1 INTRODUCTION

A vast body of literature exists on 19th-century daguerreotype technology, extending back to the announcement of the daguerreotype in 1839. In the last 15 years, conservators and scientists have studied the complex modes of deterioration of the silver and silver amalgam image of the daguerreotype. One very important aspect of these artifacts is the surface tarnishing of the plate, but due to the extreme thinness of the tarnish layers, this aspect is difficult to examine and analyze. Recent published research by Barger and White (1991) investigated daguerreotype surface corrosion using atomic emission spectroscopy, auger electron spectroscopy, energy dispersive x-ray spectroscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, x-ray diffraction, and scanning electron microscopy. This research followed important earlier research by Swan et al. (1979).

The use of ultraviolet illumination in the examination of daguerreotypes was investigated as part of the stabilization and rehousing of the Walter Johnson Collection of postmortem daguerreotypes, a project carried out at the Strong Museum in Rochester, New York, during the summer of 1992. It followed the example of the George Eastman House's condition and monitoring project for its Southworth and Hawes Collection of daguerreotypes. However, unlike the George Eastman House project, both long-wave and short-wave ultraviolet radiation were investigated.

This study is an investigation into and preliminary analysis of one surface phenomenon observed during this project on 19th-century daguerreotype plates. The phenomenon is characterized by a bright fluorescence when illuminated by short-wave ultraviolet radiation. It was first observed by the authors in July 1992 at the Art Conservation Department, State University College at Buffalo. It has not been established if the phenomenon is actually a surface tarnish, corrosion product, or accretion, but for now we are referring to it as a surface tarnish, as it most closely resembles other tarnish patterns observed on daguerreotypes.

1.1 ULTRAVIOLET ILLUMINATION

Ultraviolet radiation extends from about 10 to 400 nm in the electromagnetic spectrum. The ultraviolet portion of the spectrum is divided into four sections, two of which are used for the examination of artifacts. The most commonly used section is long wave, or UVA, which extends from 320 to 400 nm. The less frequently used section is short wave, or UVC. It extends from about 200 to 280 nanometers in wavelength (Eastman Kodak 1968).

Ultraviolet cannot be seen by the human eye. Photographic emulsions, however, are sensitive to much of the ultraviolet spectrum, and by using a filter that absorbs all visible light, it is possible to make a photographic exposure of ultraviolet radiation reflected from a subject. The most common use of ultraviolet illumination in conservation, however, is not the documentation of its reflection or absorption, but rather the observation and photographic documentation of the visible fluorescence produced by some materials when excited by it. This visible fluorescence is caused by the absorption of this higher-energy ultraviolet radiation by the electronic structure of atoms and the release of this absorbed energy through conversion into visible light. Visible fluorescence from ultraviolet excitation at certain wavelengths is a characteristic of certain substances.

Long-wave ultraviolet radiation is frequently used in paintings conservation, for example, to help distinguish overpaint from original paint or to ascertain the presence of natural resin varnishes, shellac, and pigments such as zinc white. It has also been applied to the examination of artifacts composed of paper, ceramics, and wood. Objects conservators may be more familiar with the use of short-wave ultraviolet. This illumination is used, for example, to identify inorganic materials such as some minerals and glasses.

Until recently, the use of long-wave ultraviolet for the examination of photographs has paralleled the uses in examination of paintings and of works of art on paper: identification of coatings, locating areas of foxing, or identification of hand-colored additions of a photographic image. The International Museum of Photography at the George Eastman House, however, began using long-wave ultraviolet illumination in a condition-monitoring and photodocumentation project for the Southworth and Hawes Collection of daguerreotypes in May 1989.

1.2 THE DAGUERREOTYPE

The daguerreotype was the first commercially popular and successful form of photography. From its announcement in Paris in 1839, the making of daguerreotypes quickly evolved into a precise technology, employed by the scientist, the artist, and layperson. Between 1839 and the late 1850s, thousands of daguerreotypes were produced all over the world. The process was especially popular in America, where 2,000 persons were reported to be practicing the art by 1850 (Taft 1938). The fascinating history of the process has been well documented elsewhere and will not be addressed here (e.g., Taft 1938; Gernsheim and Gernsheim 1968; Buerger 1989; Barger and White 1991).

The daguerreotype plate structure consists of a silver layer adhered to a copper substrate. The copper and silver layers were prepared by the cold roll-cladding method (Barger and White 1991). Beginning in the early 1850s, daguerreotype plates were also electroplated, a process that added a layer of silver to the roll-clad silver. This electrolytic silver layer was thought to increase the hardness of the surface, thus enabling the daguerreotypist to achieve a superior polish. The polished plate would be processed by sensitizing with iodide and bromine vapors, exposing the plate in the camera, developing with warm mercury vapors, and, finally, fixing with sodium thiosulfate.

The final image of a daguerreotype is composed of silver amalgam particles and silver. The highlight and midtone amalgam particles range in size from .1 to 1 μm in diameter (Swan et al. 1979; Barger and White 1991); the exact structure and size of the particles vary from plate to plate and from point to point on individual plates. The darkest areas of the image remain predominantly unaltered, pure polished silver; thus, if the plate is held so that it reflects back to the viewer a dark background, these areas will appear dark. The amalgam particles forming the midtone and bright areas of the subject scatter light diffusely, and thus these areas will appear lighter in tone. As might be expected, viewing the plate incorrectly so that the background reflects back a bright surface will cause the image to appear as a negative.

The most common forms of deterioration found on daguerreotype plates, as cited by Swan et al. (1979), and Barger and White (1991), and also observed by the authors, are: (1) tarnish in the form of silver sulfide from air and pollutants; (2) physical damage such as finger smudges, brush marks, scratches, and gouges; (3) corrosion products from copper erupting from the substrate; and (4) localized accretions of debris from glass and case materials and deposits and residues of polishing compounds.

Fine detail and deep, rich shadows in the final image were achieved in part by a superior polish to the silver surface; scratches or blemishes in the surface affect the reflectance, causing physical and visual disruptions to the image. In addition, the silver amalgam particles forming the highlights are extremely delicate and easily abraded; physical damage to these highlights will result in localized loss of the image. Gold toning or “gilding” of many daguerreotypes enhanced the image by strengthening the detail and contrast in addition to improving the permanence of the final image layer.

Like the surfaces of other silver objects, daguerreotypes will tarnish if not protected from the environment. The development of complex corrosion products can also develop throughout the daguerreotype structure. Imperfections in the plating and polishing processes can leave tiny pinholes or polishing residues in the surface. Over time, these sites can become the nuclei for copper corrosion or tarnish blemishes (Swan et al. 1979). Historical enclosures, evolved from case housings of 18th-century miniature portraits, were used to protect the delicate silver surface from both physical abrasions and tarnishing.