Preparation of fossil specimens has two goals, the exposure of specimens for research, and the final devel-opment of the best specimens for display or photography.

Ultrasonic cleaning is a fast and usually safe method for cleaning many hard objects that are not glued together, and it is thus useful in paleontological laboratories. It is relatively ineffective for cleaning soft materials such as rubber, cloth, and fibers. Ultrasonic cleaning machines use sound waves, or mechanical vibrations, that are above the human hearing range, and operate at frequences up to 55,000 cycles per second. The sound waves are generated by a transducer (Figure 1), which changes high frequency electrical energy to mechanical energy. This mechanical energy, or vibration, is then coupled into the liquid in the cleaning tank. The vibrations cause alternating high and low pressure waves in the liquid. This action forms millions of microscopic bubbles, which expand during low pressure waves and form small cavities. During the high pressure waves, these cavities collapse, or implode, creating a mechanical scrubbinglike action, which loosens dirt on all surfaces in contact with the cleaning solution. This action can take place up to 55,000 times a second, making it seem as though the dirt is being blasted from the surface and cavities of the object being cleaned. Ultrasonic cleaning is effective wherever capillary action will take the solution. Complete cleaning usually requires from 30 seconds to two minutes (Anonymous, 1983).

Rapid temperature change can be used to free fossils from some types of rock. Using this method, rocks are alternately heated over a gas burner and quenched in cold running water. The method is especially useful for obtaining specimens of small species (5mm–7mm), but whole specimens up to 50mm long have been released from rock. Heating and quenching should be tried on those limestones that break through both fossils and matrix when the rock is struck with a hammer or broken with a rock splitter. In such limestones, it is often difficult and time consuming to remove specimens using needles and grinding wheels (Sohl, this volume, chapter 19). Many more specimens can be obtained for study more rapidly by heating and quenching such rocks than can be obtained by mechanical preparation. In most instances, only one side of a specimen is exposed when limestone is split mechanically, and the opposite side of the specimen adheres tightly to the remaining rock. In such cases, heating and quenching can free the entire specimen of all rock.

Freezing and thawing is accelerated mechanical weathering. We have little firsthand experience with this technique, but it has been written about in English by several authors who have used water, or water with a dissolved sodium salt (Hanna and Church, 1928; Sohn, 1959, 1961; Sohn, Berdan, and Peck, 1965; and Surlyk, 1972).

AIR-ABRASIVE, also called Airbrasive (a trademark of the Pennwalt S.S. White Co.), units are in effect miniature sandblasting machines. The units propel an abrasive, by means of a gas, through a flexible hose attached to a handpiece. Originally designed for dental and industrial uses, these units are now widely used for preparing fossils. The air abrasive can be used by itself or in combination with other techniques to excavate fossils from matrix, to remove matrix from areas of specimens for detailed study, and to elevate specimens above the surface of the matrix for display and photography. The technique of preparing fossils using the air abrasive has been previously described and discussed by a number of authors, notably by Stucker (1961), Stucker et al. (1965), Rixon (1976, p. 75–77), and Converse (1984, p. 42–49).

The technique of freeing fossils from carbonate rocks by the use of various acids has been known for many years, but still is not used as widely as might be expected. Cooper and Whittington (1965) cited early uses of acid to prepare graptolites and eurypterids (Holm, 1890; 1898). Common practice among early paleontological users of acid was to pick up a silicified specimen, or carefully chip one out of the matrix by hammer and chisel, and use the acid to clean it or to expose internal features. For example, Waagen (1882, p.354) mentioned exposing the internal features of silicified brachiopods by use of “dilute acid” (kind unspecified) from Permian rocks in the Salt Range of India (now Pakistan).

The minerals pyrite and marcasite (broadly termed pyritic minerals) are iron sulfides that are common if not ubiquitous in sedimentary rocks, especially in association with organic materials (Berner, 1970). In most marine sedimentary associations, pyrite and marcasite are associated with organic sediments rich in dissolved sulfate and iron minerals. Because of the rapid consumption of sulfate in freshwater environments, however, pyrite formation is more restricted in nonmarine sediments (Berner, 1983). The origin of the sulfur in nonmarine environments must lie within pre-existing rocks or volcanic detritus; a relatively small, but significant contribution may derive from plant and animal decomposition products.

All of the taphonomic and other factors involved in fossilization, exposure and collection of fossils affect the preservation state, the condition in which the fossils become available for collection. These factors also determine what should be done to, at the very least, at least maintain that preservation state, and ideally provide a stabilized specimen that can, if desired, be restored. Weathering of rocks and their contained fossils is always a combination of mechanical and chemical processes. The collection of fossils in the field, and later work in the laboratory, attempts to halt or slow those processes (Rixon, 1976; Converse, 1984).

Screen-washing bulk samples of fossilliferous matrix is the method of choice for concentrating relatively large numbers of small fossils from unconsolidated sediments. The technique is most effective when used to disaggregate dry and unconsolidated sand, silt, and silty clay. Ideally, when the dry fossil-bearing particles will pass through an appropriate size screen-mesh, leaving behind a residue of fossils and coarser grain sizes. Organic-rich sediments, overconsolidated silt, silty clay, maris and clays may require soaking in dilute chemical solutions to encourage disaggregation. The extraction of fossils from these more intractable sediments, therefore, generally will be limited to the processing of quantitatively small samples (several hundred grams), in the laboratory.

When visiting the dinosaur displays in museums such as the American Museum of Natural History, the Tyrrell Museum of Palaeontology, or the United States National Museum of Natural History, one is overwhelmed by the abundance, diversity, and completeness of dinosaur skeletons. What is not obvious to the average visitor is that the majority of those skeletons are not from an individual animal. They are almost invariably composite skeletons containing bones from at least two individials, and many missing bones have been filled in with sculptured models constructed of plaster or fiberglass. How is it possible for paleontologists to reconstruct and mount these titanic skeletons? Perhaps even more fundamentally, how do such enormous fossil bones find their way to museums to begin with?

Latex casting of macroinvertebrate fossils, when filling natural molds, produces a positive, or cast, reproduction of the organism. The use of latex molds to cast fossils in plaster or plastic materials will be discussed elsewhere in this volume. The purpose of this paper is to describe the preparation of latex casts for study and photography. The procedure for preparing a latex mold is essentially the same. Latex casts, carefully made, reproduce surficial detail with remarkable fidelity and a photograph of a whitened, stained-latex cast is often times indistingushable from a photograph of a well- preserved whitened body fossil.

The procedure for permanently enlarging natural latex casts and molds up to 50% larger without appreciable material distortion was first reported by Fuehrer (1939). The technique was later modified for casts and molds of polybutadiene rubber by Gunther et al. (1979).

Specimens of all types and sizes can be molded with room temperature vulcanizing silicone rubber (R.T.V.). Mold making procedures, when using R.T.V., differ little from those followed when molding with latex, polysulfide, or polyurethane rubbers. The most important difference is that when using R.T.V. separators are not, under normal circumstances, required either in the making of the mold or in the production of the cast. This allows for the ultimate in surface detail replication in the cast. Thus R.T.V. compounds are ideal for molding small specimens and portions of larger specimens requiring examination using electron microscopy, typically used in vertebrate paleontology for the examination of micro-striations on teeth (for masticatory functions and dietary studies) or bones (for pathologic and taphonomic examinations).

In view of their inherent simplicity single piece molds should always be the first consideration when deciding on casting methods. However, the complexity of shape or the intricate contour of a specimen often dictates that a multiple-piece mold be made. This is especially true when a three dimensional cast is required and a simple surface impression or “peel” will not suffice.

Plaster of Paris is a general term for gypsum plasters and gypsum cements. Plaster, which has been in widespread use for producing casts of fossils since the Nineteenth Century (e.g., Green, 1832; Ward, 1866), is easily used for making rigid, long-lasting, and inexpensive casts of study specimens, and for making field casts from natural molds. Good general descriptions of the use of plaster are given in Clarke (1938) and Rich (1947), as well as in many recent books on sculpture (e.g., Miller, 1971; Chaney and Skee, 1973; Andrews, 1983), and in various brochures distributed by manufacturers (e.g., United States Gypsum Company, 1987a, 1987b). Using plaster for casting paleontological specimens was previously discussed in works by Quinn (1940), Keyes (1959), Heintz (1963), Rigby and Clark (1965), Rixon (1976), and Chase (1979).

Fossils are characteristically uncommon, making it impossible to distribute original specimens to all interested investigators. Dissemination of information, therefore, often depends upon the circulation of accurate reproductions. Such reproductions or casts can be made from a variety of materials including plasters and two-part resins. Using resins for the casting of paleontological specimens was discussed by a number of authors, including Burke and Jensen, 1961; Madsen, 1974; Parsons, 1973; Schrimper, 1973; Reser, 1981; and Burke et al., 1983 (see additional references in Hannibal, this volume, chapter 6). Plaster casting is covered by Babcock (this volume, chapter 34).

Hollow casts are advantageous because they require less casting material and are, therefore, economical. They are also light weight and can be made in a variety of materials including plaster or resins. Although hollow casts can be produced simply and economically using plaster, the fragility of the final product speaks strongly against the use of this medium. Resinous casting compounds require somewhat greater effort but the durability of the casts makes them superior to plaster casts in all respects.

Preparation of appropriate photographs is absolutely essential in conveying paleontological information. The effort expended in adequate cleaning and preparation of specimens is not only reflected in exposing the detail of material so that it can be properly described but also in permitting the morphologic information to be transmitted to others through photography. Therefore the purpose of this chapter is to describe the general procedures involved in preparing high quality, publishable photographs because special techniques related to photography of microfossils will be treated elsewhere, the emphasis within this chapter will be upon photography of macrofossils, specimens large enough to be photographed using a normal spectrum of photographic lenses and extension tubes. Because nearly all paleontological material is illustrated as black and white photographs, no reference will be made to the preparation of color illustrations.

The purpose of photographing fossils for study and for publication is to provide information regarding the morphology of the fossil. Typically, this morphology is expressed as topographic relief. Tonal patterns and color rarely serve to enhance the expression of morphology and may, instead, provide false information. As a result, it is desirable typically to eliminate, or reduce, the effects of variations in tonal pattern over the surface to be photographed to emphasize aspects of relief of the specimen and, therefore, actual expressions of morphology. Furthermore, uncoated specimens may be lustrous so that lighting the specimen for photography may produce undesirable highlights. A simple, and non-destructive, technique for whitening fossil specimens involves the application of ammonium chloride sublimate to the surface of the specimen (Figure 1). This coating is non-destructive, produces a uniform, nonlustrous white surface that enhances detail, and can be readily removed without damage to the specimen.

Because many fossils are preserved in relatively high relief, that is they are not preserved on flat surfaces, it is often desirable to prepare a stereoscopic photographs which permit viewing the specimen as a three dimensional object. This could be easily accomplished by considering that stereoscopy is achieved simply by superimposing two images of an object upon one another, the images having been viewed from slightly different perspectives.