Chemical Preparation

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:

General Procedures

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. Read more on adhesive and consolidant properties.

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.

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." 

Rinsing

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.

Drying

The specimen should be allowed to air-dry in between runs.  It can then be re-coated and the run repeated as necessary. 

Final Bathing

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.

Consolidation

Acid prepped bone can be extremely fragile and may require consolidation to withstand handling. Read more about consolidants. 

Documentation

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

Specimen safety

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.