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Preferably, a fossil is kept in its natural, untreated state. After all, the fossil may be damaged by treatment, or information may be lost. Only if there is no other possibility (e.g. because the fossil is instable), preserving the piece can be taken into consideration. Whenever possible, reversible techniques are used, so the fossil can always be restored to its original state (in order to be able to apply a more advanced preservation technique at a later date, for example).

Sometimes it's not easy to decide whether preservation is necessary. Relatively fragile or unstable fossils may, as a precaution, be impregnated with a reversible consolidant. In the following specific cases it is advisable to always apply a preservation technique:

  • Fossils recovered from saline environments (e.g. saline waters)
  • Fossils containing pyrite or other sulfur compounds
  • Fragile fossils, showing instable cracks


Fossils from salt water: desalination

The problem with fossils that have been lying in salt water is, that they are completely saturated with a saline solution. As long as the fossil is kept wet, there is nothing to worry about, but when the fossil dries, the dissolved salt will crystallize on the inside, which usually results in an volume increase, increasing internal pressure. The fossil can then over time (this can take a long time) be pushed apart from the inside out. To avoid this, one should try to desalinate the fossil. This can be done by placing the fossil for a long time (at least a couple of weeks) in fresh water that is refreshed regularly. The salt will dissolve and/or be diluted, and will be removed when the water is refreshed. Very large fossils should of course stay in the water for a longer period of time.

Hint: place fossils you wish to desalinate in the watertank of the toilet. This way the water is constantly refreshed and you will conserve water at the same time.

After desalination, it is sometimes advisable to impregnate the fossil with a glue solution. For example, bones which have been fished up from the North Sea, wood glue is often applied for preservation. To do this, you dissolve water resistant white wood glue (D3 water-resistant polyvinyl acetate) in water in a 1: 25 ratio. Immerse the fossil in this solution for a few hours until no more bubbles are seen escaping from the fossil. Then leave the piece to dry in a cool room.


Fossils containing pyrite or sulfur

Fossils which contain pyrite or marcasite (FeS2) can react over time with air and moisture and consequently fall apart. The sulphur oxidizes and combines with water to form sulphuric acid. This irreversible process is called 'pyrite bloom' . To make matters worse, toxic degradation products are formed. Blooming pyrite fossils may also 'infect' other fossils. The best solution therefore when pyrite bloom occurs, is to remove the piece from the collection and to dispose of it. A collection of pyrite fossils should therefore regularly be checked for signs of pyrite bloom.

The sensitivity of pyrite fossils for pyrite decay is highly variable; some fossils are stable, others can suddenly bloom after years. The sensitivity varies from location to location:  ammonites from the albian of Cap Blanc Nez for example, are very sensitive, perhaps due to the presence of marcasite, the orthorombic form of FeS2, which is noticeably more sensitive to pyrite bloom. When in doubt, it is therefore best to treat the fossil preventively.

Prevention might be done by the following two measures. It should be noted that there is no solution with a 100% success rate:

1) storage: the fossils must be kept in a dry environment, preferably sealed off and the fossils should be separated from each other (e.g. in micromount boxes). A regular check for signs of pyrite bloom is necessary. If the fossils are kept separate from each other and if blooming fossils are removed quickly, the chance of pyrite bloom spreading decreases.

2) treatment: by keeping the fossils completely airtight, the chance of pyrite bloom is greatly reduced. In the past, methods such as various kinds of lacquer products, archeoderm, hair spray and glue were used. These treatments are not recommended because they are insufficiently effective and irreversible! Since no method is 100% effective, it is important to regularly check the fossils. One of the better methods so far seems to be to submerge the fossils in a mixture of melted paraffin (wax) with petroleum (ratio 1: 1). After 10 minutes they may be removed and they can cool off. After buffing the fossils with a cloth, they are ready. The fossils do tend to get a darker colour with this method. Another good method is treatment with Paraloid B72. When the decay continues, further treatment is futile.


Fossils that are brittle or have unstable fractures

Fossils which are fragile or which have cracks that are likely to continue can be impregnated with a diluted adhesive. This means that you let the dissolved glue enter into the cracks and pores of the fossil and matrix (in case it is sufficiently porous). To do so, they can either be immersed, or the glue solution can be applied with a brush. In the latter case, the solution will often penetrate less deeply. Fossils that are not porous but which should be stabilized can be fitted with a superficial adhesive. Finally, an unstable matrix sometimes can be fixed at the bottom, e.g. by using an epoxy glue.

The shell of this gastropode in clay has been superficially fixated with a reversible glue solution.

The knowledge and skills about preservation evolve very quickly, and techniques that were popular in the past are often already outdated. This applies in particular to the amateur circuit, where conservation techniques are passed on over generations rather than that they are based upon the latest scientific insights. On the other hand, an amateur collector does not always have access to information about specialized techniques and products, the latter of which are often only available in large, wholesale quantities. In addition, both the budget and the qualitative requirements of the collector is often limited. All this means that the "wrong" techniques are often used. A well-known example is the use of a 'glue-acetone’ solution for impregnation: a method that is not only difficult to reverse, but which is known to sometimes result in a yellow discoloration over time.


With this fossil, the matrix consists of brittle chalk marl which continues to crumble. The full piece is impregnated with an adhesive solution in acetone.


The choice of adhesive is very important and depends upon several factors. Generally speaking the glue must be a combination of the following properties:

-reversible: ideally the adhesive should be removable. This seems not obvious, but it is nonetheless very important. This is demonstrated, for example, when a used glue in time proves to be unstable or corrosive, or when it affects or discolors the fossil, or when better conservation techniques become available, or if the fossil needs to be further prepared for research, and so on ... Reversibility of treatments is a key factor in fossil conservation.

-stable: the adhesive must be stable for long storage and should discolor or change chemically as little as possible.

This means that most adhesives such as the ones found in hardware stores or do-it-yourself stores do not qualify. The composition of most common commercial glues is often not known or communicated exactly, and it is not clear how the product will react over time. Although it is not possible for most amateurs to develop a specialized laboratory in order to always use the state-of-the art techniques, we propose several adhesives here which are clearly superior, particularly Osteofix and Paraloid B72. Both are available in granules which are easy to store and which must be resolved in pure acetone (this you can get in the DIY-store). It is easy to adjust the proportions for every application: thin for impregnation (e.g. 1: 10) and thicker for adhesives (e.g. 1: 1). The use of acetone does require a well-ventilated area. For those who need to stabilize regularly, it is wise to invest in a stash of one of these adhesives.

The transportation of this ammonite has gone terribly wrong. The irregularity of the fractures is not a problem, but it does call for a strong and filling adhesive. This piece is best restored with an epoxy adhesive.


For fixing irregular fractures or for fractures where extreme strenght is needed (for example, because of the weight of the fossil or when it needs to be prepared further) it may be best to use a two-component adhesive (epoxy). This also fills up the fracture nicely. Even though this method is not very well reversible, it sometimes is the best choice. Here too, the classic brands from the hardware store are clearly of a lesser quality than some of the professional alternatives.

Do you have additional information for this article? Please contact the Fossiel.net Team.