Preparation, Molding and Casting
Although fossils may be visible during excavation because of partial removal of their matrix, they do not come out of the ground ready to use. Usually they are brought back to the institution in a field jacket, still embedded, and require what is known as preparation, the process by which the fossil is exposed, to enable them to be studied.
Preparation can also require the use of adhesives to reattach broken or detached elements as well as a consolidant, usually a resin, to harden and strengthen a specimen sufficiently to allow handling and study.
Preparation
The level of preparation will be determined by the goal for that particular specimen and is usually undertaken with a specific aim in mind. This may be a research goal e.g. to expose features for identification or for further study. Alternatively, the specimen may be prepared for exhibition; depending on the nature of the fossil, this could involve leaving the specimen partially embedded in matrix, or completely removing the matrix and mounting the fossil on a supporting armature. Preparation can irrevocably ruin a specimen and, as a result, should only be conducted by someone who understands the specimen, the materials they are working with and the techniques available to them.*
The Society of Vertebrate Paleontology has resources online to aid Preparators in their work. For more information click here. Additionally, Appendix U: Curatorial Care of Paleontological and Geological Collections from the NPS Museum Handbook, Part I has a section on preparation.
A future module specifically on preparation techniques is in development for the Paleontology Portal website. Please check back later to access this resource.
Preparation may either be mechanical, involving the application of a physical force to dislodge material, or chemical, applying particular compounds, or combinations of compounds, to the specimen to dissolve the surrounding matrix.
Mechanical preparation
There is a wide range of tools and equipment that can be used in mechanical preparation. Some of the most common and their effects are detailed here in this table:
| Technique | Material removed | Potential effect on specimen | ||
|---|---|---|---|---|
| Brush | Loosely adhering | Slight, applicable for very delicate specimens | ||
| Dental picks, awls, etc. | Various types | Can scratch the surface | ||
| Scrapers | Soft – medium consolidants | Can mar the surface | ||
| Hammer, chisel, airscribe | Medium to hard | Impact specimen, crack or dislodge pieces, mar surfaces | ||
| Air abrasive | All types depending on abrasive used | Remove pieces, etch surface |
Chemical preparation
Watch a preparator in action
Click here to view a short film of a preparator from the American Museum of Natural History preparing a fossil specimen from Mongolia’s Gobi Desert.When using chemical preparation techniques the material that the preparator does not want touched is coated with an acid-resistant barrier layer. The specimen is then immersed in acid. It must be regularly rinsed and re-coated to prevent unintentional loss. Acid preparation is generally used for fragile specimens prepared in hard calcareous matrix.
There are a number of potential problems that need to be considered when using this technique, Although it is usually possible to protect the surface of a specimen, the internal structure can be attacked and the specimen weakened. The generation of gasses from the reaction of the acid with the matrix can result in forces being applied to the specimen. After treatment it is difficult to wash out all the acid, which will continue to act even if very dilute. Finally, if the specimen is not properly rinsed, it is also possible to introduce acid into the storage environment.**
| Chemical | Used on | |
|---|---|---|
| Acetic Acid | Carbonate matrices | |
| Formic Acid | Carbonate matrices | |
| Thioglycollic (Mercaptoacetic Acid) | Iron-rich matrices | |
| EDTA (ethylenediamine tetraacetic acid) | Iron-rich matrices | |
| Hydrofluoric Acid | Silaceous matrices |
Solvents (e.g. water, acetone, ethanol, toluene) can also be used to remove materials such as tar, old adhesives and consolidants. Solvents also present some possible dangers to fragile specimens and some have implications for the health and safety of the preparator. For this reason, their use should only be undertaken with proper training and health and safety precautions.
Preparation while on loan
In order to carry out a particular program of research on a fossil, it may be necessary to carry out additional preparation. If this is the case the institution must decide whether to allow visiting researchers or borrowers to undertake this preparation at their home institution. In order to have all of the relevant information required to make this decision, the researcher should be asked to submit a formal proposal which covers the following points:
- The purpose of the research and its scientific merit
- The specimen(s) to be prepared
- The proposed method of preparation
- Evidence of the researcher’s competence, or access to a trained preparator, including proven experience in preparing the material under request
- Why the specimens in question are essential to the study
Generally, all products of preparation, e.g., matrix, samples, acid preparation residues, SEM stubs, thin sections, are regarded as part of the specimen and must be returned with it. The borrower should also provide a fully illustrated condition and preparation report describing the work undertaken on the object.
Molding and casting
Another way to study the specimen and obtain additional information is by producing a cast. Casts are reproductions of the object, made by pouring a liquid material, such as plaster or a resin, into a mold which has been made from the surface of the specimen. While not an inherently destructive process like sampling, molding can be damaging if not done properly. It can also be very time intensive and, for large fossils, expensive.
As with sampling and preparation, an institution should have a policy on whether this can be done by outside researchers or while a specimen is out on loan. The researcher should be asked to submit a written proposal that addresses the following points:
- The purpose of making the mold and the proposed use of the cast(s) e.g. research or display
- The method to be employed
- Evidence of the researcher’s competence, or access to a trained preparator, including proven experience in molding the material under request
Certain categories of research (e.g., tooth micro-wear studies) require the making of temporary molds, often during the course of a visit to the collection. Although the end product is a lot simpler and lasts a much shorter time than a conventional mold, it is a good idea to make such molding subject to the same approval procedures as are used for more permanent molds.
For more information…
- Society of Vertebrate Paleontology’s (SVP) Preparators Materials & Methods
- Preplist - This is a list devoted to the exchange of information, questions, opinions, etc. about preparation of vertebrate fossils
- A technical publication on Adhesives and Consolidants in Geological and Paleontological Conservation: A Wall Chart is available as a two part pdf download on the SPNHC website. Download Part 1 or Download part 2.
- The website of the Vertebrate Paleontology Preparation Laboratory at the Yale Peabody Museum gives information on the preparation of some important collections in their collection.
- National Park Service Conserve-O-Gram series has an article on Vertebrate Skeletons: Preparation and Storage
- The Stony Brook University Vertebrate Fossil Preparation Laboratory website outlines basic preparation techniques.
- Information on fossil preparation techniques from The Vertebrate Paleontology Department Of The Florida Museum Of Natural History
- For a good site for school children to learn about fossil preparation go with Flat Stanley on a visit to the University of California Museum of Paleontology and their Paleo Lab.
References
Howie, Francis M.P. 1984. “Materials used for conserving fossil specimens since 1930: a review”, Adhesives and consolidants: preprints of the contributions to the Paris Congress, 2-8 September 1984. pp 92-97.
Footnotes
* Society of Vertebrate Paleontology, et al. Conservation of Fossil, Mineral, and Rock Collections. The Society of Vertebrate Paleontology Professional Development Workshop 2005 October 17-18 Mesa, Arizona. Prepared and presented by Robert Waller, Gerald Fitzgerald, Chris Collins. p. 5.
** Society of Vertebrate Paleontology, et al. Conservation of Fossil, Mineral, and Rock Collections. The Society of Vertebrate Paleontology Professional Development Workshop 2005 October 17-18 Mesa, Arizona. Prepared and presented by Robert Waller, Gerald Fitzgerald, Chris Collins. pp. 5-6
