Chapter 13
Microscopy and Analysis

This chapter includes description of a camera set-up for closeup copying.


The identification of several of the attributes of photographs requires close examination under appropriate illumination and with some degree of magnification. Following are some suggested methods:

1) A hand magnifier and a pencil flashlight.
Hand magnifiers are available from magnifications of about 4x to 20x. At lower magnifications the best illumination for faithful color rendition is daylight. The higher powers require more light, and because the working distance becomes quite short, the light needs to be tightly focused. Grazing angle lighting is useful in revealing layers such as in Woodburytypes and carbon prints. Transmitted light can reveal paper fibers in the highlight regions of unmounted salt prints and albumen prints such as tissue stereos.

2) Close-focusing cameras
A camera equipped for macrophotography and mounted on a copy stand can be a useful inspection tool as well as a recorder. Many 35mm reflex cameras can be equipped with bellows focusing attachments or with combinations of extension tubes. Some asymmetrical lenses can be reversed with a mounting adapter to give more magnification. The mounting stand should be rigid and free of vibration, particularly for copying, because movement of the mirror mechanism in reflex cameras during exposure can blur the picture.

The following home-made setup has proven useful to this writer in many cases:

A 35mm reflex camera with automatic exposure control was equipped with 75 mm of extension tubes and a reversed 25 mm f1.9 Kodak Cine Ektar lens (from a 16mm movie camera), mounted on an enlarger column with laboratory clamps. The specimens are laid on a laboratory scissors jack for focusing, and illuminated with microscope lights. Automatic exposure control makes it easy to take record shots. This combination gives a magnification of 6x with excellent definition over the field.

Illumination can be with microscope lights or miniature halogen lamps, being careful not to expose the specimen to excessive time-intensity levels. It should be remembered that old photographs are subject to fading, and that the damage is cumulative. Many archival organizations do not permit copying of original photographs on office copiers for this reason.

3) Binocular inspection microscopes.
These are designed for good working distances at magnifications up to about 90x. They are mounted on swing arms that can reach the center of large photographs, and some can mount cameras for permanent records. They are useful general purpose laboratory tools.

4) Biological microscopes.

It is seldom that magnification up to several hundred is needed, but it is available with biological microscopes at the expense of very shallow depth of field. It is possible to focus down through the paper fibers into the embedded image of salt prints. Biological microscopes are usually mounted on rigid C-frames, which prevent access to centers of photographs as large as cabinet cards. The optical heads can often be transferred to other mounts for large area examination.

Chemical and Physical Analysis

Some of the attributes listed in Section 1 of Chapter 14 can be identified by inspection, and this will often suffice. When inspection leaves doubts, and when the value of the unknown picture is high (historically or monetarily), there is a good probability that modern analytical methods can find the answer. Two case studies are discussed in Appendix I and II. The discussion below is a resume of available techniques.

The photosensitive material and the binder are the attributes most likely to require analysis for identification.

1. Binder Identification
The solvent tests described by Rempel [124] have already been mentioned in Chapter 2. They are simple to perform, but they are destructive; this disadvantage can be minimized by limiting the test to a small area outside the actual image. Infrared spectrophotometry is capable of identifying any of the organic binders nondestructively. The difficulty is in adapting the instrument to a specimen the size of a photograph, since cutting off a corner may be even less acceptable than the solvent tests.

2. Identification of Photosensitive Material

The compositions of most surviving 19th century photosensitive materials except diazo dyes were based on metals. Classical wet chemistry can identify the metals, but only destructively. It can be done on microscopic zones by the use of colorometric spot tests such as those given in Feigel [52].

One of the most useful non‑destructive analytical methods that is applicable to specimens the size of photographs is x‑ray fluorescence analysis. There are several types of instrumentation, depending on the means of excitation. Basically they depend on exciting the specimen to emit its characteristic x‑ray spectrum, then analyzing the wavelength or energy distribution of the spectrum.

Scanning Electron Microscopy
The x‑ray spectrum can be excited by bombarding the specimen with electrons whose energy is a few kilovolts. Scanning electron microscopes (SEM) generate their magnified images by electron bombardment, with the emission of both secondary electrons and x‑ rays from the specimen. The secondary electrons are used to form the topographical images; the by‑product x‑rays can be analyzed to give the composition. This kind of analysis has to be done in a vacuum, an environment that may damage photographs except all‑metal Daguerreotype plates. Specimen size that can be accommodated in electron microscopes is limited to a few inches. Appendix I describes a scanning electron microscope analysis of a Daguerreotype.

Radiation‑Excited X‑ray Analysis
Of more general use is the x‑ray fluorescence technique, whose application to the analysis of photographic emulsions and papers was reported in 1983 by Enyeart et at [50]. Excitation is by gamma radiation from radioisotopes or by X‑rays from vacuum tubes. The analysis has been shown not to damage photographs or to leave any residual induced radioactivity in the specimens. It is safe, portable, non‑destructive, and can be used on any size specimen. It will detect any of the sensitizing elements in photographs except organic dyes. It cannot distinguish between gelatin, collodion, and albumen except indirectly by their impurity content. For example, albumen contains detectable sulfur, and collodion may contain a variety of preservatives as mentioned in Chapter 7. The instrumentation is widely used in forensic and medical analysis as well as numerous industrial applications; it has figured prominently in the authentication of many art objects.

Cost and Availability
Scanning electron microscopes, X‑ray fluorescence, infrared and ultraviolet spectrophotometers are beyond the reach of most individuals for their capital cost as well as for the necessary professional operators. But there are thousands of such instruments in industrial and college laboratories, and in consulting scientific laboratories where analyses can be performed for a reasonable fee. There have even been instances where graduate students or friends have been persuaded to donate a noon hour or weekend for the analysis of a specimen of historical interest.

The purpose of this discussion is to call attention to the existence of appropriate analytical technology to archivists and advanced collectors. For additional information, college libraries have textbooks on the above‑mentioned instruments, and the technical periodicals listed in Chapter 12 regularly contain relevant research papers.