Thursday, February 7, 2013

Spectral analysis of two gelatin silver prints: from negative to final product

The earliest record of an individual using gelatin in photography was Niepce in 1847.  He and many others were not initially successful.  When gelatin first became successfully employed, it was used merely to coat and preserve collodion plates.  Over time, gelatin silver printing became an independent photographic process and lead to the disuse of collodion altogether.  Since the late nineteenth century, gelatin silver prints have become and remain the most common black-and-white photographic process. 

This photograph was taken by Keara Teeter in fall 2011 at the California Palace of the Legion of Honor in San Francisco.  

The Legion of Honor negative (above) is composed of a thin layer of gelatin emulsion and a thick polyester baseThe text on the perforated edges says “Kodak 400TX” designating it was manufactured Kodak Professional Tri-X 400 film.  By looking at the presence of silver (Ag) in the spectrum (below), one can discern a significant difference between maximum and minimum density readings.  The silver is dispersed within the gelatin emulsion of the negative.  The nickel (Ni) and rhodium (Rh) peaks for all spectra can be disregarded because they are generated by the instrument.

The Legion of Honor print (above) was developed using fiber paper and toned using an indirect sepia toner.  Indirect toning requires two steps but usually produces a final product in less time than direct toning.  The first step is to immerse the print in a solution which “bleaches” the image by converting the silver metal to a silver halide.  After briefly washing the print under running water, the second step involves immersing the bleached image in a redeveloping solution.  This solution rejuvenates the image density with a sepia color.  In the spectrum (below), the silver peaks again indicate there is a different concentration of silver in the maximum density reading than in the minimum density reading.  Because this print was developed on fiber-based paper, the barium (Ba) and sulfur (S) peaks are also visible.  Barium sulfate (BaSO4) is found in the baryta coating on the fiber paper.  A baryta coating covers the fibrous texture of the paper in preparation for the emulsion layer.  A strontium (Sr) peak is present because this element forms compounds with barium.

The second example is of a negative and gelatin silver print on resin-coated (RC) paper.  Resin-coated paper was developed in the 1960s for military purposes.  This paper was more time efficient and appealed to their need for rapid processing during sensitive operations.  This paper is capable of producing an image more quickly than fiber-based paper because the resin layers (consisting of pigmented polyethylene on the emulsion side and clear polyethylene on the reverse) encapsulate the paper fibers.  This polyethylene barrier prevents the paper fibers from absorbing liquid and allows the surface to wash and dry in shorter time intervals. The Dominican University (below) print underwent a direct selenium toning process.  Selenium toning tends to darken prints and heighten image contrast.
This photograph was taken by Keara Teeter in fall 2011 at Dominican University of California in San Rafael.

As before, both spectrum of the negative and the final print demonstrate significant differences between maximum and minimum silver (Ag) densities.  For the resin-coated print, however, does not show barium (Ba), sulfur (S), or strontium (Sr) peaks as seen previously in the fiber-based print.  Instead, the resin-coated print shows titanium (Ti), zinc (Zn), and tin (Sn) peaks.  The titanium and zinc peaks are consistent with the presence of titanium dioxide and zinc oxide pigments in the resin.

Eaton, George T.  Photo Chemistry in black-and-white and color photography.  Rochester: Eastman Kodak Company, 1957.
Hammond, Arthur.  How to Tone Prints.  Boston: American Photographic Publishing Co., 1946.
Harrison, William Jerome.  A History of Photography.  New York: Scovill Manufacturing Company, 1887.
Kennel, Sarah, Diane Waggoner, and Alice Carver-Kubik.  In the Darkroom: An Illustrated Guide to Photographic Processes before the Digital Age.  Washington: National Gallery of Art, 2009.
“Kodak Professional Tri-X 320 and 400 Films.” Eastman Kodak Company No. F-4017 (2007): 1-16.
Sheppard, S. E.  Gelatin in Photography.  Rochester: Eastman Kodak Company, 1923.
Tan, Syliva, ed.  The Film Preservation Guide: The Basics for Archives, Libraries, and Museums.  San Francisco: National Film Preservation Foundation, 2004.  
Wall, E.J. and Franklin I. Jordan.  Photographic Facts and Formulas.  Garden City: American Photographic Book Publishing Co., Inc., 1976.  

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