Fossils are generally found partly encased in sedimentary rock. Rarely are fossils found entirely free from the rock in which they were encased. Because of this, fossils must be prepared to reveal their full scientific information or to make them look attractive for displays. Fossil preparators employ a variety of methods to extract or expose fossils from their rock matrix.
In the field large bones are encased in a plaster jacket which provides protection for the fossil during transport. This plaster jacket has to be removed before work on the fossil can proceed. A special plaster saw, much like those used in hospitals, is used to cut the jacket open. The specimen is then readied for preparation by removing all the loose soil and rock from around the bone.
The most common approach is to use mechanical means to chip away at the rock, slowly and carefully exposing the fossil. At the Queensland Museum we use a variety of air-powered micro-jackhammers which remove the rock. Each of these has a stylus which chips the rock away. Larger styli are used for coarse work and finer, thin styli are used for delicate work. Rarely a rotary drill is used to loosen material.
During field work and the preparation process the fossil may be strengthened by applying a soluble plastic to the material. This is absorbed by the bone and hardens, making the bone stronger and more resistant to the vibrations from the preparation tools. Breaks in the fossil are repaired with epoxy resin glues.
It can take between a few hours and a few months to mechanically prepare a fossil specimen. Large skeletons can take many years to prepare, with each bone taking many months to extract from the rock.
Some specimens are preserved in limestone and yet are composed of different chemicals such as silica or phosphate. Using the difference in chemistry between the calcium carbonate of the limestone and the chemistry of the bone or shell, organic acids can be used to dissolve the rock away and leave the fossil intact. The most common acid used is a very weak solution of acetic acid not unlike household vinegar. Care must be taken to protect bones extracted using this method, and soluble plastics are applied to protect delicate bones. Acid etching and acid preparation are very slow processes and take a number of months. Unlike mechanical preparation, they are not as labour-intensive. Acid preparation of fossils should never be tried at home.
Casts and moulds
Fossil footprints are impressions left by walking animals. These are replicated in the field by using liquid latex rubber. The latex is spread very thinly across the print so as to mirror the fine detail preserved in the rock. This thin layer is left to cure. Further layers are added to make a thick rubber mould. Often reinforcing gauze or fabric is added to the mould to help it hold shape. In a similar way latex replicas can be made on small fossils which are preserved as natural moulds. These are very accurate and preserve great detail of the fossils they mould.
These are but a few of the many fossil preparation techniques used by scientists at the Queensland Museum. It takes many years of practice and patience to develop good preparation techniques.
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