
water)
Code (used for this study); Agent; Description
P; Phosphoric Acid (H3PO4); 2 ounces of 85% reagent grade per 1 gallon
water
C; Citric Acid; 20 grams per 1000 ml water (2% solution)
S; Sodium Sulfide; 20 grams per 1000 ml water (2% solution)
F; Sprint Fixer Remover; label directions (1 part plus 9 parts water)
E; EDTA(Na4); 40 grams per 1000 ml water (4% solution)
W: water; 0.5 um filtered tap water
Note that HCl (Hydrochloric Acid) was not included because previous use
has demonstrated that it can bleach the highlights of the print and
mainly because I did not have any at the moment. However, the HCl may
work fine (without bleaching) at the short clearing times this study
demonstrates and should not be dismissed.
Potassium Meta Bisulfite worked well but gave off too much sulphurous
fumes so was eliminated from most of the tests and is not reported.
Several Ferric Oxalate powders were investigated. However, results were
a factor of the additives rather than the powder used. The Vizcay
powder was selected as its composition and purity is best known. The
addition of Oxalic Acid and EDTA made a significant difference. 3%
Oxalic Acid was included because previous study indicated an advantage
of having from 2% to 5% and I typically use 3%. 3% EDTA was selected
being the same concentration as the Oxalic Acid. When used together, 1%
EDTA was found to be adequate.
Sensitizer Code; Description
V; Vizcay Ferric Oxalate at 24% (Vizcay's preparation procedure is in my
guide)
VO; Vizcay Ferric Oxalate at 24% plus Oxalic Acid at 3%
VE; Vizcay Ferric Oxalate at 24% plus EDTA at 3%
VOE; Vizcay Ferric Oxalate at 24% plus 1% EDTA plus 3% Oxalic Acid
(VOE = 24.00 g FO + 1.00 g EDTA + 3.00 g Oxalic Acid + H2O to make
100 ml)
Note: EDTA used was ethylene diamine tetra-acetic acid
Synonyms: EDTA; Complexone II
Molecular Formula: C10H16N2O8
Molecular Weight: 292.25
CAS: 60-00-4
Purity Grade: pure
(This was NOT the same EDTA(Na4) used as a clearing agent. The
EDTA(Na4) tested for clearing was some sent to me by John Melanson.
John may have more info as to the source.)
The metal salt solutions were K2PdCl4 (16.2%) and K2PtCl4 (20.6%) in a
ratio of 5 parts Pd and 2 parts Pt. The strengths of these solutions is
that calculated to work with the 24% FO sensitizer strength.
PROCESSING:
All samples were processed as follows:
coating solution mixed
brush coated (with coating edge indicated and as instructed in my guide)
dried
identified with marks and then cut into samples
1 minute in Potassium Oxalate (developer and all baths at ambient
temperature)
2 minutes in water
# minutes in clearing agent
5 minute wash in water
dried
evaluated
Potassium Ferricyanide (1% solution) applied (smeared on with clean
Q-tip) to part of coated area
exposed for 12 minutes with UV lamps (about double typical printing
time)
10 minute wash to remove all yellow from Potassium Ferricyanide
dried
evaluated
NOTE: If one uses a different developer or different conditions, those
should be used for the test.
IDENTIFICATION & NOMENCLATURE:
Samples are referred to as X# where
X = the code letter of the clearing agent
# = number of minutes in the clearing agent
Observations of the evaluations are coded as follows:
For areas without Potassium Ferricyanide:
x = no sample
0 = identical to uncoated paper
1 = very faint gray color (the slightest distinction)
2 = light gray color
3 = gray color
4 = light brown or light yellow-brown color (may include gray)
5 = brown or deep yellow-brown color (similar to coating color)
For areas with Potassium Ferricyanide:
no code = no change
a = faint blue color
b = blue color
Note: Consideration should be given for unnecessary cross contamination
and any "0a" rated samples might be considered completely clear.
For example:
3b - means the coated area was a gray color and the area with Potassium
Ferricyanide was a blue color.
1 - means the coated area was only a very faint gray color and the area
with Potassium Ferricyanide showed no change.
OBSERVATIONS:
samples = observations
For V sensitizer (FO only):
P5, P10, P20, P30, P40 = 5b, 4b, 3b, 2b, 1a
C5, C10, C20, C30, C40 = 5b, 4b, 3b, 1b, 1b
S5, S10, S20, S30, S40 = 5b, 4b, 3b, 2b, 1a
F5, F10, F20, F30, F40 = 2b, 1b, 1a, 0a, 0a
E5, E10, E20, E30, E40 = 4b, 3a, 2a, 2, 2
W5, W10, W20, W30, W40 = 5b, 5b, 4b, 4b, 4b
For VO sensitizer (FO + OA):
P5, P10, P20, P30, P40 = 2b, 1b, 1b, 1b, 1a
C5, C10, C20, C30, C40 = 1b, 1b, 1a, 1a, 0a
S5, S10, S20, S30, S40 = 2b, 2b, 1b, 1b, 1a
F5, F10, F20, F30, F40 = 1b, 1b, 0, 0, 0
E5, E10, E20, E30, E40 = 1a, 1a, 1a, 0, 0
W5, W10, W20, W30, W40 = 3b, 1b, 1b, 0b, 0b
For VE sensitizer (FO + EDTA): (Note change in times)
P5, P10, P15, P20, P30 = 1a, 1a, 1, 0, 0
C5, C10, C15, C20, C30 = 1a, 1, 1, 0, 0
S5, S10, S15, S20, S30 = x, x, x, 1a, 0
F5, F10, F15, F20, F30 = 1a, 1a, 0, 0, 0
E5, E10, E15, E20, E30 = 1a, 0a, 0, 0, 0
W5, W10, W15, W20, W30 = x, 1b, x, x, 1a
For VOE sensitizer (FO + OA + EDTA): (again note change in times)
P5, P10, P15, P20 = 0a, 0a, 0a, 0a
C5, C10, C15, C20 = 0a, 0a, 0a, 0a
S5, S10, S15, S20 = 0a, 0a, 0a, 0
F5, F10, F15, F20 = 0a, 0a, 0, 0
E5, E10, E15, E20 = 0a, 0a, 0, 0
W5, W10, W15, W20 = 1b, 1b, 0b, 0a
For comparison, with old developer, VOE gave the following results:
P5, P10, P15, P20, P30 = 2b, 2a, 2a, 2a, 1a
C5, C10, C15, C20, C30 = 2b, 2b, 2a, 1a, 1a
S5, S10, S15, S20, S30 = 2b, 2b, 1a, 1a, 1a
F5, F10, F15, F20, F30 = 2, 2, 1, 1, 1
E5, E10, E15, E20, E30 = 2, 2, 2, 2, 1
W5, W10, W15, W20, W30 = 2a, 2a, 2a, 2, 2
CONCLUSIONS:
The addition of either Oxalic Acid or EDTA to the Ferric Oxalate (FO)
sensitizer solution improved clearing and reduced clearing times.
Best results were obtained when both Oxalic Acid and EDTA were added.
Note that even water (without any clearing agent) cleared completely in
20 minutes plus rinse and wash when both Oxalic Acid and EDTA were
added.
A used developer will dramatically hinder clearing.
The choice of sensitizer additive and a fresh developer is more
important to good clearing than the choice of clearing agent.
RECOMMENDATIONS:
For proper clearing of Pt/Pd prints it is recommended that Oxalic Acid
(3%) and EDTA (1%) be added to the Ferric Oxalate sensitizer solution.
Note that previous and ongoing work from the Threshold Study has shown
the addition of up to 5% of Oxalic Acid and up to 3% EDTA to the
sensitizer does not seem to degrade the quality of the print.
Since Potassium Meta Bisulfite produces a strong sulphurous odor
(especially when used in an open tray), it is NOT recommended.
Sprint Fixer Remover is a good choice for the selected paper as it
provided the best and fastest clearing in every case. A different paper
may favor another clearing agent.
A wash time of 10 minutes with at least one change of fresh water is
recommended.
The recommended times for clearing the selected paper are:
10-15 minutes for the Sprint Fixer Remover or EDTA(Na4) giving a total
processing (develop through wash) time of 23-28 minutes.
15-20 minutes for Sodium Sulfide giving a total processing (develop
through wash) time of 28-33 minutes.
20 minutes for Phosphoric Acid or Citric Acid giving a total processing
(develop through wash) time of 33 minutes.
It should be routine practice to replace the developer rather than
continuously replenish.
Have a clear day.
-- Jeffrey D. Mathias http://home.att.net/~jeffrey.d.mathias/
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