Study of the Clearing of Pt/Pd Prints

Considerations Materials Processing Nomenclature Observations Conclutions Recommendations

This study has documented some very significant findings concerning the clearing of Pt/Pd prints.  This study makes use of the revised clearing test 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.

Top of Page

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, which may appear to be incomplete clearing.

• 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 may react with or lodge onto 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 initial clearing indicator of a 10% solution of Potassium Ferricyanide proved to be 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.

Top of Page

MATERIALS:
For simplicity, only one paper was used, Crane’s paper (lot# 5302).  This is a thick and traditionally long to clear paper (typically 40 to 60 minutes, sometimes longer).  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 Bisulfate	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.  However, the HCl may work fine (without bleaching) at the shorter clearing times this study demonstrates and should not be dismissed without further testing.

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 to the sensitizer made a significant difference in clearing ability.  3% Oxalic Acid was included because previous study indicated an advantage of having from 2% to 5% and 3% is typically used.  3% EDTA was initially selected being the same concentration as the Oxalic Acid.  When used together, 1% EDTA was found to be adequate.

NOTE:  The EDTA was subsequently reduced to about 0.4% from the 1% used in the Clearing Study.  The reason was that some "bleeding" of the metals may occur in the print noticeable as washing off or flowing through the substrate.  The "bleeding"  did not seem to occure at the lower amount.

Sensitizer Code	Description
V	Vizcay Ferric Oxalate at 24% (Vizcay’s preparation procedure)
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 3% Oxalic Acid plus 1% EDTA

VOE = 24.00 g FO + 3.00 g Oxalic Acid + 1.00 g EDTA + H2O to make 100 ml

                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
               Note:  This is NOT the same EDTA(Na4) used as a clearing bath.  The EDTA(Na4)
                             tested as a clearing bath was some sent to me by John Melanson.

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.

Top of Page

PROCESSING:
All samples were processed as follows:

• coating mixture mixed (as for a 4x5 print)
• brush coated (with coating edges indicated and as instructed in Test for Clearing)
• 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
• exposed for 12 minutes with UV lamps (about double typical printing time)
• Potassium Ferricyanide (1% solution) applied (smeared on with clean Q-tip) to part of coated area
• 2 minutes rest
• 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.

Top of Page

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: (colors shown are approximate)
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: (colors shown are approximate)
no code		no change
a		faint blue color
b		blue color

Note: Consideration should be given for unnecessary cross contamination and any slightly
         “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 (remained a very faint gray color).

Top of Page

OBSERVATIONS:
The following clearing results as a function of agent and time are shown for several sensitizers.  The last data set compairs use of an old developer.
 

Time (minutes) >

5

10

15

20

30

40

For V sensitizer (FO only):

P

5b

4b

x

3b

2b

1a

C

5b

4b

x

3b

1b

1b

S

5b

4b

x

3b

2b

1a

F

2b

1b

x

1a

0a

0a

E

4b

3a

x

2a

2

2

W

5b

5b

x

4b

4b

4b

For VO sensitizer (FO + OA):

P

2b

1b

x

1b

1b

1a

C

1b

1b

x

1a

1a

0a

S

2b

2b

x

1b

1b

1a

F

1b

1b

x

0

0

0

E

1a

1a

x

1a

0

0

W

3b

1b

x

1b

0b

0b

For VE sensitizer (FO + EDTA):

P

1a

1a

1

0

0

x

C

1a

1

1

0

0

x

S

x

x

x

1a

0

x

F

1a

1a

0

0

0

x

E

1a

0a

0

0

0

x

W

x

1b

x

x

1a

x

For VOE sensitizer (FO + OA + EDTA):

P

0a

0a

0a

0a

x

x

C

0a

0a

0a

0a

x

x

S

0a

0a

0a

0

x

x

F

0a

0a

0

0

x

x

E

0a

0a

0

0

x

x

W

1b

1b

0b

0a

x

x

For comparison, with old developer, VOE gave the following results:

P

2b

2a

2a

2a

1a

x

C

2b

2b

2a

1a

1a

x

S

2b

2b

1a

1a

1a

x

F

2

2

1

1

1

x

E

2

2

2

2

1

x

W

2a

2a

2a

2

2

x

Top of Page

CONCLUSIONS:

• The addition of either Oxalic Acid or EDTA to the Ferric Oxalate (FO) sensitizer solution improved clearing and reduced clearing times.

 
• Best clearing 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 wash when both Oxalic Acid and EDTA were added.

 
• A used developer can dramatically hinder clearing.

 
• The choice of sensitizer additive and a fresh developer is more important to good clearing than the choice of clearing agent.

Top of Page

RECOMMENDATIONS:

• For proper clearing of Pt/Pd prints it is recommended that Oxalic Acid (3%) and EDTA (0.4%) 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 to the sensitizer does not seem to degrade the quality of the print.  Note also that work subsequent to this study reduced the EDTA from 1% to 0.4%)  NOTE (added 8/2001): Further study of the "bleeding" of metal during processing has indicated that too much EDTA seems the culprit.  It is now recommended that EDTA in the sensitizer be kept to a solution strength of between 0.04% and 0.1%.  The actual amount can vary with different papers, so the smallest amount of EDTA to add to the sensitizer to assist with clearing should be determined for each paper.

• 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.  However, I personally continue to use Phosphoric Acid.

 
• A total wash time of 10 minutes with at least one change of fresh water is recommended.

 
• 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 Bisulfate 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 after a large amount of coatings while  replenishing for developer carried off by prints.

Have a clear day.

Top of Page

Back to Contents