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