This study centers on determining what amount of metal would be the threshold for making the highest metal containing print using the Ferric Oxalate (FO) sensitizer.  The threshold being that point at which further increase of metal would no longer present a perceivable improvement in the print.

The threshold is dependent on many factors including the paper selected, the area coated, the coating procedure and coating efficiency.  It is imperative that the coating coverage and area be determined and the coating mixture be restricted to the area.

Note:  Pencil lines are a helpful way to delineate the area.

If one desires more flexibility and less restrictive accuracy, solution concentrations may be used which are higher than the threshold.  This may accommodate various papers, inaccuracies in efficiency, coverage, or areas, and some inconsistencies in coating procedure.  This convenience will be traded for the cost of some unused metal and sensitizer.  Using less material than required to meet the threshold will result in a noticeably weaker print.
 

The Study

The sensitizer was initially mixed as a 30% solution.  This solution was the Vizcay "Balanced Solution" from the Comparison of FO Powders study.

7.65 grams Vizcay FO powder (contains 0.15 gram Oxalic Acid & 7.50 grams FO)
1.00 gram EDTA (Na4)
0.80 gram Oxalic Acid
H2O to make final volume of 25.0 ml when completely dissolved

The drop sizes of all the solutions were determined to be 0.05 ml/drop.
A bulk base coating mixture was mixed consisting of:

70 drops sensitizer (described above) (30 % solution)
50 drops K2PdCl4 solution (20.2 % solution, warmed)
20 drops K2PtCl4 solution (25.7 % solution, warmed)

Both metal solutions must be warmed to keep dissolved.  These are the metal solution concentrations for use with a 30% solution of Ferric oxalate.  The Metal Solution Formula Calculator may be used to determine the metal solution formulas to use with other sensitizer and concentrations.

Prints were made from the following coating mixtures based on the following sensitizer concentrations.

M = bulk base coating Mixture
W = H₂O

 

# of drops	mixture	equivalent % concentration of sensitizer
15	15M	30 %
15	14M+1W	28 %
15	18M+2W	27 %
15	13M+2W	26 %
15	15M+3W	25 %
15	16M+4W	24 %
15	11M+4W	22 %
15	10M+5W	20 %

• Ambient conditions were temperature of 68-70^oF, relative humidity of 34%.
• The paper used was Crane's Parchment Business Card Stock (AKA: Cover-90; CP)
• Coatings were by brush into an area 5 inches by 5 inches (25 inches² or 161 cm²)
• All were dried by the dry method (see Drying the Coating).
• All were exposed with the same negative and a 21-step for 10.25 minutes under UV lamps.
• All were processed the same:
• developed in Potassium Oxalate
• 2 minutes water bath
• 30 minutes total clearing in H3PO4 (3 baths).
• Buffered rinse and 10 minute wash.

Observations

Note:  When discussing results, the %# refers to prints make with the equivalent % concentration
            of sensitizer solution and associated metal solution.

• The 20% was noticeably slightly weaker and showed a very slight solarization effect.
• The darkest areas of the 22% and 24% (as well as the 20%) were not solid (having a "pin holes" sort of look when viewed with transmitted light), and the 25% had a slight hint of this effect, unlike the other prints which were uniformly very solid.
• The prints were identical other than the above differences.

All of the prints made from a mixture at or above 26% FO in the sensitizer and with the corresponding metal solution were identical by any visual observation.  All of the prints below the 26% had some lesser quality observed.
 

Conclusions and Recommendations

It is concluded that for the materials and conditions used, a threshold of 26% is to be expected.  Another paper may have a different threshold.

It is recommended that, for the materials and conditions as above, a sensitizer solution with a Ferric Oxalate concentration of 26% be used, along with the respective concentrations of metal solutions.  The metal solutions for a given sensitizer may be found using the Metal Solution Formula Calculator or from the tables in Chapter 6, Optimized Formulas for Metal Solutions.

It is further recommended that a FO concentration slightly greater than 26%, may be used for greater flexibility and diversity.  However, it must be remembered that at higher concentrations, it can become more difficult to dissolve the metal solutions.  27% is a recommended FO concentration.

These recommendations are being verified and may change as noted in the
Verification of FO Powder Composition Study.
 

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FO means Ferric Oxalate, a light sensitive sensitizing agent for the Pt/Pd process.