Photographic Techniques and Specimen Preparation

Very little can be added to the description of the physical principles involved and the technique of fossil photography given by Franco Rasetti in the Handbook of Paleontological Techniques (Kummel and Raup 1965). As a physicist, I have appreciated Rasetti's suggestions tremendously and have endeavored to translate them into practice as much as possible.

Although it is not generally recommended for high resolution work, 35mm. film was used for all of the photographs. The use of slow, fine-grained film and careful development procedure has, however, enabled enlargements to 8" X 10" print size without loss of quality. For this purpose, a now irreplaceable negative film was used—ADOX KB 14, 20 ASA. Many of the photographs that appeared in the first edition of this atlas were taken before this film was discontinued. For most of the new plates presented here, the film of choice was Kodak Technical Pan, 25 ASA. The ADOX film was developed in AGFA Rodinal developer, diluted 1:100, for 20 minutes at 20° C. The Kodak film was developed with the same developer, dilution, and temperature, but for only 5 minutes. Microscopic examination of the developed image in these films showed a grain size and lack of graininess (coalescence of groups grains) much superior to any other presently available negative material. A recently introduced color negative film, Kodak Ektar 25 ASA, was occasionally used for black and white printing with excellent results. This, however, required use of high contrast paper. For those specimens photographed on location, often in less than optimal lighting conditions, the Ilford films XP1, XP2, and Delta, 400 ASA, were used with some success.

I seem to have found my best solutions to the problems of fossil photography in the use of materials and equipment now discontinued. Among the latter is an item responsible for the best focusing in the pictures presented here, a Leitz device called Reprovit. It consists of a sliding mount, carrying the camera body (a Leica) and a ground glass placed at the level of the film plane in the camera. Beneath the sliding mount, the lens is usually carried by a focusing extension or, as modified by the author, a bellows controlled by rack and pinion motion. Focusing is achieved by projecting the image of a flat marker, encased in the ground glass, onto the surface of a specimen, as if the device were a photographic enlarger. By doing so, the field of view seen by the lens is also illuminated, and the specimen can be properly centered or positioned. The lens stop is then closed to the desired setting, the projector removed and the camera made to slide into position above the lens. The focal plane shutter is open and the exposure takes place by switching the illuminating lamps on and ofT. Depending on the size of the specimen and depth of field required, different lenses were used, usually stopped to the largest f: number. Among the lenses used are Leitz Elmar 50mm (f: 16), Schneider Componon (f:22), and Schneider Symmar 135 mm (f:45). The latter two are projection lenses, originating from the author's bubble chamber film scanning projectors.

For printing, a Leitz enlarger with 50mm Elmar lens was used, as well as an Omega enlarger with a Schneider 80mm lens. The enlargements for the first edition were printed on Agfa Brovira paper, mostly extra-hard, developed in Kodak Dektol developer diluted 1:2. All new pictures presented here were printed on Ilford Multigrade paper.

Specimen illumination was achieved generally with two high-intensity desk miniature lamps, at times with a parallel beam of light from a microscope illuminator, and occasionally with a ring fluorescent lamp. The paleontological convention of illuminating samples from the Northwest has not been adhered to in several cases, when lighting from other directions may have led to better photography.

As described in Rasetti's article, the immersion of specimens in xylene often enhances the contrast between the specimen and matrix. This technique was employed to particular advantage when mineralized or pigmented details were embedded in surface incrustations of calcite or quartz. The optical contact between these minerals and the xylene, having approximately the same refractive index, enables light to penetrate unreflected and unrefracted through the surface layers, revealing the underlying structure of the fossil. Immersion in xylene yielded quite spectacular results also in enhancing the contrast of the pyritized appendages of the Utica Shale trilobites. In several instances, when color contrast existed between specimen and matrix, it was found worthwhile to take a color photograph on a Kodachrome slide and then print directly from the slide. The resulting print is a negative, but this is irrelevant in fossil photography. The high contrast factor of Kodachrome (not Ektachrome) yields extremely sharp prints.

Valuable sources of further technical information on the photography of small objects in general are two Kodak technical publication manuals—N 12A on close-up photography, and N 12B on photomacrography.

Specimen Preparation

The preparation of the specimens for photography has often been a painstaking operation, usually carried out under a stereoscopic microscope. Some of the methods adopted followed the advice of Rasetti and Palmer in the Handbook of Paleontological Techniques (Kummel and Raup 1965). Different procedures have been employed in exposing tri-lobites preserved in shale from those preserved in, say, limestone. Soft shale—for example, the Silica Shale—disintegrates in water, and so does the trilobite. Here the matrix is easily removed to expose the trilobite details by using a variety of scraping tools and soft bristle brushes. Metal brushes are to be avoided at all times. The use of varnish or shellac to waterproof or consolidate the exoskeleton has been avoided in order not to cause unwanted reflections. Repairs of flaked parts have been successfully obtained by using Duco cement diluted in amyl acetate. (The suggestion is from Rasetti and Palmer.) This diluted glue dries without leaving a glossy surface. Final cleaning of the trilobite was obtained by using a moist cloth. A somewhat harder shale is the Waldron Shale or Wheeler Shale. Here prolonged soaking in a solution of

Quaternary O (Geigy Industrial Chemicals) is the best preparation. Caution should be taken to avoid any procedure which could scratch the calcite of the carapace. Unless the specimen is whitened, chisel marks are very disturbing to good photography. In general, care also has been taken to present the matrix in a fairly natural condition.

The hardest preparation is that of trilobites in limestone. Only rarely, as in specimens from Grafton, Illinois, does the separation of trilobite and matrix obtain naturally. More often the trilobite can only be exposed by resorting to rather arduous chiseling and grinding and to the aid of vibro-tools. In these cases, the matrix surface is usually left scarred by white marks. In such instances, careful illumination has been used in order to form shadows on the unwanted scratches.

The trilobites presented are, in many instances, the only survivors of unsuccessful attempts to prepare adequate samples from a much larger initial body of specimens. For obvious reasons, the specimens borrowed from museums or from private collections had to be left untouched. It is standard procedure in paleontology to whiten fossils for photography. This approach has been followed here occasionally by exposing the specimen to magnesium oxide vapors from a burning magnesium ribbon (Rasetti 1947).

In conclusion, this book contains a spectrum of both orthodox and unorthodox approaches to the photography of fossils. These notes contain no implication whatsoever that any of the methods employed here are preferable to the accepted standards in paleontology. My personal tastes and the means at my disposal have been the only guidelines here.

Photographic Techniques and Specimen Preparation

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  • farrah
    How do i care for my trilobite fossil?
    8 years ago

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