4 ANALYSIS OF PLATINUM AND PALLADIUM PHOTOGRAPHS BY ENERGY DISPERSIVE X-RAY SPECTROSCOPY

4.1 APPLICATION OF EDX TO PHOTOGRAPHIC CONSERVATION

Scanning electron microscopy (SEM) with energy-dispersive x-ray analysis (EDX) was used for elemental analysis of photographs. Data were obtained using a JEOL 840 spectro-photometer. Samples equal to or less than the size of the sample cell, 25.4 � 25.4 mm in the present study, can be analyzed by EDX. For this reason, and because it is a destructive technique, EDX can be applied only to small fragments in study collections; at the same time, it is essential to know which print or group of prints a fragment came from.

The scanning electron microscope used with EDX allows very small regions of a sample, for example a visible defect, to be analyzed in addition to the entire sample. Most samples can be analyzed without prior preparation: they are simply taped down onto a graphite disc, usually with both aluminum and copper tape, which serve to calibrate the instrument. Brittle or powdered samples are prepared for analysis by placing them on the graphite disc and overlaying them with an adhesive, such as polymethyl-methacrylate (PMMA), that will not interfere with the analysis.

Currently, the spectroscopic technique most in use in conservation is x-ray fluorescence spectroscopy (XRF). Several authors have compared the benefits of SEM-EDX and XRF (Russ 1971; Gilfrich et al. 1973; Bertin 1975; Jenkins 1976; Whiston 1987; Jenkins 1988). The two technique are comparable, although XRF is more often preferred, especially for its lower cost and nondestructive nature.

4.2 RESULTS OF ANALYSIS BY EDX OF FIVE PHOTOGRAPH FRAGMENTS

Broken fragments from five photographic prints, dating between at least 1870 and 1910, in the study-research collection of the Art Museum, Princeton University, were analyzed by EDX. Pretreatment was not required: the samples were simply taped to a graphite disc as described in section 4.1. Samples are described in table 3; historical information, in some cases inferred, was provided by the museum.

TABLE 3 DESCRIPTION OF PHOTOGRAPH FRAGMENTS

Table 4 shows the elements detected by EDX analysis. Table 5 classifies the elements in table 4 according to their relative abundance in water and the earth's crust. On this basis, the following elements can be said to constitute background noise to any material being analyzed: aluminum, calcium, chlorine, iron, magnesium, manganese, phosphorus, potassium, silicon, sodium, sulfur, zinc, and possibly chromium and nickel. Important nonphotographic sources that may add to this background are photographic paper, desiccants, storage materials, and air.

TABLE 4 EDX ANALYSIS OF IMAGE SURFACES OF FIVE PHOTOGRAPH FRAGMENTS

TABLE 5 BACKGROUND ELEMENTS IN EDX ANALYSIS

In principle, assignment can now be made as to the origin of the elements in each print, using historical and spectroscopic information. For each element shown by EDX to be present, all the chemicals, listed in Gottlieb (1993), that contain that element can be identified. In each case, only the most plausible sources of an element are discussed. Chemicals mentioned frequently in platinum processes are assumed to have also been used in palladium processes, even if never explicitly mentioned for use with palladium.

All samples contain or probably contain the following elements: aluminum or bromine; silicon; lead or sulfur; calcium, indium, or tellurium; iron; copper. These are discounted, as they provide no information particular to any specific sample. On the basis of table 5, peak 1 in table 4 should represent aluminum; peak 4, sulfur; and peak 9, calcium. Given peak 14, peak 16 should represent copper rather than zinc; peak 10, calcium. Elements that cannot be discounted are shown in bold and provide the basis for analysis of the print fragments.

Sample 1 is a palladium print. The presence of chromium (Cr) probably indicates either potassium dichromate, used in the sensitizer or developer as a contrast control agent (of 32 references to chromium in the literature, 23 are as potassium dichromate in the sensitizer or developer), or chromic potassium sulfate, a paper sizing and additive in platinum intensification of silver prints. Table 5 indicates that chromium may also be a background element (Gottlieb 1993). Silver may be present in the sample, which, if positively identified, could indicate either a silver-intensified palladium print or, less likely, a palladium-intensified silver print. Uranium and molybdenum are less probably present; there is a single mention of ammonium molybdate as an ingredient in platinum intensification of silver prints, whereas uranyl nitrate is mentioned frequently as a toner for platinum prints (38 of 56 mentions of uranium). Mercury may be also be present and would probably have come from mercuric chloride in the sensitizer or developer (69 of 100 mentions of mercury).

Sample 2 is a platinum print. The presence of molybdenum and gold is less likely. Palladium is also possibly present; it could indicate a sensitizer containing both platinum and palladium, palladium intensification of a platinum print, or platinum intensification of a palladium print. If the greater permanence of platinum were known at the time, palladium intensification should not have been common. Gold may also be present; gold chloride was a common toner for platinum prints (26 of 38 mentions of gold) as well as an occasional sensitizer ingredient. Chromium may also be present; its likely origins are the same as those discussed for sample 1. The absence of lead and silver possibly dates this print no earlier than 1883, when these metals disappeared from widespread use in the platinum printing process (Gottlieb 1993). Nickel may be a background element and is never mentioned in platinum photography; it is, however, an impurity in unrefined platinum.

Sample 3 is a silver print. Uranium and molybdenum are less probably present. Processes in which the binder is organically based cannot be distinguished from one another by EDX, which cannot verify whether this is an albumen print; visual information is required. The difficult detection of iron and silicon, both certainly present, possibly indicates advanced deterioration of the print. Chromium would not be expected in a silver print. If present, it probably indicates the paper sizing is chromic potassium sulfate, or it is a background element.

Sample 4 is a silver print. Once again, no in organic elements are present that could indicate which process was used to form the image. The discussion of sample 3 applies here as well.

Sample 5 contains both platinum and silver. It is either a platinum-intensified silver print (126 mentions), a silver-intensified platinum print (24 mentions), or a platinum print prepared with silver in the sensitizer (15 mentions). The possible presence of gold does not simplify the choice: as discussed for sample 2, gold could very plausibly have been used to tone or sensitize a platinum print. Combined gold and platinum toning baths for silver prints appear to have been popular in Germany and Great Britain between 1894 and 1902 (Gottlieb 1993). Matte collodion prints, which are silver printed-out images, were typically toned with gold and/or platinum. Mercury may also be present; its likely origins are discussed for sample 1. The possible presence of chlorine and potassium, both generally highly water soluble in ionic form and largely absent from the four other samples, may indicate that this print has been relatively well preserved, at least with respect to moisture.

The paper side of sample 1 was also analyzed by EDX (table 6). Qualitative comparison with, rather than subtraction of, the spectrum from analysis of the image side may provide information about differential absorption of chemicals from the storage environment by paper and by image and about migration of chemicals from the image layer through the paper, either within one print or between adjacent prints stored back-to-front. The two spectra are nearly identical. Questionable peaks for mercury, uranium, and copper in the image are absent in the paper, and the iron peak, certain in the image, is questionable in the paper. All the elements found on the paper surface were also present on the image surface, although possibly not vice versa. As iron is a background element, no meaningful conclusion can be drawn concerning peak 12. As discussed earlier, peaks 5 and 6 together suggest peak 8 in the image sample does not represent uranium. It is interesting to note that mercury, uranium, and copper are all toners for platinum and palladium prints, which, as image-forming substances, would not be expected to migrate.