VII. The Identification of Materials and Processes Used in the Manufacture of Orotone, Hand-Colored Orotone, and Silvertone Photographs

Introduction

Popular from the late nineteenth century to the 1940s, the orotone (also known as a Curt-tone, Doretype, and goldtone) consisted of a positive photographic image developed on glass and coated with a varnish modified by a yellow metal flake that gave the image its characteristic brilliancy. A silvertone was a photograph with the same layer structure as an orotone but contained a white metal flake instead of a yellow metal flake. The orotone photographic process was popularized by Edward S. Curtis (1868–1952), whose orotones served as a major source of income for his studio. Orotones were primarily popular in the western United States and are still somewhat rare in museum and library collections,¹ making them a unique art form about which there is limited available knowledge.

The aim of this project was to provide a broader understanding of the materials and methods used in the production of orotone photographs and to aid in considerations for their long-term preservation and exhibition. The project focused on the material analysis of the orotone collection at the University of Washington (UW) Libraries by the Lasseter Clare Lab at Portland State University within the Pacific Northwest Conservation Science Consortium (PNWCSC). The PNWCSC is a collaboration of five regional museums and the Lasseter Clare Lab that provides scientific expertise and instrumentation to consortium partners for research on a range of artistic and historic works. Graduate students in the Clare Lab partner with conservators and curators at these institutions to explore conservation science questions and projects in a real-world context. The UW Libraries’ collection consists of thirty-two orotones, including ten hand-colored orotones, two silvertones, twenty monochrome orotones, and one glass positive framed with a yellow metallic cardboard backing board rather than an applied varnish with metal flake (Fig. 1). These orotones range in date from the late 1890s through the 1970s.

History of the Orotone and Silvertones

The orotone and silvertone process has its origins in earlier photographic techniques used to create positive images on glass, particularly photographically based lantern slides popular in the last half of the nineteenth century.² As positive transparencies on glass designed to be viewed through projection, lantern slides were monochromatic and could be tinted, chemically toned, or hand colored.³ These projections were a popular form of entertainment and were used by Edward Curtis in his travelling lectures.⁴

The first printed record of the orotone process appeared in November 1858 in the Journal of the Photographic Society of London in a letter to the editor by R. M. Grier, in which he described his invention of a new photographic process utilizing a positive image on glass backed with a yellow metal “bronzing powder.”⁵ However, the process Grier described did not appear to have gained popularity at the time of publication. Further experiments with yellow-metal photographic images emerged in the late-nineteenth century when Hanbeh Mizuno (1852–1920) introduced the now-obscure maki-e process. The photographic process combined maki-e, a traditional Japanese decorative process of sprinkling metallic powder on lacquered wood, with a French photographic printing technique known as the dust-on method.⁶ A mixture of ammonium dichromate, gum arabic, and sugar was applied to a black-lacquered wood substrate and exposed to light while in contact with a negative. Nonexposed areas would retain their tackiness and were sprinkled with gold, which would form the light parts of the image. Finally, collodion was applied to protect the surface.⁷ One type of maki-e photograph utilized a glass support, though the gold flake in these images was dusted onto the unexposed areas before the black lacquer was applied, while orotones were created by a positive image before a yellow metal layer was applied.⁸

The studio of Curtis, one of the orotone’s most prominent practitioners, was a major producer of orotones beginning in 1916.⁹ By the mid-1910s the orotone process had gained in popularity and was described in contemporary photographic trade journals under various names, including dorotype and doretype.¹⁰ The process reached its pinnacle of popularity from the late nineteenth century into the 1920s and remained somewhat prominent through the 1940s.¹¹

Orotones were created as works of art and were often sold in characteristic art-nouveau style frames.¹² The frames completed the aesthetic presentation and provided protection for the fragile glass support. The images were sometimes hand colored and often portrayed natural landmarks or individuals in studio portraits.¹³ The popularity of orotones coincided with the American Pictorialism movement. Pictorial photography emphasized the artistic use of the medium over its mechanical and scientific applications. The movement sought to elevate photography to an art form. Pictorialism was marked by an emphasis on craftsmanship, with photographers employing a range of techniques, including selective lighting, composition, and focus, as well as the application of hand-coated emulsion and creative photographic-finishing processes. The warm hue, soft focus, and carefully crafted processing of orotone photographs reflect the Pictorialist style.¹⁴

The Orotone and Silvertone Photographic Process and Composition

The characteristic layer structure of orotone and silvertone photographs consists of a glass image support, a light-sensitized emulsion layer, and a varnish layer with metal flake. (Fig. 2). The glass support on which the emulsion was applied was likely soda-lime silica glass, containing 65–75% silica, 10–20% alkali and the remainder lime.¹⁵ This was the most common and durable glass available in the early twentieth century. While the photographic development process used for creating orotones is not documented, it is likely to have been one of two methods of glass-plate photography contemporaneous with orotone production: the wet-plate collodion process, popular from 1851 to 1885,¹⁶ or the gelatin dry-plate process, popular from 1880 to 1925.¹⁷ Both utilized a well-established method of dispersing silver halides in the emulsion, exposing them to light, then developing and fixing the silver clusters to create the photographic image.

The chemicals used in the photographic development process and their reaction byproducts often left residues in the emulsion. A description of the relevant chemicals in this development process can therefore inform the interpretation of compositional analysis. Briefly, the emulsion layer contained a silver halide dispersion¹⁸ containing either iodine, chlorine, or bromine.¹⁹ Exposure to light converted the silver ions within the silver halides to silver atoms which form silver atom clusters.²⁰ A potassium nitrate by-product produced during light exposure was washed away with water. The final imaging medium consisted of these silver atom clusters, which were enlarged and consolidated using a developing agent. A potassium halide restrainer removed residual halide ions²¹ while a bath of sodium sulfite, acetic acid, and potassium aluminum sulfate (colloquially referred to as alum) hardened the emulsion layer at the end of the process.²²

A toning agent may have been used before the emulsion was hardened to shift the tone of the entire image by replacing either the silver or halide ions. To provide a sepia (brown) tone rather than black, sulfur toning was done, which was described by one source as a step in the orotone development process and involved converting the silver atom clusters back to silver halides using potassium ferricyanide and a potassium halide, which bleached the entire image. The image plate was then immersed in sodium sulfide, converting the silver halides to silver sulfide, a dark solid that is more stable than reduced silver atoms.²³ Metals such as platinum and gold have also historically been used as toning agents in similar photographic processes, replacing silver ions and impacting the hue and permanence of the imaging medium.²⁴ Given the limited documentation available for orotone processing methods, it is unclear to what extent orotones underwent toning.

If an orotone was hand-colored, pigments were painted onto the emulsion layer. A varnish was then applied consisting of collodion dissolved in amyl acetate, acetone, and benzine. Sometimes castor oil was added to increase flexibility and wax to decrease glossiness. This solution was termed “banana liquid” due to the characteristic banana scent of amyl acetate.²⁵ A metal flake was mixed into the varnish before application and pouring over the back of the orotone,²⁶ though it may also have been applied after as a leaf or dust.²⁷ Despite the terms orotone and silvertone implying the use of gold and silver metals, neither have yet been identified in these photographs.²⁸ The terms orotone and silvertone refer to the color of the metals rather than their composition.

Previous Scientific Research

There are only two technical reports published on orotones and no published studies of silvertones. The first analysis of orotones was conducted in 1988 by Siegfried Rempel, who studied an orotone titled Wisconsin Dells (1897) by Henry Hamilton Bennett (1843–1908) using XRF spectroscopy. He concluded that the “bronze powder,” or metal flake, was a brass alloy of copper and zinc.²⁹

The second study was conducted by Richard Stenman in 2011, in which he analyzed four orotones using XRF spectroscopy and scanning electron microscope/energy dispersive X-ray spectroscopy (SEM/EDS). He concluded the following: the image material for three out of the four images studied was a silver halide; toning agents may have been present in one orotone, indicated by the detection of sulfur in image areas containing silver; the elements potassium, arsenic, strontium, barium, iron, and calcium were detected and hypothesized to originate from the glass support; and the metal flake was composed of copper and zinc. Using FTIR spectroscopy, Stenman determined that the emulsion was composed of gelatin in three orotones from the 1920s and 1930s, attributed to the gelatin dry-plate process, while an older orotone contained a collodion emulsion associated with the older wet-plate collodion process. Collodion was identified by FTIR in the varnish layer of all orotones studied, along with metal stearates, added as a stabilizer.³⁰

Rempel’s and Stenman’s contributions to the available knowledge of orotones was significant; however, they both noted that analyzing a larger sample set would add to their findings.

Specific Aims and Scope

This project aimed to build upon Rempel’s and Stenman’s work by performing materials analysis on a range of orotones and silvertones from the UW Libraries. The materials analyzed included the glass image supports, elements associated with the photographic development process, pigments in hand-colored orotones, metal flake in orotones and silvertones, and the emulsion and varnish layers. Eighteen photographs in the UW Libraries’ permanent collection were analyzed including sixteen orotones, six of which were hand-colored, and two silvertones. One orotone did not contain an applied metal flake, and instead was backed with a gold-colored board. All were given unique numbers for this study, formatted as PNWCSC #O1 (Table 1). An orotone from the Portland Art Museum (PAM) was also analyzed which has undergone delamination. By comparing the orotone from the PAM with those from the UW Libraries, it was hoped that the cause of delamination could be identified.

Materials and Methods

Visual Examination of the Metal Flake and Varnish Layer

The backs of all orotones were photographed and a subset of six orotones (PNWCSC #O2, #O9, #O16, #O19, #O20, and PAM 2001.122) was examined to determine if the application of metal flake could be inferred by the overall appearance of the flake and varnish layers. Varnish microsamples containing metal flake were collected from these six orotones and imaged using a Leica MZ6 stereomicroscope using the program Leica Application Suite Version 4.12.0 (Leica Microsystems, Deerfield, IL, USA) and utilizing a Volpi Intralux 4000-1 fiber optic light source (Volpi Group, Auburn, NY, USA). By comparing the stereomicrographs of the microsamples with the photographs of the orotone backings, the flake application method could be inferred.

Glass Density Calculations

The dimensions and masses of orotones PNWCSC #O2, #O8, #O9, and #O18 were measured to calculate glass density and determine whether glass was soda-lime-silica or a more dense glass such as lead glass. Varnish and backing layers were accounted for by assuming a backing thickness of 0.003 cm, which is double the thickness of previously published measurements of gelatin films on glass.³¹ Assuming a density of 0.77 g/cm^3^ for collodion and 1.27 g/cm^3^ for gelatin, masses were calculated for each layer and subtracted from the total orotone mass while film thickness was subtracted from measured thickness. Correcting for this layer only increased the calculated glass density by <1%. Therefore, a simple calculation of total mass (g) / total volume (cm^3^) was used to calculate glass density for the four orotones.

XRF Spectroscopy

XRF spectroscopy was utilized to qualitatively determine the elemental composition of all orotones and silvertones in this study. This method was chosen as it allows for non-destructive analysis of elements within the layers of the orotone and, with the use of filters, can be optimized for collecting elements with wide-ranging atomic masses. It significantly reduces the requirement for sampling. Finally, it reproduces the method of analysis used by Stenman in his study of orotones³² and has been documented as a successful method for characterizing the composition of photographic materials.³³

Spectra were collected with a Tracer III-SD XRF spectrometer (Bruker, Billerica, MA, USA) equipped with a rhodium (Rh) source silicon drift detector (SDD), and connected to a 3V vacuum pump (Bruker, Billerica, MA). All spectra were collected at a pressure of less than 30 Torr and a beam spot size of approximately 10 mm. A red filter was used for most orotones, which is composed of three stacked filters (0.001-inch copper and titanium filters and a 0.012-inch aluminum filter). When using a red filter, voltage was set to 40 kV, current to 7.60 μA, and acquisition times to 20, 60, or 300 seconds. In some cases, a blue filter, or 1-mm titanium filter, was used, and voltage was set to 20 kV, current to 55 μA, and acquisition time to 60 seconds. If no filter was used, acquisition parameters were set to 40 kV, 7.60 μA, and acquisition time set to 30 or 60 seconds. Settings were selected to maximize count rate without saturating the detector. The unframed orotones from the UW Libraries’ collection were placed on a polyethylene foam support to reduce background noise and covered by a mylar sheet with ~7-mm diameter holes over the test areas. The instrument was mounted on a tripod and positioned perpendicular to the orotones at a distance of approximately 2 mm and aligned to each location using crosshairs on an alignment map on mylar. Nine of the photographs from the UW Libraries’ collection were analyzed, both from the front through the glass and from the back over the metal flake; the other eight photographs were only analyzed from the back.

Background spectra were collected at each instrument setting used for analysis. All spectra were normalized to the Compton peak unless collected using only a titanium filter, in which case the spectra were normalized to the background titanium peak. After normalization, background spectra were subtracted from orotone spectra that had been acquired under the same parameters. Spectra collected from the front, accounting for the glass, were also subtracted from spectra collected from the back, leaving only elements associated with the metal flake and imaging materials in the emulsion layers. Peaks not associated with the metal flake in the corrected spectra were associated with photographic materials in the emulsion layer. If a spectrum was not collected from the front, elements were assigned to layers based on how they tracked with the image, what the literature indicated, or how the results compared with other orotones.

Results were reported as signal-to-noise ratios (S/N) and were standardized to a 60-second acquisition time, as the S/N increases with the square root of the total acquisition time. Reporting data as S/N ratios provides objectivity to the identification of elements within the orotone and provides a consistent benchmark for both instrument performance and detection limits when reporting minor or trace elements. S/N values have consistently been used in forensic analysis of glass and art materials.³⁴ Comparing S/N values in the metal flake provided compositional information allowing for more detailed comparisons of alloys across orotones. Furthermore, by assigning a detection limit value for S/N, data were eliminated that did not meet criteria for detection. In this case, an S/N value of 10 or greater was used as a benchmark for detection, while values less than 10 were attributed to trace elements.

SEM/EDS

SEM/EDS was carried out on the silvertone microsample to verify XRF results and determine the metal flake composition. EDS is more sensitive to elements of low molecular weights and is therefore suited to detecting metals such as aluminum. The microsample was mounted onto carbon tape affixed to an aluminum sample holder and surrounded at a short distance by strips of copper/nickel tape to minimize charging. The sample was analyzed by SEM/EDS using a Sigma variable-pressure field emission SEM (FESEM) (Zeiss, Oberkochen, Germany) equipped with an Ultim Max 65-mm^2^ energy dispersive X-ray detector (Oxford Instruments, Abingdon, UK). EDS elemental mapping was performed with AZtec software (Oxford Instruments, Abingdon, UK). Acceleration voltage was set to 20 kV, working distance to 8.3 mm, and processing time to 3 for adequate dead time.

FTIR Microanalysis

Microsamples of the emulsion and varnish layers from PNWCSC #O2, #O9, #O16, #O20 and PAM 2001.122 were analyzed using µFTIR spectroscopy to obtain compositional information. Additionally, two pigmented microsamples collected from the edge of PNWCSC #O3 were analyzed to aid in pigment identification—one from a red pigment in a tree and the other from a blue pigment in the sky. Both samples included the pigment, the emulsion layer, and the varnish layer with metal flake. µFTIR spectroscopy does not destroy the sample, allowing for further analysis and verification of findings. The orotones chosen for sampling represented a range of artists and dates from the early twentieth century. Analysis utilized a Nicolet Continuum FT-IR microscope coupled to a Nicolet iS10 infrared spectrometer (Thermo Fisher Scientific, Waltham, MA) with a 50-μm nitrogen-cooled mercury cadmium telluride type A (MCT/A) detector and operated using Omnic software (version 8.3.103). The varnish and emulsion samples were compressed onto a diamond cell. Spectra were acquired in transmission mode between 750 and 4000 cm^-1^ at 4 cm^-1^ spectral resolution. 64 scans were averaged per spectrum. The data were transformed using an N-B strong apodization function and Mertz phase correction. Once collected, the spectra were converted to absorbance and compared to the IRUG databases.³⁵

Raman Spectroscopy

Raman spectra were collected to aid in pigment identification. Spectra were collected using a LabRAM HR Evolution Raman spectrometer (Horiba Scientific, Ltd., Kyoto) with a Synapse detector, using a 532-nm mpc6000 laser (Laser Quantum, Fremont, CA) operating at 4.8 mW, a neutral density filter at 2.5%, and a grating of 600 lines/mm. The spatial resolution was 1.3 μm under a 50× objective with a 400-μm confocal hole. The acquisition time was 5 seconds for 30 accumulations in the range of 100 to 3000 cm^-1^ with a 5.7 cm^-1^ spectral resolution. The spectra were collected and baseline corrected using LabSpec6 (version 6.3.40.9).

Py/GC/MS

Pyrolysis/gas chromatography/mass spectrometry (Py/GC/MS) was carried out on backing samples from PNWCSC #O2, #O9, #O16, #O20 and PAM 2001.122 as well as on mock-up varnishes created using cellulose nitrate and amyl acetate. This mass spectral method provided verification of FTIR findings as well as greater sensitivity, allowing for identification of major and minor organic components of the emulsion and varnish layers. Amyl acetate, which may remain in trace amounts in the orotone varnishes, is likely only detectable using this method.

Py/GC/MS was carried out with a Model 4000 Pyroprobe (CDS Analytical, LLC, Oxford, PA) coupled to a 6890N GC system and a 5973 inert standard turbo electron ionization (EI) mass selective detector (Agilent Technologies, Santa Clara, CA) with a quadrupole mass analyzer and electron multiplier detector. The chromatographic column was a J&W DB-5 fused silica capillary GC column (30 m × 250 μm × 0.25 μm) with a (5%-phenyl)-methylpolysiloxane stationary phase (Agilent Technologies, Santa Clara, CA).

The Py/GC/MS method was adapted from literature.³⁶ Pyrolysis was carried out at an initial interface temperature of 75°C for 0.1 min followed by a ramp of 100°C/min and a final temperature of 125°C held for 3 minutes. Pyrolysis was initiated when the final interface temperature reached 125°C. The initial temperature of the pyroprobe was 125°C, followed by a ramp of 10.0°C/msec and a final pyrolysis temperature of 600°C held for 10 seconds. The front inlet temperature of the GC was held at 250°C. The carrier gas was helium, the flow rate was 1.0 mL/min, and the nominal inlet pressure was 7.62 psi. The chromatograms were collected in splitless mode, so the purge flow was adjusted to 49.9 mL/min to achieve the same total flow (53.6 mL/min) as the literature method. The GC oven was programmed with an initial temperature of 50°C held for 5 min followed by a ramp of 5°C/min to 240°C held for 5 min, and then finally a ramp of 5°C/min to 300°C held for 8 min. This was followed by a cool-down ramp of 50°C/min to 200°C, a hold of 2 min, then a ramp of 50°C/min to 100°C, and a hold for 2 min. A temperature of 50°C was held for 1 min post-run. The MSD transfer line was kept at 250°C. The mass spectra were collected with EI at 70 eV over the m/z range of 40–600 without a solvent delay. The MS ion source was held at 230°C and the MS quadrupole was held at 150°C. The data were collected and processed using ChemStation E.02.02.1431 (Agilent Technologies, Inc., Santa Clara, CA). Preliminary identification of compounds was achieved by comparing collected spectra to the NIST/Wiley (W8N08) MS library (John Wiley and Sons, Inc., Hoboken, NJ). User-created mass spectral libraries of reference binders were also used to identify some compounds.

Results and Discussion

Metal Flake Analysis

Visual Analysis

Visual analysis of backings and microsamples indicate that the six orotones examined represent cases of both mixed-in metal flake and dusted-on flake. Figure 3 demonstrates how both backings and microsamples appear for each type. The dusted-on orotones are characterized by an uneven reflective yellow color caused by localized differences in metal flake density during the dusting-on process, and microsamples from these orotones are distinguishable for being especially reflective and friable (Fig. 3b and d), compared to those from mixed-in orotones. The mixed-in metal flake varnishes were typified by very even metal flake distribution, in one case containing striations from the pouring-on process, while microsamples were solid without friation and with flake distributed throughout (Fig. 3a and c).

Visual examination indicated that the metal flake of three orotones had been dusted on after varnish application (PNWCSC #O9, #O18, and #O20, PAM 2001.122) while the metal flake of two other orotones had been mixed into the varnish and poured on (PNWCSC #O2 and #O16). PNWCSC #O9 appeared to have an even distribution of flake (consistent with a mixed-in flake varnish); however. the loss of metal flake from abrasions without the loss of the image or underlying varnish as well as the tendency of the microsample to shed metal flake indicated that the metal flake was dusted-on (Fig. 4).

Two orotones with dusted-on metal flake are attributed to Edward Curtis (PNWCSC #O18 and PAM 2001.122), while two mixed-in flake varnishes were found in orotones by his brother Asahel Curtis (1874–1941) (PNWCSC #O2) and John Steen (PNWCSC #O16). John Steen was known to be an employee of Asahel Curtis, who in turn worked in the Edward Curtis studio in Seattle producing orotones.³⁷ Notably, the brothers used different metal flake application methods in the orotones mentioned. Documentation implies that Edward Curtis primarily mixed-in the metal flake and poured it on,³⁸ while results of examination indicate that at least some of his orotones were created with the dust-on method.

Elemental Analysis

Copper and zinc were detected in every orotone analyzed in this study using XRF spectroscopy (Table 2), indicating that brass metal flake was used. Dividing S/N values of zinc by copper revealed all brass metal flake to be similarly composed (with ratios ranging from 0.10–0.20) except for PNWCSC #O21, which had a ratio of 0.40.

Aluminum was tentatively identified by XRF spectroscopy in PNWCSC #O27 based on an unusually high-intensity peak with an S/N value of 17. Given that aluminum’s emission energy is near the lower detection limit of the XRF instrument, this indicated that the metal flake was likely composed of aluminum, but verification with another technique was required. Elemental maps were collected of a microsample from silvertone PNWCSC #O27 using SEM/EDS and identified aluminum in the lower left region of the sample (Fig. 5e), verifying that the metal flake used in this silvertone was an aluminum metal flake.

Glass Analysis

Elemental Analysis

XRF spectroscopy identified strontium, iron, and zirconium in the glass of most orotones and silvertones. These elements have been identified as common elements in glass.³⁹ Calcium was identified in the glass of all orotones, likely originating from the lime, or calcium oxide, added to soda-lime-silica glass to improve its chemical durability.⁴⁰ Calcium along with oxygen, sodium and silicon were identified in the glass of PNWCSC #O27 using SEM/EDS elemental maps, verifying that soda-lime-silica glass was used (Fig. 5a–d). Arsenic was identified by XRF in the glass of five orotones. Arsenic is added to glass to provide clarity, improve color and transmittance, or prevent the formation of bubbles.⁴¹ Chromium was detected in the glass of PNWCSC #O6 and may have been used as a glass colorant.⁴² Lead was identified in PNWCSC #O2, implying the glass may be lead glass or soda-lime-silica glass with a lead impurity or additive.

Glass Density Calculations

Glass density was calculated to determine if the presence of lead indicated lead glass or a soda-lime-silica glass with impurities. A density of 3.1 to 5.9 g/cm^3^ is expected for lead glass while soda-lime-silica glass has a density of 2.4 g/cm^3^.⁴³ All four orotones studied (PNWCSC #O2, #O8, #O9, and #O18) had glass densities less than 2.4 g/cm^3^, ranging from 1.55–2.07 g/cm^3^, indicating that they were soda-lime-silica glass with small amounts of lead and arsenic added to improve glass properties. Lead is often added to glass for its fluxing effect or for its higher refractive index.⁴⁴

Imaging Media and Processing Chemicals: XRF Analysis

Silver was identified as the imaging medium for 9 out of the 11 analyzed orotones (Table 2). This was further verified by differences in peak intensities between dark and light image areas in PNWCSC #O2 and #O7. Silvertone PNWCSC #O27 and orotones PNWCSC #O8, #O9, and #O18 contained high silver peaks in dark and mid tone image areas and no silver in light image areas, further verifying that the imaging medium was silver. Bromine was identified in three orotones, originating from either silver bromide crystals in the emulsion that were not washed away after sensitization or residual potassium bromide used at multiple stages in the developing process. Trace mercury (Hg) was identified in PNWCSC #O9, although its origin is unclear. Iron was present in five orotones, likely left over from the potassium ferricyanide bleaching agent. Three orotones (PNWCSC #O2, O18, and O26) were analyzed at high-density and low-density image areas, yet the iron peaks in both spots were of about the same intensity. This suggested that the iron was evenly dispersed in the emulsion and was likely from a potassium ferricyanide bleaching agent applied to the whole photograph.

Analysis of Pigments in Hand-Colored Orotones

Elemental Analysis: XRF Spectroscopy

Seven areas of hand-colored orotone PNWCSC #O7, corresponding to seven different pigments, were analyzed using XRF spectroscopy (Fig. 6a, Table 3).

A high-intensity mercury peak with an S/N value of 590 was identified in the red pigment at Spot 02, indicating vermilion (Fig. 6b). Chromium peaks, with S/N values ranging from 20 to 32, were identified in all test areas. These values are much higher than those found in other orotones, which contain S/N values around 9, indicating only trace amounts of chromium due to bleaching agents or chrome alum hardeners. The intensity of the chromium peaks in PNWCSC #O7 compared to other orotones, as well as a distinct yellow cast compared to the others, indicates a likely chromium yellow wash over the whole orotone.

Cadmium was identified in a dark green pigment in PNWCSC #O7 (Spot 03), suggesting a mixture of cadmium yellow with a blue pigment. Spot 03 also contained a notable iron peak, suggesting the blue pigment might be Prussian blue.

Iron was detected throughout orotone PNWCSC #O19 and was determined to originate from pigments, but specific iron-containing pigments could not be identified by XRF.

µFTIR Analysis of PNWCSC #O3 Pigments

Two pigmented microsamples collected from the edge of another orotone, PNWCSC #O3, were analyzed via µFTIR spectroscopy. The red pigment could not be identified, while the blue pigment contained a strong amide I band at 1645 cm^-1^ and an amide II band at 1543 cm^-1^ (Fig. 7a) corresponding to a protein backbone as shown in the reference protein spectrum (Fig. 7b), as well as a peak at 2083 cm^-1^ attributed to a nitrile functional group. This functional group is present in Prussian blue (Fe~4~[Fe(CN)~6~]~3~) (Fig. 7c) as well as the bleaching agent used in the photographic process, potassium ferricyanide.

Raman Spectroscopy of PNWCSC #O3 Pigments

In order to determine the origin of the amide bands and nitrile group in the blue pigment and aid in identification of the red pigment, both were analyzed by Raman spectroscopy. The Raman spectrum contained peaks corresponding to the Fe^II^-CN-Fe^III^ stretching vibration, Fe-C stretching vibration, and Fe-CN-Fe deformation vibration (Fig. 8).⁴⁵ The presence of these peaks confirmed that the blue pigment was Prussian blue. The red pigment was not identified using Raman spectroscopy, though the lack of heavy metals from the XRF analysis suggests an organic red pigment.

Py/GC/MS and µFTIR Analysis of Emulsion and Varnish Layers

Backing samples from four orotones were analyzed via Py/GC/MS and µFTIR, namely PNWCSC #O2, #O9, #O16, and #O20, and delaminating orotone PAM 2001.122. The entire layer structure was analyzed together via Py/GC/MS, while the emulsion layer was analyzed separately from the varnish layer using µFTIR spectroscopy (Table 4).

Emulsion Layer: Gelatin

Proteins were detected via Py/GC/MS in samples from PNWCSC #O16 and #O20 along with PAM 2001.122 (Fig. 9). All samples contained pyrrole and its derivatives along with unidentified peaks matching mass spectra from gelatin and animal-glue reference samples. Pyrroles are derived from the pyrolysis of proline and hydroxyproline, two of the most abundant amino acids in collagen, the primary component of gelatin.⁴⁶ When the emulsion layers from orotones PNWCSC #O2, #O16, and #O20, as well as PAM 2001.122, were analyzed using µFTIR spectroscopy, all contained a strong amide I band near 1645 cm^-1^ and an amide II band near 1543 cm^-1^. The peak at 1450 cm^-1^ corresponds to the δ(CH~2~) and δ(CH~3~) absorptions and the peaks from 1280–1233 cm^-1^ correspond to the amide III band (Fig. 9b–e). All peaks result from a protein backbone as shown in the reference protein spectrum (Fig. 9a), indicating the emulsion layers are proteinaceous and likely gelatin. All orotones which were found to contain gelatin emulsions date from the early twentieth century, further supporting the conclusion that the gelatin dry-plate process was used in their production.

Varnish Layer: Cellulose Nitrate

A few microsamples were taken for µFTIR spectroscopic analysis to gain insight into the varnish layers. Cellulose nitrate was identified in the varnish layers of PNWCSC #O2, #O9, and #O16, while the varnish layer of PNWCSC #O2 also contained a polymethacrylate polymer and phthalate plasticizers (Fig. 10), which is further discussed in the next section. The presence of cellulose nitrate was verified by the nitrate vibration bands near 1653, 1280, and 840 cm^-1^ as well as a COC stretching peak at 1070 cm^-1^. A peak at 1725 cm^-1^ in the spectrum from the PNWCSC #O2 varnish sample corresponds to camphor, a common plasticizer in cellulose nitrate.⁴⁷ Camphene, a camphor derivative and common plasticizer in cellulose nitrate, was detected in PNWCSC #O2 using Py/GC/MS and µFTIR, though its presence was detected in the emulsion (not as a component of the vanish layer) using µFTIR (Fig. 9b).

To further characterize the cellulose nitrate varnish layer, the degree of substitution (DS) was calculated. This value indicates the number of hydroxyl groups in the glucose monomer of cellulose that have been replaced or modified, in this case by nitrates. The highest DS value for cellulose nitrate is 3, indicating total substitution; however, most cellulose nitrate films have a DS value less than 3.⁴⁸ First, a baseline correction was carried out as described by Nunes et al.,⁴⁹ then the ratio of the intensities of the NO~2~ peak at 1662 cm^-1^ to the COC stretching peak at 1070 cm^-1^ was calculated. The result was then input into the calibration curve created by Nunes et al. This yielded DS values of 1.59 from orotone PNWCSC #O2, 1.76 from orotone PNWCSC #O16, and 2.19 from orotone PNWCSC #O9. Calculating the DS of an IRUG database reference sample, a sixty-three-year-old film of cellulose nitrate, yielded a DS value of 2.09. PNWCSC #O9 (produced in the early 1900s) appears to be most similar to the aged reference despite being significantly older. It is also the only orotone of the three that has a dusted-on metal flake layer. The low DS value of 1.76 determined for orotone PNWCSC #O16 suggests some nitrate loss. PNWCSC #O2 has the lowest DS value, indicating it may have undergone degradation in the form of nitrate loss. Results from Nunes et al. found that a roll of cellulose nitrate film had a much lower DS value in the interior of the roll than on the exterior. They theorized that being so encapsulated by layers of film prevented off-gassing and loss of nitric acid byproducts, which were instead retained and caused further chain scission and embrittlement of the film.⁵⁰ Perhaps the low DS value for PNWCSC #O2 is related to its encapsulation by an additional polymethacrylate varnish layer. The orange-peel surface texture of the orotone indicates that the polymethacrylate was sprayed on, implying its application as a protective varnish or flake consolidant. This varnish may instead have promoted deterioration of the underlying cellulose nitrate layer by preventing loss of acidic byproducts.

Varnish Layer: Polymethacrylates and Phthalate Plasticizers

Characterization of methacrylate types via µFTIR suggested that some combination of ethyl methacrylate, methyl methacrylate, and butyl methacrylate was utilized in some of the varnish mixtures. PNWCSC #O2 contained a clear phthalate ester absorption at 1281 cm^-1^, while this peak was present only as a shoulder in PAM 2001.122. Both spectra contained a strong carbonyl peak from 1720-1730 cm^-1^ characteristic of methacrylate polymers. Positions of the C-H stretching vibrations matched those reported for ethyl/methyl methacrylate copolymers in the case of PNWCSC #O2 and poly-isobutyl-methacrylates in PAM 2001.122 (Fig. 10).⁵¹ PAM 2001.122 was the singular orotone in which cellulose nitrate was not detected in any of its layers.

Findings from µFTIR spectroscopy were verified using Py/GC/MS, with PNWCSC #O2 found to contain majority ethyl methacrylate while PAM 2001.122 contained majority n-butyl methacrylate. The relative abundance of methacrylates and phthalate plasticizers were measured as in Babo et al. (Table 5).⁵² Both orotones contained a significant methyl methacrylate component, a common additive to poly-ethyl and poly-butyl methacrylates, which when used as a copolymer dramatically improves the stability of the polymethacrylates.⁵³ Phthalate plasticizers were detected in both orotones, though they were present in greater amounts in PAM 2001.122, comprising 20% of the chromatographic peak area when compared with the methacrylate peaks, compared with 6% plasticizer peak areas in PNWCSC #O2.

The varnish of PNWCSC #O2 appears pristine, and the entire backing is well adhered to the glass. In contrast, PAM 2001.122 is delaminating from the glass around the edges. The poly-methacrylate copolymer in PNWCSC #O2 contains a high percentage of methyl methacrylates, a known plasticizer when used as a co-polymer. Additionally, its layer structure is similar to other orotones in this study, differing only in the application of a polymethacrylate over the cellulose nitrate varnish. The ratios and types of methacrylates and phthalates present in PNWCSC #O2 compare well to Paraloid B72, a common conservation material used for consolidation and coatings. It would appear that the orotone had been treated, and the polymethacrylate applied long after the orotone was produced in order to protect the varnish and metal flake layer from deterioration.

In contrast, the varnish layer of orotone PAM 2001.122 contains predominantly poly-butyl-methacrylates, a more hydrophobic polymer prone to oxidation, chain scission, cross-linking, and embrittlement.⁵⁴ Cross-linking would have increased the rigidity of the varnish, while the more hydrophilic gelatin-emulsion layer would have expanded and contracted over time with changes in relative humidity. This combination caused the backing to lose adhesion to the glass surface, resulting in delamination. The PAM 2001.122 samples collected for analysis were more brittle than the PNWCSC #O2 samples, further indicating this loss of plasticity in the varnish layer.

A notable observation about the PAM 2001.122 orotone involves its assigned date. While the orotone is dated 1903, the presence of poly-butyl-methacrylate varnish puts this date into question. The earliest widespread use of poly-butyl methacrylates such as Acryloid® B67 dates from the 1940s.⁵⁵ The image itself appears to be an enlargement of the original photograph in the UW Special Collections (NA610) by Edward Curtis in which Edmond Meany is shown posing with Chief Joseph and Red Thunder. The orotone in this study was likely made at a much later date than 1903, enlarging only the image of Chief Joseph and using a poly-butyl-methacrylate varnish contemporary to its creation, leading to subsequent deterioration and delamination.

Conclusions

This survey of materials from eighteen photographs in the collection at the University of Washington Libraries and one orotone from the Portland Art Museum greatly broadens the available technical knowledge of orotone and silvertone photographs. All orotones studied contained gelatin emulsions, indicating they were created using the gelatin dry plate method. High-intensity silver peaks in XRF spectra taken from dark image areas compared with low-intensity or absent silver in light areas confirmed the imaging medium was silver. The prevalence of bromine, along with evenly distributed elements associated with silver bromide processing methods such as potassium and iron, indicated the silver salt used to sensitize the emulsion was a silver bromide. No evidence of gold or platinum toning was observed on any of the orotones. Sulfur toning could not be verified nor excluded.

The metal flake in all orotones contained copper and zinc, indicating brass was used to impart the characteristic “gold” tone. The silvertone PNWCSC #O27 was found to contain an aluminum metal flake. Both methods of metal flake application to the varnish, either dusted on or mixed in, were identified visually in the collection of orotones studied.

Low-density values of 2 g/cm^3^ or below coupled with an abundance of calcium indicated that the glass supports were commonly available soda-lime-silica glass. A few glass supports contained minor lead and arsenic components, additives that improve glass properties during production.

Analysis of pigments from hand-colored orotones indicated the use of both inorganic and organic pigments. The palette of inorganic pigments in this set of orotones included cadmium yellow, vermilion, Prussian blue, and iron-containing pigments. In addition, chromium yellow was used as a wash to impart an overall yellow hue to one image.

Py/GC/MS and µFTIR analysis of the orotone varnishes identified cellulose nitrate in three orotones. PNWCSC #O2 was found to contain a sprayed-on ethyl methyl methacrylate varnish over the cellulose nitrate varnish, suggesting a previous treatment has been carried out. Determining the degree of substitution for the NO~2~ group in cellulose nitrate for three orotones and a naturally aged reference sample indicated that the ethyl-methyl-methacrylate varnish over PNWCSC #O2 may have accelerated the loss of nitrates by encapsulating the varnish layer, promoting the retention of acids formed during degradation processes.

PAM 2001.122 was found to contain no cellulose nitrate varnish, instead only containing a poly-butyl-methacrylate coating applied directly to the emulsion layer. The delamination observed around the edges of the orotone was attributed to chemical-mechanical changes in the backing that differentially swelled/de-swelled at the edges. Furthermore, the image itself was determined to be an enlargement of a photograph in the University of Washington Special Collections (NA610). The unusual backing paired with this finding led to the conclusion that the assigned production date of 1903 is likely incorrect and should be modified to sometime after the usage of poly-butyl-methacrylate coatings in the 1940s.

The documentation of materials presented in this collection of orotones, along with an example of a successful treatment with the application of an ethyl methyl methacrylate to PNWCSC #O2, provide previously unpublished information crucial to the execution of successful treatments of orotones. Given the vulnerability of dusted-on metal flake to removal by abrasion, future treatments with a consolidant could prove to be beneficial to orotones by preventing further loss of flake.

A greater variety of production techniques than previously published was implied by determining the metal-flake application methods used in orotones in this study. While literature indicates that Curtis mixed in his metal flake, findings in this study indicate that Curtis sometimes used the dust-on method, while his brother Asahel mixed the brass metal flake into his varnish. By finding these prominent examples of the dust-on method in American orotones, an influence by early pioneers of gold-toned photography such as Hanbeh Mizuno is implied.

The University Libraries collections are used frequently for teaching, research, and exhibition; the results from this work will be integrated into those scholarly activities. Further studies of orotones could expand upon and illuminate the relationships between photographers implied by this study and encourage interest in the medium and its history.

Acknowledgements

This project received funding from the Mellon Foundation. The authors would like to thank the Preservation Staff at the University of Washington Libraries (Justin Johnson, Stephanie Lamson, Kathryn Leonard, and Judith Johnson), Nicolette Bromberg (Visual Materials Curator, Special Collections), and Yan Ling Choi (Conservation Intern) for collaborating with us on this project. Thank you to the Portland Art Museum and Samantha Springer for providing the orotone samples. We acknowledge Greg Baty, from the Center for Electron Microscopy and Nanofabrication at Portland State University, for analyzing the silvertone sample and Trine Quady for collecting Raman spectra.

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