An empirical study to determine the appropriate ratio of metal double salts to sensitizer for the POP (Printing Out Process) Pt/Pd process equation supported a ratio of one-to-one.  The solution concentrations for various metal double salt solutions were then calculated.  That study produced prints which demonstrated that for given sensitizer solutions (40% and 60% Ammonium Ferric Oxalate [AFO]) certain metal solution concentrations existed that below which produced progressively  weaker prints and above which produced identical prints.  These metal solution concentrations corresponded to those as calculated.

A question remained as to what amount of metal would be the threshold for making the highest metal containing print using the AFO sensitizer.  The threshold being that point at which further increase of metal would no longer present a perceivable improvement in the print.

For the DOP (Develop Out Process), the Ferric Oxalate (FO) sensitizer provides a limiting function as it can only be mixed to about a 27% solution under standard conditions (25^oC, 1 atmosphere pressure).  To date all attempts have demonstrated that the highest concentration of FO (27%) and the respective calculated metal solutions produces the highest quality prints.  If lower concentrations are used, noticeably weaker prints are observed.  It can be stated that any threshold for DOP cannot be attained due to the limit set by the maximum strength of the FO solution.

For AFO, a solution strength of 60% is easily obtained.  However another limit comes into play in that this 60% solution with the corresponding calculated metal solution produces a coating solution which begins to have problems, including:

• streaks in the print from brush marks because of the thick solution;
• crystallization of the coating mixture if not applied immediately after mixing;
• graininess in the prints perhaps from unnoticed crystallization;
• bleeding of metal from the print in the water pre-clearing bath.

Unlike DOP, a question emerges as to what is the limiting concentration of the AFO sensitizer, and does a metal amount threshold occur within this limit.  The purpose of this study is to answer these questions and formulate a coating recommendation for POP.
 

The Study

Prints were made from the following coating mixtures (see end of Preparing the Coating Solution for a description of nomenclature used).

The AFO was a solution of 60%.
The Li₂PdCl₄ was a solution of 36.73%.
The W indicates H₂O.
 

	A	B	C	D
1	6AFO60-6L36-0W
2	5AFO60-5L36-2W
3	4AFO60-4L36-4W	6AFO60-6L36-6W
4	3AFO60-3L36-6W	6AFO60-6L36-12W	4AFO60-4L36-8W
5	2AFO60-2L36-8W	6AFO60-6L36-24W	4AFO60-4L36-16W	3AFO60-3L36-15W
6	1AFO60-1L36-10W	6AFO60-6L36-60W	4AFO60-4L36-40W	3AFO60-3L36-30W

• The mixtures of the A column start at 60% (A1) and diminish in concentration through 40% (A3) to 10% (A6).
• The mixtures of the B column are diluted identical to their counter parts of the A column but keeping an identical amount of metal as the first mixture (the calculated 60% AFO mixture).
• The mixtures of column C are diluted identical to their counter parts of the A and B columns while keeping an identical amount of metal as in the calculated 40% AFO mixture.
• The mixtures of column D are diluted identical to their counter parts of the A, B, and C columns while keeping an identical amount of metal as in a 30% AFO mixture (A4).
• Each of these mixtures was coated into an identical area of about 25 square inches (enough room for a 4x5 negative a 21-step).  Keeping the area of coverage constant is critical, and in order to accomplish this for some of the mixtures in the B and C columns, a  different coating method was devised (see Quasi Multi-Coating Method).

The purpose of column A was to observe an incremental decrease in the concentration of coating mixture solutions.  It is important that the sensitizer to metal solution ratio remains one-to-one as this was determined to be proper from previous study.

One purpose of columns B, C, and D was to compare the dilution of the coating mixture at values of 60%, 40%, and 30% AFO with the reduction of mixture (column A).

Another purpose of columns B, C, and D was to investigated changes in total metal content independent of mixture concentration and without problems associated with the stronger solutions.  This investigates a row such as 4, 5, or 6.

• Ambient conditions were temperature of 73-75^oF, relative humidity of 65-67%.
• The paper used was Crane's Parchment Business Card Stock (AKA: Cover-90; CP)
• Single coatings received a very brief pre-mist; multi-coatings received no pre-mist.
• All were coated by brush and "Wet" dried.
• All were exposed for 12 minutes under UV lamps.  A6 was repeated with a longer exposure to

     verify that the weakness was not from speed loss.
• All were processed the same:
•      2 minutes water bath
•      20 minutes total clearing in H3PO4.
•      Buffered rinse and 8-10 minute wash.

Observations

Strength and Depth of image (dark values)
• B5 & B6 had excellent strength and depth with good solid dark areas more so than all the others.
• B3, C4, C5, & C6 had excellent strength and depth with good solid dark areas more so than the A group but not quite as much as B5 & B6.
• A2 & A3 had excellent strength and depth with good solid dark areas but not quite as much as those above.
• A4 had very good strength and depth with good solid dark areas but noticeably less than those above
• A5 was slightly weak.
• D5 & D6 were definitely weak, not as weak as A6 but weaker than A5.
• A6 was definitely weaker than all.
• A1 & A2 were the only prints to “bleed” metal into the water (pre-clearing) bath.
• A1 was not included in this comparison as some weakening was noticed likely due to other factors.

 

 

Overall Substance and Presence

• B5, B6, C4, C5, A2, & A3 showed the most Substance and Presence.
• A6, D5, and D6 showed the least Substance and Presence.
• B5 came the closest to matching (identical) the Substance and Presence of a DOP print.

 

 

Image Color

• A1, A2, & A3 were neutral color; A4, A5, & A6 were barely slightly warmer.
• B3 & B4 were neutral color; B5 & B6 were slightly warmer.
• C4, C5, & C6 were in-between the neutral and warm colors with C5 the warmest.
• D5 & D6 were neutral color.

 

 

Image Graininess of large tonal areas (21-step)

• A1 was noticeably grainy; A2, A3, & A4 slightly grainy; A5 & A6 were smooth.
• B3 was slightly grainy; B4 barely showed some grain; B5 & B6 were smooth.
• C4 barely showed some grain; C5 & C6 were smooth.
• D5 & D6 were smooth.

 

 

Paper Surface

• The surface of B6 was slightly rougher (probably due to the long brushing of five coats).
• The surfaces of those receiving three or four coats (B5, C6, & D6) felt barely, but noticeably, rougher.

 

 

Speed

• All the coatings seemed to be close to the same speed.  Too close to be evaluated by the 21-step, however a few appeared slightly darker throughout the print (B5, B6, & C5).

 

 

 Contrast

• A1 through A6 showed a graduated change in contrast with A6 being about 1 grade more contrast.
• The B group showed the same gradation of contrast with B6 being the highest but about a half grade less than A6.
• The trend followed in the C and D groups (6 the most contrast); and in each row, A having the most contrast.  In all rows the contrast progressively changed about one and a half grades.
• Two coats by the Quasi Multi-Coating Method consistently produced a half grade less contrast than a single coat.

Quasi Multi-Coating Method

This author has had mixed results from multiple coatings in the past.  A thought was that some of the past difficulties could have been related to drying the mixture and then re-coating.  Coatings of mixtures containing 50 % to 100 % additional mixture had been brushed into the same areas without problems.  So it was supposed that additional mixture could be added to the coating before it dried completely.  (Hence the term Quasi.)

The Quasi Multi-Coating Method is basically as follows:

1. Pour out a typical amount of mixture on to the paper as with a single coating;
2. Brush this into the area desired as if it were a single coating;
3. Allow to dry beyond the point of loss of gloss, but before complete dryness (rather damp);
4. Pour out some more of the mixture (about the same amount);
5. Brush this into the area desired as if it were a single coating;
6. Allow to dry beyond the point of loss of gloss, but before complete dryness (rather damp);
7. Continue until all of the mixture is used.
8. Dry to desired dryness.

Each pouring should be enough to completely coat a layer over the full area.  Do not extend any mixture beyond the area.  With this Quasi Multi-Coating Method, it is of the utmost importance that the coating be restricted to the correct area.  Spreading the mixture too thin (past the area) will result in a weakened coating which, if at the threshold, will produce a weakened print.  Not spreading the mixture to fully to cover the area will result in a stronger coating.

The coating of mixture B6 was accomplished with five pourings (5 quasi coats) and took a little more than half an hour to coat and dry.  These coatings produced excellent prints.  All of the A group had one coat; B5 had three coats; B4 two coats; B3 one coat; C6 four coats; C5 two coats; C4 one coat; D6 three coats; D5 one coat.  Drying times are significantly lengthened with multiple coats.

a few of the coatings showed some difficulty (large lighter blotchy areas) which was likely because of too much drying of the first coating.  Those coatings were repeated, taking care not to over dry, and produced excellent results.  It was also noted that none of the prints (including the 5 coat) showed any signs of solarization effect.
 

Discussions

COVERAGE:

Consistency of the coating coverage is of critical importance.  Coverage is expressed as cm²/ml and defined as:

[Coverage] = [area of coating in cm²] /
[number of drops of sensitizer at specified solution concentration] * [ml per drop]

An increase in coverage will weaken the print, and a decrease in coverage will waste chemistry.  Because multiple coatings may be made and may use a diluted mixture, the total number of mixture drops is not an accurate reference.  The appropriate reference is the number of drops of sensitizer solution, which is identical to the number of drops of metal solution.  The purpose of coating is to get a known amount of active chemicals evenly distributed throughout the coated area.  Coverage indicates that amount.

Coverage will vary by paper and must be determined for each paper.  Knowing the coverages will permit the easy transition from one paper to another without the worry of insufficient chemistry.  The Coverage for the paper used in this study is calculated as follows.
Coverage of CP = 161 / 6 * 0.05 = 1.34 cm²/ml

This means that an 8x10 with with half inch boarders (100 inches² or 645 cm²) will require the number of drops of sensitizer as follows.

drops = [area coated] / [Coverage] * [ml per drop] = 645 / 1.34 * 0.05 = 24

Add to this amount of sensitizer, an identical number of drops of the appropriate metal solution(s), an optional drop of contrast agent, and any optional dilution water, to make a coating mixture for an 8x10 on CP.

THRESHOLD:

Column A alone indicates that A4 (a 40% AFO solution with respective metal solution) may be  close to the threshold.

When evaluating the rows, it is noticed that column D (a 30% AFO solution with respective metal solution) is far below the quality of column C (a 40% AFO solution with respective metal solution)

Columns B (a 60% AFO solution with respective metal solution) and C (a 40% AFO solution with respective metal solution) are very close.  In row 6, B is definitely better than C.  In rows 4 and 5 the prints look identical with B5 being a bit warmer.

It is suspected that the appearance of depth and substance may be enhanced by a slight warm color.  Without this color B5 and C5 would look identical.

Without finer tuning, it is most likely that the 40% AFO solution with respective metal solution is the closest to the threshold (30% being definitely too weak).  It is possible that slightly less than 40% may be used and should be verified.

QUASI MULTI-COATING:

It appears that multiple coatings by the method above may produce a fuller advantage of the materials than a single coat.  C5 appears superior to C4 and A3 (especially considering smoothness).

Two coats by the Quasi Multi-Coating Method consistently produced a half grade less contrast than a single coat.  However, further coats showed an increase in contrast, suspected to be caused by an overriding influence of dilution.

Further Data and Discussion

Additional data was sought from coatings based on a 35% AFO solution with respective metal solution.  To accomplish this two prints were made with the following coating mixtures.

• 12 drops of the mixture 7AFO60-7L36-10W
• 24 drops of the mixture 7AFO60-7L36-36W

Note: For reference 7AFO60-7L36-10W is the equivalent of 12AFO35-12L21 which is twice
          the amount needed for the area coated.

The prints fit into the above assembly of prints where expected.  These prints were extremely close to C4, C5, and A3.  They were noticeably better than D5 and D6 and slightly better than A4.

These prints appear to be of a quality good enough to consider them closer to the threshold than those of the 40% AFO.

These prints had a neutral color and did not appear to have as much depth or substance as B5 and B6.  However, it is becoming more convincing that the depth and substance perceived is a function of more than the amount of metal in the print (i.e., the color has some influence).
 

Conclusions and Recommendations

A reminder that this information is paper specific.  The sensitizer solution strength (and the associated metal solution) and coating mixtures for other papers should be evaluated by this or a similar procedure.  However, the ratio of sensitizer to metal solution will remain unchanged.

It is unlikely that a change to another type of metal solution will influence these results, if that metal solution is the calculated optimized solution for the solution of sensitizer (see Optimized Formulas).

For POP, a 35% AFO solution with respective metal solution provides enough metal to reach the discussed threshold and should be the coating mixture used and applied as either a single coat or a two coat Quasi Multi-Coating.

This translates to the following coating solutions and equivalent coating mixture recommendation.

AFO made as a 35% solution (AFO35).
Li₂PdCl₄ made as a 21.43% solution (L21).
Na₂PdCl₄ made as a 24.05% solution.
K₂PdCl₄ made as a 26.7% solution.
K₂PtCl₄ made as a 33.9% solution.

Note:  Metal solutions may be mixed or interchanged freely.

For single coat:
(#drops) (AFO) - (#drops) (metal solution)[ - 1 drop (contrast agent)] optional

For double Quasi Multi-Coating:
(#drops) (AFO) - (#drops) (metal solution) - (2 times #drops) (H₂O)[ - 1 drop (contrast agent)] optional

An advantage of the double Quasi Multi-Coating:
As some of the metal solutions above will not stay in solution (unless heated), they could be mixed at half their concentration, twice the #drops put into the mixture, and a #drops deleted from the H₂O.

For double Quasi Multi-Coating with metal solutions at half their strength:
(#drops) (AFO) - (2 times #drops) (metal solution) - (#drops) (H₂O)[ - 1 drop (contrast agent)] optional
 

Additional Pondering

Not only has this study been paper dependent, results are also affected by coating efficiency.  The Wet Dry Drying Study has roughly predicted a coating efficiency of 81% for the brushing techniques used.  A different brushing technique or the use of a rod may have a different coating efficiency.  A higher efficiency would allow the solution strengths to be reduced, and a lower efficiency would require the solution strengths be increased.  It is highly recommended that a standardized coating technique be practiced and used at all times.

An important aspect of this study has been to determine and to have technique to consistently put the maximum beneficial (threshold)  amount of metal into the print.
 

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