KENT GEOLOGISTS' GROUP
Preparation and Preservation of Macrofossils
Although a fossil may well have remained safe underground for hundreds of millions of years, once it is exposed to the weather it will immediately start to deteriorate, in some cases even in the safety of a collector's cabinet. This deterioration will be greatly accelerated in the open, and especially on a beach. The collector can therefore justifiably claim to have 'saved the specimen for posterity'. An important distinction must be made between specimens 'saved for scientific purposes' and those 'saved for aesthetic or monetary purposes'. Only in the former case is provenance essential, though for maximum monetary value provenance is becoming increasingly important. The minimum data that should accompany every specimen are:-
- The exact location where the fossil was found, and
- The rock layer in which the fossil was found.
All the other information, name and age for example can be found and added later. As quarry names frequently change with change of ownership, a grid reference or name found on a local map will be invaluable to later owners and researchers. A link to this information should be fixed to the specimen, preferably using a painted number.
Most fossils will have some of the enclosing rock still covering part of the specimen which, if only for aesthetic reasons, may need to be removed. There are two main methods, chemical and mechanical.
All fossils that have been exposed to the sea will have absorbed some salt, which must be removed as soon as possible to prevent surface damage. Salt absorbs water from a damp atmosphere and re-crystallises when it dries. This will seriously damage the surface of a fossil and may crystallise as an insoluble, and difficult to remove, coating. Where possible the whole piece should be soaked in several changes of fresh water, finishing in distilled water, until all salt has been removed. For a fist-sized block, this treatment could last several days. This immediate soaking may not be possible if the substrate is a clay that disaggregates in water, until some consolidation with a dilute glue such as paraloid in acetone (propanone), has been done. To remove small amounts of clay from a solid fossil, e.g. from a pyritised ammonite, dry the specimen first and then add to water. The clay should then fall off. Acetic (ethanonic) and formic (methanoic) acids at 5% concentration will dissolve calcite but not phosphate or silica. Bone and teeth frequently retain their original calcium phosphate so can be removed from a limestone or cement stone matrix using these acids. However, much of the strength of a fossil bone may be provided by a calcite infill or there may be minute cracks filled with calcite which should not be removed. Diluted paraloid can be used to protect cracks and damaged ends during treatment. The technique is to remove as much matrix as possible using one of the mechanical methods to avoid wasting acid. Then protect exposed fossil with paraloid and soak in acid for 24 hours on a wire or nylon mesh to facilitate removal [Danger]. Wash well in water and brush away any softened matrix. Dry and paraloid any newly exposed fossil and repeat the treatment. Have patience, as even a small specimen will take several weeks. Finally soak well in changes of water to remove all traces of acid and use acetone (propanone) to remove excess paraloid. If the fossil has any pyrite in it, this treatment is not recommended.
Note: Most chemicals are not available to the general public and those that are can be dangerous. All chemicals should be handled with caution.
Gentle brushing with an old toothbrush and water will remove clay, especially if it is thoroughly dried first. Harder matrix will scrape away with a scalpel or craft knife blade, but obviously care is required. Very hard material needs chipping, the size of hammer and chisel depending on the amount of material to remove and the proximity to the specimen. Very small chisels can be made from masonry nails, sharpened on a grindstone, and a needle held in a pin vice is useful for very fine work. When close to the fossil there is a danger of the tool slipping and damaging the fossil so an air pen or vibro tool is particularly valuable. Small tools for the latter can be made from old gramophone needles. Preferably work with magnification and remove a small amount at a time. There is always a danger from inhalation of dust and chippings flying into eyes so take suitable precautions. A vacuum cleaner nozzle placed close to the working area can make an excellent dust extractor.
The majority of fossils, once they have been
cleaned and desalinated, need little further
treatment. Where stabilisation and repair of
flaking and broken fossils is required, try to use
easily reversible glues. If a join made with
superglue is imperfect there is no going back,
whereas with a solvent-based glue you can have a
second try. Unless it is absolutely necessary do
not coat a specimen with glue and certainly not
with varnish. Pyrite, (iron sulphide) is a common
mineral in clay and many fossils are partly or
completely pyritised. When pyrite is exposed to
damp air it reacts forming oxides and hydroxides of
iron and sulphuric and sulphurous acids. These
decay products destroy the fossil and encourage a
similar fate in other adjacent specimens. Acids
emanating from some materials such as oak, some
chipboard and wool will accelerate
[Never keep silver cutlery in a felt-lined drawer, as it will tarnish silver.]
At one time varnishes, anti-bacterial coatings and storage in paraffin or glycerine were recommended, but none of those treatments actually works. So far, the most successful treatment seems to be careful desalination and drying, followed by storage in a constantly low humidity atmosphere. The old methods 'worked' sometimes because of subsequent storage rather than the treatment. A centrally heated room that is seldom lived in has a constant 60% humidity, which is close to the optimum of 50% humidity required for ideal storage. A sealed glass container with silica gel is ideal. Polythene will allow water vapour to enter. If a small amount of decay has occurred, shown by yellow sulphur or ash like powder, prolonged exposure to strong ammonia vapour (Danger) may neutralise acids, and after cleaning and drying, the specimen may be saved. Unless it is a rare or interesting specimen it may be wiser to throw it away. Photography and making a resin cast is another way of preservation if loss is inevitable.
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