Thursday, April 25, 2013

Stabilization and Gold Toning in Salt Prints

Salt prints were first produced by William Henry Fox Talbot in 1834, and this photographic process was the first documented image produced from light and permanently fixed upon paper.  Salt prints became more commonly used from the late 1830s to the mid 1860s.  Talbot was a scientist, chemist and mathematician who – frustrated by the camera lucida and his inadequate drawing skills – turned to scientific means of producing imagery.  His work was not only a major contribution to the Royal Society (a scientific organization to which he belonged), but also inspired the groundwork for future photographers, scientists, and other professionals for centuries.  In the early 1840s, improvements to his process became referred to as calotypes.  The word calotype comes from the Greek word kalos meaning beautiful.

Salt printing is a two-step process: first the paper is coated with sodium chloride and then – after the paper has dried – it is coated with silver nitrate.  These solutions combine to form silver chloride and sodium nitrate.  Traditionally, this printing-out process requires a contact printing frame to expose the image.  When exposed to light, the silver chloride dissociates and releases particles of metallic silver.  The metallic silver binds to the surface of the paper fibers to form an image.  Salted paper prints are matte and can exhibit a broad range of hues including golden-yellow, reddish-brown, and purple tones. The hues result from differences in paper sizing, light exposure sources, humidity, and washing/toning methods.

These two 8"x10" salted paper prints are photograms produced by Keara Teeter in the spring of 2013 at Gawain Weaver Art Conservation.

The prints above were produced on uncoated Bergger COT-320 paper.  To ensure sharper results, gelatin sizing was used in the salt solution.  Sizing prevents the metallic silver from sinking too far into paper fibers.  The metallic – or reduced – silver does not react to light, but free silver halides will continue reacting if not stabilized, toned, or fixed.  The golden photogram was washed in a sodium chloride stabilizaton solution (Talbot's stabilization method) and the purple photogram was toned using gold chloride (published by Jean-Baptiste Gustav Le Gray in 1850).  Talbot's stabilization process does not remove silver halides, but it makes them far less susceptible to light.  Gold toning effectively removes free silver halides.

By analyzing the two prints with x-ray fluorescence spectroscopy, it is possible to see the differences between their chemical compositions.  Both the stabilized print and gold toned print were analyzed at 45 kV and 34.00 µA with the green filter.  The spectra appear different as a result of the significant change in the height of silver (Ag) peaks.  

The first two spectra compare sections of a single print, and thus, these spectra compare the prints to each other indirectly.  The spectrum above is comparing maximum (red) and minimum (blue) densities for the stabilized print, and the spectrum below is comparing maximum and minimum densities for the gold toned print.  The stabilized print exhibits large silver (Ag) peaks in both the maximum and minimum densities.  The gold (Au) peak is identified here only as a comparison to the gold toned spectra.  In the gold toned print, the minimum density gold and silver peaks are significantly shorter than the maximum density peaks for the same elements.  Nickel (Ni) and rhodium (Rh) peaks are generated by the instrument.

The final two spectra are comparing the prints directly to each other.  The first spectrum compares the maximum density of the stabilized print (blue) to the maximum density of the gold toned print (red).  The second spectrum compares the minimum density of the stabilized print (blue) to the minimum density of the gold toned print (red).


Barnier, John, ed.  Coming Into Focus.  San Francisco: Chronicle Books, 2000.

Farber, Richard.  Historic Photographic Processes: A Guide to Creating Handmade Photographic Images.  New York: Allsworth Press, 1998.

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.

Young, Ellie.  The Salt Print Manual.  Victoria: Gold Street Studios, 2011. 

Monday, February 18, 2013

Examining a tintype from Des Moines, Iowa

The tintype depicted above was produced during the mid-to-late nineteenth century by a studio photographer in Des Moines, Iowa.  As with other case images (the daguerreotype and ambrotype), tintypes such as this were frequently hand-colored and displayed within a mat or frame.  

The tintype process was discovered by Hamilton Smith in 1854.  Smith was a chemistry professor at Kenyon College, and two students of the college became manufacturing competitors after the process was patented in 1856.  The first student, Peter Neff, shared in the patent rights and was the original manufacturer of the “melainotype”. The second student, Victor Griswold, began producing the “ferrotype” after the process gained popularity.  

“Melaino” means dark or black (referring to the varnish coating the support) and “ferro” refers to iron (the metal support itself).  These terms were used interchangeably and both provide a more accurate description of the process than the term “tintype.”  Rather than using tin, an iron plate is used as the metal support and coated in black japanned varnish.  This plate is first coated with black japanned varnish, then coated with collodion, immersed in a bath of silver nitrate, and exposed, and finally developed in progallic acid.  

The spectrum below identifies the iron (Fe) peak from the metal support and the silver (Ag) peak from the particles suspended in collodion.  Iron has a higher peak for maximum density (red) and lower peak for minimum density (blue) as expected.  The silver (magnified in the upper right corner) displays a higher peak for minimum density and a lower peak for maximum density.  The silver peaks are inversed because – as with ambrotypes – the photograph produced is actually a negative image (because it is developed on a dark surface, however, it appears as if positive to the viewer).  The maximum and minimum density readings from the Des Moines tintype were taken from the top and bottom edges respectively.  These edges were selected, rather than the center, to prevent the instrument from analyzing the paper label adhered to the backside.

Because this process was more cost-effective than a daguerreotype, less fragile than an ambrotype, and relatively quick to develop, tintypes became quite popular within the United States.  The tintype never did become influential in European countries.  But, after the carte-de-visite became popular overseas, tintypes in the United States started being mounted in albums and on cards.  A second major contributor to the long-term success of this photographic process was the rise of the American Civil War.  Working class families wanted to have their loved ones photographed before they left for war and could afford the price of a tintype.    

The demand for tintypes had decreased immensely by the 1870s.  The major downfall of the tintype was its flat appearance.  Since the highlights are never completely white, the images lack the luster and vibrancy of other photographs.  The tintype continued being produced as novelty images until manufacturers discontinued its production in the 1930s.

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.
Mace, O. Henry.  Collector’s Guide to Early Photographs.  Iola: Krause Publications, 1999.
Taft, Robert.  Photograph and the American Scene.  New York: Dover Publications, Inc., 1938.

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.