Fossil preparation
Fossil preparation is a complex of tasks that can include excavating, revealing, conserving, and replicating the ancient remains and traces of organisms. It is an integral part of the science of paleontology, of museum exhibition, and the preservation of fossils held in the public trust. It involves a wide variety of techniques, from the mechanical to the chemical, depending upon the qualities of the specimen being prepared and the goals of the effort. Fossil preparation may be executed by scientists, students or collections personnel, but is often undertaken by professional fossil preparators.[1]
Techniques[edit]
Acid maceration[edit]
Acid maceration is a technique to extract organic microfossils from a surrounding rock matrix using acid. Hydrochloric acid or acetic acid may be used to extract phosphatic fossils, such as the small shelly fossils, from a carbonate matrix. Hydrofluoric acid is also used in acid macerations to extract organic fossils from silicate rocks. Fossiliferous rock may be immersed directly into the acid, or a cellulose nitrate film may be applied (dissolved in amyl acetate), which adheres to the organic component and allows the rock to be dissolved around it.[2]
Film pull[edit]
The film pull technique is a means of recovering carbonaceous compression fossils for study under transmitted light microscopy. An acid is applied to the surface of the rock to etch away the matrix from the surface, leaving carbonaceous tissue protruding. (Surfaces not to be etched can be coated in a wax (e.g. Vaseline or grease). This is usually accomplished by placing the rock upside-down in a weak, continually stirred acid, so that any debris can be washed away. Nitrocellulose is then painted on to the fossil-bearing surface, and once dry may be peeled from the rock, or the rock dissolved in hydrofluoric acid.[3]
The method was pioneered by John Walton, in collaboration with Reitze Gerben Koopmans, in 1928 as a method to derive serial thin-sections without the time, expense and lost material incurred by dissolving the rock.[4] An improvement on the method, using gelatine (with glycerin and formalin) instead of cellulose, was reported in 1930, and is especially suitable for larger samples.[5] This solution-based method was largely superseded by the use of pre-formed sheets of film, similar to those used in overhead transparencies; cellulose nitrate and cellulose acetate can be used, although the latter is preferable.[6] By wetting the reverse surface of the film with acetate, the film becomes more labile and makes a better contact with the material. The peel can be washed in acid to remove any remaining matrix before mounting onto a slide with resin for further study.[7] The method is somewhat destructive, as the acid etching used to remove the rock matrix can also destroy some finer detail; the fizzing caused by the reaction of the acid with the matrix breaks up less-robust cellular material.[7] A second peel without further etching, a "rip peel", will remove any cell walls that are parallel to the surface, and would otherwise be destroyed when subjected to acid.[7]
Details of the modern application of the method can be found in reference ([8]). Even the latest technique does have some disadvantages; most notably, smaller fossils that may lie between cell walls will be washed away with the acid etch, and can only be recovered by a thin section preparation.[9]
In order to mount the slides for microscopy, a series of steps are necessary:[7]