Understanding Photographic Processes

Silvia A. Centeno

March 2, 2011

«As a chemist in the Museum's Department of Scientific Research, I work closely with Anna Vila-Espuña, also in the Department of Scientific Research, and Nora Kennedy, in Photograph Conservation, on collaborations with Met curators to increase our understanding of methods and materials used to create paintings, works of art on paper, and photographs. This knowledge not only enlightens us about the artists' techniques, but it also aids in the care and preservation of the works.» As we see in the current exhibition Stieglitz, Steichen, Strand, fine art photographers in the early twentieth century were able to choose from a variety of commercial, manufactured photographic papers. They could also hand-coat selected papers with light-sensitive compounds mixed from scratch. Platinum, cyanotype, direct carbon, and gum dichromate processes could be used in an individualized, painterly manner, allowing an artist's hand to be purposely visible. In some cases, artists used multiple processes in combination, one on top of the other, then embellished a photograph with brushwork during processing, and completed the work with hand-applied coatings.

Anna Vila-Espuña employs micro X-ray fluorescence spectroscopy (XRF) in the nondestructive analysis of photographs.

It is possible to identify many photographic processes with the naked eye or by studying a print's surface characteristics under a microscope. Art historical information about the time period, the geographic region, and the artist's working habits also contributes valuable clues. When visual and historical identification is inconclusive, however, we turn to nondestructive, noninvasive methods of scientific analysis such as X-ray fluorescence spectroscopy (XRF) or Raman microspectroscopy. XRF is used to identify the elemental composition, indicating, for example, the presence of imaging materials such as the silver and gold particles present in a nineteenth-century albumen print or the platinum in a platinum print. Raman microspectroscopy is an analytical technique that can be used to identify pigments, binders, and other materials used by artists. In Raman measurements, the work of art is irradiated with a laser beam and the energy and number of scattered photons—which interact differently with each type of molecule—is analyzed in the spectrometer.

Edward Steichen (American, b. Luxembourg, 1879–1973). Alfred Stieglitz and His Daughter Katherine, 1904. Platinum print. The Metropolitan Museum of Art, New York, Alfred Stieglitz Collection, 1949 (49.55.228)

In preparation for Stieglitz, Steichen, Strand, we discovered that scientific analysis is particularly helpful in unraveling the working methods of Edward Steichen, who used a variety of processes, sometimes combining two or more—one on top of the other—to make a single print. XRF analysis of Steichen's photograph Alfred Stieglitz and His Daughter Katherine, for example, revealed peaks for platinum and mercury, confirming a platinum print with mercury incorporated within the sensitized paper or during the developing step.

XRF spectra recorded in Alfred Stieglitz and His Daughter Katherine. The blue spectrum was acquired in a dark image area and the purple one in a light image area.

Platinum prints may range from neutral black to cool gray to warm sepia in tonality. The process is valued for its long tonal range, which allows photographers to represent subtle variations in light, sensitively depicting tonal gradations. The simple physical structure of the platinum print consists of platinum metal dispersed in the paper fibers, which typically produces a rich, matte surface. In many of the warm-toned papers used by Steichen, mercury and platinum metals are both present. In some cases the prints may be coated with a wax or resin—or even a mixture of the two.

Edward Steichen (American, b. Luxembourg, 1879–1973). Untitled, 1904. Gum bichromate print. The Metropolitan Museum of Art, New York, Alfred Stieglitz Collection, 1933 (33.43.13)

Gum dichromate prints, commonly called "gum-bichromate," or simply "gum" by turn-of-the-century artists, are pigment prints. In such works, the photographer combined a pigment of the desired color with a solution of gum arabic and dichromate and brushed this mixture onto a gelatin-sized paper; watercolor papers were sometimes selected for their sizing and rougher surface texture. The pigment-coated paper was allowed to dry, then exposed to bright light in direct contact with a photographic negative of the size desired. After exposure, the sheet would have been placed in a water bath to dissolve those areas not exposed to light and reveal the white paper beneath, thereby forming the light portions of the image. The darker tones of the image represent the areas of the dichromate coating, which hardens and becomes less soluble when exposed to light.

The untitled photograph by Steichen (shown above) is unusual in that he inscribed it "Experiment in Multiple Gum," noting for himself—or perhaps for Alfred Stieglitz—technical details about the printing process. The inscription reads:

Experiment in Multiple Gum / showing color coating on edges / 1st printing solid lamp Black (contrasty / 2nd printing terre verte (flat) / 3rd sepia + black (very pale) / The three printings developed mechanically / by floating paper on cold water / no local manipulation.

Pigment particles are seen distributed across the paper surface and caught in interstices of the paper fibers. Note that the darks are rendered by the pigment, the "whites" by the paper, and the mid-tones are rather grainy with pigment particles, a characteristic of the high contrast of the gum dichromate process.

Gum prints are characterized by a grainy appearance and a general lack of detail. Although they do not render mid-tones well, they are perfect for the hazy, dreamlike aesthetic favored by the Pictorialists, or for images with bold contrasts in imitation of painting or charcoal or red-chalk drawing.

Edward Steichen (American, b. Luxembourg, 1879–1973). The Photographer's Best Model—George Bernard Shaw, 1907. Gum bichromate over platinum print. The Metropolitan Museum of Art, New York, Alfred Stieglitz Collection, Alfred Stieglitz Collection, 1949 (49.55.166)

In this playful portrait, Steichen utilized the strengths of both the gum and platinum processes. While evidence of his technique is visible to the naked eye—a higher sheen in the dark areas, where the pigment and binder are thicker, for instance—scientific analysis tells us even more. The warm-toned platinum-mercury image, confirmed by XRF analysis, is readily apparent in the gentle brown tonalities of the sitter's face, yielding far greater detail and subtlety than would a gum print alone. By contrast, the heavy pigment application of the gum process rendered Shaw's dark clothes as a deep, solid form and allowed Steichen to manipulate the background to give a suggestion of painterliness.

Details of Shaw's nose reveal small spots of pigment from the gum dichromate process overlaid onto the smooth, continuous platinum image. The platinum is distributed as particles too fine to be seen under magnification, thus rendering image details of infinitesimal proportions.

XRF analysis on the photograph detected chromium from the dichromate used in the gum process. Particles, only visible under magnification, were revealed with Raman analysis to be carbon-based black and indigo blue pigments.

Raman spectra of indigo blue colorant and carbon-based black pigment

While we may have appreciated the beauty of Steichen's photographs, the scientific analysis of his methods increases our appreciation of his achievement and gives us greater understanding of his intentions as an artist.

Silvia A. Centeno is a research scientist in the Department of Scientific Research.

Anna Vila-Espuña is an Annette de la Renta Fellow in the Department of Scientific Research.

Nora W. Kennedy is Sherman Fairchild Conservator of Photographs in the Department of Photographs.

Silvia Centeno

Silvia A. Centeno is a research scientist in the Department of Scientific Research.