What exactly occurs during the POP process is not fully known.  In order to verify the POP Process Equation and optimize the solutions used in the coating, an experimental test was conducted.  Conditions and parameters were carefully controlled and recorded.  The ability to make consistently identical prints is a must for this type of test.  Evaluations were made based on the study of actual prints.  Prints were compared side by side and with standard DOP prints.

Some of the data in this test were obtained as part of a larger more comprehensive study.  The interest at this point was to study the Lithium double salt of Palladium and two types of sensitizer on three frequently used substrates and to verify empirically the process equations.
 

SET UP

For these comparisons the following materials and conditions were used.  Only Palladium solutions were used; no tests were made with Pt so as to limit parameters.  All subsequent reference is by the respective notations given.

SUBSTRATES:
The following substrates were chosen as they are  ones used often and  represent some diversity.
B    = Hunt Co., Bienfang 360
CP = Crane's Parchment Business Card Stock (Lot No. 5302) ("Cover 90") (or "platinotype")
SI   = Sea Isle fabric

COATING CHEMISTRY:
The notation used is [sensitizer - contrast agent - Pd salt - Pt salt]
     sensitizer [# of drops, type]
          types used were:
               FO .......... Ferric Oxalate 27% solution
               AFO40 ... Ammonium Ferric Oxalate 40% solution
               AFO60 ... Ammonium Ferric Oxalate 60% solution
     Pd solution [# of drops, type of salt, approximate % solution]
          type of Pd salt used was:
               Li ... Lithium - Li₂PdCl₄
     Pt salt [# of drops of 20% solution]
          none was used in order to eliminate a variable
     contrast agent [one drop of x% solution, type]
          none was used in order to eliminate another variable

The following coatings were evaluated for this test.
6FO-6Li36-0-0         6AFO40-6Li36-0-0         6AFO60-6Li36-0-0
6FO-6Li30-0-0         6AFO40-6Li30-0-0         6AFO60-6Li30-0-0
6FO-6Li24-0-0         6AFO40-6Li24-0-0         6AFO60-6Li24-0-0
6FO-6Li18-0-0         6AFO40-6Li18-0-0         6AFO60-6Li18-0-0
6FO-6Li12-0-0         6AFO40-6Li12-0-0
6FO-6Li6-0-0           6AFO40-6Li6-0-0
 

AMBIENT CONDITIONS:
Other testing has shown the POP process to be only consistently dependable at an ambient relative humidity (RH) below 65% and an ambient temperature below 75^oF.  The ambient temperature was between 68^oF and 73^oF.  The ambient relative humidity was set at 62 % +- 3%.

EXPOSURE:
Exposure was identical for all prints under artificial lamps (described in my Guide).  This was approximately the exposure this negative would typically get.  The time was kept constant so that speed and contrast could be readily compared using a 21-step.  It would have been just as valid to match the print and 21-steps and note time differences, although it would have been difficult and cumbersome to accurately define any contrast differences.  Consideration was made when evaluating prints as to any differences in speed and contrast.  Results have been verified using other negatives and printing times that give the optimum print.

PROCESSING:
Processing was standardized but tailored for each type of substrate and sensitizer.
     For FO on CP:
          Developer was Potassium Oxalate 1 minute
          Rinsed in tray of water for 2 minutes
          Cleared in Phosphoric acid baths for total of 30 minutes
     For AFO40 on CP:
          Developer was tray of water for 2 minutes
          Cleared in Phosphoric acid baths for total of 20 minutes
     For FO on B:
          Developer was Potassium Oxalate 1 minute
          Rinsed in tray of water for 2 minutes
          Cleared in Phosphoric acid baths for total of 15 minutes
     For AFO40 on B:
          Developer was tray of water for 2 minutes
          Cleared in Phosphoric acid baths for total of 15 minutes
     For FO on SI:
          Developer was Potassium Oxalate 1 minute
          Rinsed in tray of water for 2 minutes
          Cleared in Phosphoric acid baths for total of 12 minutes
     For AFO40 on SI:
          Developer was tray of water for 2 minutes
          Cleared in Phosphoric acid baths for total of 12 minutes

water = 0.5um filtered tap water
Phosphoric acid bath = 2oz 85% H3PO4 in 1 gal. water
The clearing times were determined by the method outlined in this Guide.
 

RESULTS

EVALUATION:
Prints were compared by looking at, after dry, in "normal" light and direct sunlight (reflected and transmitted).  The overall evaluation was to judge the quality of the print with emphasis on darker areas.  The same image of a dark stairway with lots of shadow details and texture was used.  Also included in the print was a "21 step".

AFO40 SERIES
The series of AFO40 for Li6 to Li36 showed a transition from a noticeably week and muddy Li6 print to a healthy Li24 print with no further improvements in the Li30 and Li36 prints.  The AFO60- Li30 and Li36 prints showed a bit more substance.  There was a definite distinction between the Li18 and Li24 prints.  These results are significantly helpful as an empirical tool to finding the process equation (discussed later).  These prints all had the same color and speed.  The contrast seemed to increase about 1 grade mostly from Li6 to Li18 and a little to Li24.

AFO40 vs AFO60
A difference was noted between the AFO sensitizer solutions of 40% and 60%.   Differences were only noted for the Li36 and Li30 coatings.  No difference was detected for the Li24 and Li18 coatings.  (Recall that the number indicates the % solution of Li2PdCl4.)  These findings proved to be significantly helpful as an empirical tool to finding the process equation (discussed later).  Due to this importance, these prints were repeated and found to give consistent results.

A final comparison was made with the best of the AFO40 and AFO60 prints.  Those were the AFO40-Li24 and the AFO60-Li36.  These prints seemed to have equivalent speed.  The AFO60-Li36 seemed to have 1/4 grade (or less) more contrast.  Both had excellent solid dark areas with the AFO60-Li36 looking slightly more solid.  The AFO40-Li24 was slightly warmer and the AFO60-Li36 was very neutral in color (although not perfectly neutral).  Although both prints could be considered acceptable, the AFO60-Li36 had a bit
more substance and depth.  Portions of the image looked more alive in the AFO60-Li36 print.
 

FO SERIES
These prints with the FO sensitizer and Li metal salt all showed the same speed and contrast.  The prints exhibited an interesting orangish brown color much more obvious in the FO-Li18 and FO-Li24 prints.  FO-Li6 was weak and muddy and FO-Li12 was not far off from the mud.  FO-Li18 through FO-Li36 were fairly identical.  These findings verify the DOP process equation.
 

FO vs AFO
Comparison was made with the FO-Li24, AFO40-Li24, AFO60-Li36 prints.  The FO prints seemed to be a slight bit slower.  The prints showed the same contrast up to about Zone VI where the AFO prints then went on to show about 1 grade more contrast.  The FO prints were definitely warm in
color with the AFO prints close to neutral..  The FO prints had more solid dark areas and exhibited more depth and substance than the AFO40 prints.  The AFO60-Li36 prints seemed to have the same depth and substance of the FO prints; the differences being the color and the contrast in the upper values.
 

FINDINGS

THE POP EQUATION:
From the POP Process Equation it is calculated that the AFO40 should be totally used with a Li2PdCl4 solution of 24.49% indicating that an additional amount of Li2PdCl4 would be unused.  In the AFO40 prints, quality increased from the Li6 to the Li24 and then was constant to Li30 and Li36.  This would indicate a "point of saturation" between Li18 (18%) and Li24 (24%).  Recall that Li24 is an abbreviation and that the percentage has been rounded to 2 digits.  The actual percentage of <24> to four digits was <24.49>.  Li24 is the solution of 24.49%.  This "point of saturation" data supports the balance of one for one as given by the process equation.

In the AFO60 prints, print quality compared with AFO40 prints was improved in the Li30 and Li36 but remained the same in Li24 and Li18.  This would indicate a "point of saturation" between Li24 and Li30.  This AFO60 data is consistent with the AFO40 data and also supports the balance of one for one as given by the process equation.
 

CONCLUSIONS

• The POP and DOP processes rely on a use of one molar equivalent of metal double salt for every molar equivalent of sensitizer.  If less metal (by way of a weaker metal solution) is used, the result is a weaker print.

 
• The DOP process seems to require much less metal (by way of a weaker metal solution) than the POP process to get desirable results.

 
• The POP process can achieve unique effects, such as a neutral or cool color, but seemingly only at the expense of using much more metal (by way of a stronger metal solution).

 

 
 
 
 
 
 
 

This study sets what the concentrations for the metal solutions should be for a given sensitizer solution.  These can be found as the calculated formulas in the section, Optimized Formulas.
Further study of the amount of metal that is actually required to optimize the print quality for POP is addressed in a further study, Verification of Metal Solution Optimization.

 
Back to Table of Contents      Home