While mechanical preparation uses physical forces to remove the matrix from around a specimen, in chemical preparation various compounds are used to dissolve the surrounding matrix.
Just as a preparator must choose the right hand tool for mechanical preparation, picking the right technique for chemical prep is critical. Any preparation method can be damaging if done improperly, but acid prep can be particularly damaging because the chemicals can attack the fossil as well as the matrix. For this reason, chemical preparation should only be chosen if it will produce a superior result or if it is the only option to safely reveal the specimen.
Chemical preparation is most frequently used:
- For delicate specimens, such as fossil fish, that might be damaged using mechanical preparation methods
- For certain types of fossil invertebrates, where freeing both the inner and outer shell surfaces might reveal important morphology.
- Where complete removal of the bones from the matrix is necessary.
Chemical preparation is not an easy, “magic fix.” The process is time-consuming since the specimen must be regularly rinsed and re-coated to prevent unintentional loss of the fossil. Lack of monitoring during chemical prep can result in serious damage to the specimen.
Chemical prep techniques
Some chemical techniques include:
Acid treatment is often used on matrices partly or wholly composed of limestone, (calcium carbonate), which dissolves into carbon dioxide and calcium ions in the presence of acids. “Highly dilute (3-7%) concentrations of acetic or formic acid in water are most commonly used, along with a buffer to protect the calcium phosphate that comprises the fossil bone.” Short treatments with acid are interspersed with much longer periods of rinsing in water to remove the acid before the bone is damaged.
In some specimens, brown crusts and crystals formed from iron oxide can obscure most of the detail of the underlying specimen. Iron reduction is a method of cleaning away this crust by reducing ferric iron to ferrous hydroxide, which is more soluble. There are two techniques: thioglycollic acid and the “Waller Method”. The Waller Method, which uses no acids and is therefore non-corrosive, tends to be preferred for fossils. It works well on most silicates, carbonate and phosphate minerals, but should not be used on pH sensitive materials, such as highly soluble calcium minerals. The Waller Method is also used as a method for stabilizing specimens that have “pyrite disease”. For more on this click here.
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This technique was developed in the 1950s by Toombs and Rixon and remains in use today. In transfer preparation one half of the fossil is embedded in polyester resin. The embedded fossil is placed in a bath of formic acid where the acid slowly eats away the matrix revealing the fossil bone. Generally the process is allowed to continue until only the bones remain, supported by the resin. This technique is particularly useful when it is necessary to maintain the relationship of articulated parts after matrix removal.
Mineral acids such as hydrochloric acid are used to dissolve the bone from the rock, thereby leaving a clean matrix mold from which a peel (which becomes the positive) can be cast. This technique works well on shales, sandstones, quartzites and oil shales and is useful when bones in the matrix obscure observation.
Identifying your matrix
Chemical prep is extremely hazardous and should only be done when you have a complete understanding of the composition of the matrix and the specimen. It is essential that there be a difference in the chemical composition between fossil and matrix; this allows the acid to attack the matrix rather than the specimen. Identifying the composition of the matrix is the first step, allowing for the appropriate acid to be chosen. It is advisable to test the procedure on an expendable fragments of matrix and bone before attempting a full-scale treatment.
Applying a barrier layer
Exposed bone and any other areas that should not be dissolved are coating with an acid resistant barrier layer. Normally a thinned adhesive such as a PVAC or acrylic resin is chosen for this purpose. For more on adhesive and consolidant properties click here.
Choosing the right chemical
The choice of chemical will depend on the nature of the matrix surrounding the fossil. Some examples of treatments are shown below.
|Acetic Acid||Carbonate matrices (e.g., calcite)|
|Formic Acid||Carbonate matrices|
|Thioglycollic (Mercaptoacetic Acid)||Iron-rich matrices|
|EDTA (ethylenediamine tetraacetic acid)||Iron-rich matrices|
|Hydrofluoric Acid||Silicaceous matrices (e.g., quartz and clay minerals)|
|Water||Water-soluble minerals, such as nitrates, borates and sulfates|
Adding a buffer
Often a buffer is also added to protect the bone e.g., when using formic acid, calcium phosphate can be used as a buffering agent.
Treating the specimen
The specimen is immersed in the treatment solution for a short period of time (no more than 1-2 hours). Rutzky et.al. 1994, describes the consensus of staff at AMNH that acetic acid is “slow acting, requiring perhaps 20 immersions in acid compared with formic acid, which requires only 8 to 10 immersions… we felt that although formic acid was a bit stronger (putting stress on the bone,) there would be more stress on the bone from the greater number of immersions required by acetic acid” (p.156). For information on another alternative, download the PowerPoint presentation “Acid preparation of fossils using sulfamic acid, a weak organic acid, and its advantages over acetic and formic acid preparation” by Carlos B. Padilla & Mary Luz Parra of the Fundación Colombiana de Geobiología.
The specimen is removed from the treatment solution and rinsed in running water. It is essential that all of the acid is removed from the specimen; for this reason, the rinsing stage should last 2-4 times as long as the treatment phase.
The specimen should be allowed to air-dry in between runs. It can then be re-coated and the run repeated as necessary.
After the acid bathing has been completed the specimen should undergo a final soaking in water. This final water bath should be at least twice as long as the treatment phases and may be as much as four times as long. Specimens that have been inadequately rinsed will undergo slow acid attack from the inside out - damage that will only be visible over time. When the fossil is fully dry the bone should be checked for crystalline growth that can damage specimens and indicate insufficient rinsing.
Acid prepped bone can be extremely fragile and may require consolidation to withstand handling. Learn more about consolidants here.
Documenting preparation techniques and any relevant materials is important for all prep methods – but particularly important for acid prepared specimens. The preparator should record procedures including all chemicals and concentrations used; the immersion and rinsing times; general observations, etc. It is also a good idea to take photographs of the specimen at each stage of the treatment. This will help refine procedures and ensure that relevant information is available should the specimen require future treatment or care.
Concerns with chemical prep
There are a number of potential problems that need to be considered when using chemical preparation method:
- 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.
Preparator health and safety
When using acid treatments and chemical reduction it is imperative that proper health and safety precautions are followed. Ensure that staff understand how to use all chemicals safely and are equipped with appropriate personal protective equipment. The lab should be prepared with materials in the event of a spill or accident. For more information visit this site’s Health & Safety page.
- For more information on acid prep download the documents by AMNH preparator Dr. Robert Evander. The first gives information on several different techniques. The second gives a detailed description of acid preparation of fossil fish specimens from the Romualdo Member in the Santa Formation, an area of northeastern Brazil.
- Download “Acid preparation of fossils using sulfamic acid, a weak organic acid, and its advantages over acetic and formic acid preparation” by Carlos B. Padilla & Mary Luz Parra of the Fundación Colombiana de Geobiología originally presented at the 2009 Society for Vertebrate Paleontology Preparator’s Session.
- Watch a short clip on chemical preparation on the Stonybrook Department of Anatomical Sciences Vertebrate Fossil Preparation Laboratory website.
- The booklet from the October 17-18, 2005 Society of Vertebrate Paleontology Professional Development Workshop entitled “Conservation of Fossil, Mineral, and Rock Collections” taught by Robert Waller, Gerald Fitzgerald and Chris Collins has useful information on a variety of preparation and preservation techniques.
- There is an extensive bibliography on “Chemical, Thermal and Washing Methods of Preparation” on page 715 in Kummel, Barnhard and David Raup, 1965. Handbook of Paleontological Techniques. San Francisco: W.H. Freeman and Company as well as articles on the “Use of Acids in Preparation of Fossils”, by G.A. Cooper and H.B. Whittington (p. 294) and “A Method for Obtaining Small Acid-resistant Fossils from Ordinary Solution Residues” by James Schopf (p. 301).
- Green, Owen R. 2001. A Manual of practical laboratory and field techniques in palaeobiology. Dordrecht, The Netherlands: Kluwer Academic Publishers.
- Howie, Francis M.P., 1974. “Introduction of Thioglycollic Acid in Preparation of Vertebrate Fossils,” Curator. Number 2, pp. 159-172.
- Lindsay, William, 1995. "A Review of the Acid Technique," in The Care and Conservation of Palaeontological Materials, C. Collins (ed.), Oxford: Butterworth-Heinemann Ltd. pp. 95-101.
- Rutzky, Elvers, Maisey and Kellner,1994. “Chemical Preparation Techniques” In Vertebrate Paleontological Techniques, Volume 1, Patrick Leiggy and Peter May (eds.), New York: Cambridge University Press. pp.155-186.
- Rixon, A.E. 1949. “The use of acetic and formic acids in the preparation of fossil vertebrates”. Museum Journal, London 49. pp. 116-117.
- Toombs, H.A. 1948. “The use of acetic acid in the development of vertebrate fossils”. Museum Journal, London 48. pp. 54-55.
- Toombs, H.A. and A.E. Rixon, 1950. “The use of plastics in the “transfer method” of preparing fossils”. Museum Journal, London 50. pp. 105-107.
- Triplehorn, Don M., Bruce F. Bohor and William J. Betterton, December 2002 “Chemical Disaggregation of Kaolinitic Claystones Clays and Clay Minerals 50(6) pp. 766-770.
- Triplehorn, Don, 2002. "An Easy Way to Remove Fossils from Sandstones: DMSO Disaggregation," Journal of Paleontology 76(2): pp. 394-395.