Thursday, May 30, 2013

Richard Frederick
A Marriage of Convenience...Part 4
This is the 4th & final posting of this subject. We have learned by now that the marriage is that of Analog to Digital techniques that can produce stunning Platinum/Palladium photographic prints. This marriage allows us to overcome a significant difficulty in the original Pt/Pd process that required a very large (and unwieldy) camera to produce a large negative for printing a large photograph.
In the last posting I revealed a new technique invented by photographer Mark Nelson called "Precision Digital Negatives". It is this invention that allows us to use the computer program Adobe Photoshop, and a modern digital printer to produce a large negative that can reproduce very accurately all of the rich tonality of a scene captured with a small digital camera. Nelson determined through his research & experimentation that the inkset colors in digital printers had different sensitivities to the ultraviolet light that exposes the Pt/Pd emulsion on the print paper. His method is used to determine the exact mixture of Red, Green, and Blue colors in Photoshop that, when applied to the negative, will result in the very bright highlights in the scene to reproduce as paper white in the print. And through a necessary (and sometimes tedious) calibration process, all other tones in a scene (including deep black) will be reproduced in the print with great accuracy & precision. The calibration produces a "Curve" that is applied to the negative file before printing it.
The figure at left shows the negative I made for the Iris print with the proper "Color Density" applied. It comes as a surprise to most that a negative for a black/white print contains color. This negative was printed with a unique "Curve" applied in Photoshop. The curve is required because the Pt/Pd emulsion and specific paper combination do not respond linearly to the differing light levels in the negative of the scene. It is desirable that a 1 unit change in light passed by the negative produces the same 1 unit change in light (tone) in the print whether it be in shadow detail, mid-tone detail, or highlight detail.  
The figure at right shows the curve I generated using Nelson's invention to "linearize" the printed tones. The blue curve is an ideal linear curve, but since the Pt/Pd emulsion-paper combination is not linear the tones in the print would be seriously distorted if not corrected.
The figure to the right shows the curve-corrected negative in contact with the sensitized paper placed in a contact printing fixture (to keep it flat) under the intense light source that exposes the latent image onto the paper. The light source above the printing frame is reflected in the glass of the frame.
 Following this exposure, the paper is taken into the darkroom (all Analog now) where it is developed in a Potassium Oxalate chemical bath, as seen in the figure to the left. The developed print is then dried, flattened, & mounted on archival mat board. And...Ta Dum! Here's what you have all been waiting for...
The final print. Note that it is slightly warm toned, which is a pleasing characteristic of Pt/Pd. The degree of warmth can be controlled by adjusting the relative amounts of Pt & Pd in the emulsion. I wish I could show you the actual full sized print as it is superior to what I can post on the web.
Thanks for your interest in this age-old & modern process.


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