The Dendermonde Mammoth: fighting pyrite decay and the preservation of unique paleontological heritage

Collecting fossil vertebrates is rather popular among amateur palaeontologists. However, little interest is shown in the different stages one should undertake to treat and safely guard these specimens for the future. Loads of fossils from historical collections are currently suffering because of years of storing and neglect. This might seem strange, since the fossils themselves have spent most of their time underground in very humid conditions, but in reality, problems only start right after digging them up. Following-up on the restoration project of the “Dendermonde Mammoth”, we want to give an insight into the problems one can encounter when dealing with the restoration and preservation of Pleistocene vertebrate remains that have remained untreated for the past 20 years.

The discovery

In the historical Belgian city centre of Dendermonde (French: Termonde), we find the city’s history (including natural history) museum called the “Vleeshuis” museum (the house of meat merchants). It is located in one of the most authentic sandstone buildings in the main market square of “Dendermonde” (a province of East-Flanders). Inside the majestic wooden attic of the museum, the city’s oldest resident watches over the collection, which is packed with fossils and artefacts from the last ice age and prehistory. When walking up the impressive stone stairs that lead to the attic, visitors will encounter the paleontological pride of the “Dender” valley (the river flowing through Dendermonde). When we take a closer look at the information signs, we learn that this mammoth was found between 1968 and 1969 by Mr Hugo Depotter, who also built the framework in 1975. The missing bits have been completed with fossils from the Hofstade collection from the Royal Belgian Institute of Natural Sciences in Brussels (RBINS) (Mourlon, 1909). In 1978, samples from the enamel of the molars were carbon dated to determine the absolute age of the mammoth skull (Vanhoorne et al, 1978). Before March 2017, the skeleton was in a rather untended state. Due to the lack of conservation over the last 20 years, the poor climatological conditions under the roof and the absence of any interest by the general public, the mammoth had lost its appeal.

Pyrite decay

As a result of years of exposure and lack of any treatment, the bones of the Dendermonde Mammoth were covered with a thick layer of dust and attacked by pyrite decay. The skeleton was showing several visual outbursts of pyrite blooming out from the fossilised cartilage, especially the left shoulder blade (scapula) and right radial bone (Radius), which were heavily attacked by the typical grey-yellowish sulphur powder. This powder is the result of unstable pyrite bounding with oxygen atoms in the air. The reaction itself goes through different complex stages and eventually ends with the formation of FeSO4 (iron sulphate) and SO2 (sulphur dioxide). When you add water (moisture) to the equation (in the case of high relative storage humidity), you will also be dealing with the formation of H2SO4 (sulphuric acid). The simplified chemical reaction goes as follows:

4FeS2 + 13O2 + 2H2O -> 4FeSO4+ 2H2SO4+ 2SO2

(Pyrite) + (Oxygen) + (Water) -> (Iron sulphate) + (Sulphuric acid) + (Sulphur dioxide)

Pyrietrot (geel poeder) op de rechter Ulna (Ellepijp) van de Mammoet van Dendermonde Foto -Anthonie Hellemond
Fig. 1. Pyrite decay on the right radial bone (radius) of the Dendermonde Mammoth. (Drawing by E Coolen 2017.)

It is of the utmost importance that the influence of both water and oxygen, in combination with unstable pyrite, should be counteracted (Shinya and Bergwall, 2007). The reaction itself not only leads to the formation of corrosive products such as the sulphuric acid, but also comes with a volume expansion. This expansion is the main reason why the bones will eventually turn to powder and will destroy the internal structures of the fossilised bones. Aside from pyrite decay, the Dendermonde Mammoth also showed a large amount of desiccation cracks, as a result of the high variation in temperature. Such variations will also contribute to the fragility of the specimens, which will broaden the contact surface for oxygen and moisture, and keep the reaction mechanism going.

It would be easy to blame the storage conditions, since the mammoth is displayed directly under the wooden structure of the roof. However, we should also bear in mind that the former treatments of the fossil bones were carried out rather superficially and most internal cavities did not benefit from any previous treatment. We also noticed visual signs of desiccation in the enamel of the molars. Specimens that suffer from pyrite decay, without initial visual signs of unstable pyrite blooming out of the internal structures, can also be affected. This effect is called cross contamination and the Dendermonde Mammoth clearly suffered from it. Cross-contamination was clearly visible on both the bones and metal framework, where the sulphuric acid even started to attack inert materials.

The restoration project

The Belgian Paleontological Association (Belgische Vereniging voor Paleontologie) took the initiative to set up a restoration project with the cooperation of the Royal Belgian Institute of Natural Sciences (RBINS) and the Museum department of the city of Dendermonde (Stedelijk Musea Dendermonde). The goal was to restore the entire mammoth in just one week, with a diverse team of experts. All bones had to be treated and restored before mounting them on the metal frame. To treat all bones successfully, the following steps were needed.

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Fig. 3. Mono-ethanolamine thioglycolate treatment. (Drawing by E Coolen.)

Step 1. The first step consists of removing all visible pyrite and dust in a controlled environment. This can be done by using needles, scrapers, scalpels and toothbrushes. An experienced preparator will sacrifice only a small amount of original bone material during this first step.

The amount of visible pyrite is an indication of the amount of fossil bone that has already disappeared through pyrite decay. It is recommended that the pyrite powder is removed while working under an exhaust installation. This will prevent any corrosive airborne elements to wander around the room and be inhaled by the preparator. Personal protection throughout the entire process is important and very much so during this first step. Removing pyrite decay releases quite a pungent and unpleasant sulphuric smell and, without any protection, will irritate the throat and lungs. It is recommended that latex gloves and a surgical mask are worn when cleaning the bones with ethanol (to remove dust) and afterwards to remove the pyrite. Latex gloves will prevent any reaction with sulphuric acid, which will irritate the skin.

Fig. 4. Vertebrae of the Dendermonde Mammoth after being rinsed in ethanol (EoH). The ethanol will remove the last traces of mono-ethanolamine thioglycolate.

Step 2. During the second step, the fossil bones are treated with a mono-ethanolamine thioglycolate solution. This redox reaction will neutralise the unstable pyrite, which, in turn, will be washed away with pure alcohol (ethanol or methanol). Usually a 2% to 5% solution of mono-ethanolamine thioglycolate with ethanol (EtOH) or isopropyl alcohol (IPA) is used. The first stage is to immerse the bones in the solution for about one hour. The pyrite inside the bones will react with the solution, turning it dark red and even purple. We change the solution every hour for the following three to four hours or until no change in colour occurs. This means that the pyrite has been stabilised. After this treatment, it is important to remove all traces of mono-ethanolamine thioglycolate, by rinsing the bone(s) with ethanol (EtOH) or isopropyl alcohol (IPA). During this process, it’s important to keep an eye on previous restorations based on reversible glues. By using solvents, previous restorations can loosen or break. It’s also important not to damage or alter the original patina of the bones by letting them dry too long immediately after the mono-ethanolamine thioglycolate treatment.

There are some downsides to the use of mono-ethanolamine thioglycolate. One of the main problems is that this method is very time consuming and only works well with small bones. For the massive parts of the skeleton (like the skull, trunks or shoulder blades), we suggest the use of the mono-ethanolamine thioglycolate treatment locally by using syringes and tinfoil to prevent the solution from evaporating. Immersing all bones in large containers would be too expensive and impractical.

Step 3. All unstable pyrite will by now have been treated, but this does not mean that in the near future there won’t be any outbursts of pyrite decay. The reaction mechanism is still working and the next step is to interfere with the reaction itself. The best way to do this is to cut off the influence of oxygen and water, by treating all the bones with a polyvinylacetate solution. Therefore, we use products such as: Mowilith™, Osteofix™ and Paraloid b72™, which are dissolved in acetone. By applying this solution to the bones, the polyvinylacetate solution will penetrate deeply into the internal structures of the bone, where the acetone will evaporate and cover the bone with a strong film. This film will cut off the contact with the air and, at the same time, strengthen the internal structures of the bone itself. It is important to execute this treatment in an aerated (ventilated) environment and, at the same time, use latex gloves and oxygen masks to protect yourself from the acetone fumes.

Fig. 5. Polyvinylacetate treatment. (Drawing by E Coolen.)

Step 4. After the internal and external treatment of each bone, we can concentrate on the treatment of the desiccation cracks. After the polyvinylacetate treatment, each bone will have a glossy look and will feel a lot more solid than previously. Unfortunately, there is not a lot you can do about desiccation cracks. The most common solution is to fill up the cracks with a (pH) neutral modelling clay (calcite paste), which will not react with the remaining (stabilised) pyrite or the polyvinyl film. After applying the paste, it is important to make a decision about what colour you want to apply to it. Since the Dendermonde mammoth is part of a public exhibition, we chose to hide all restoration work by covering it with pigments resembling the original patina of the bones. We used a completely different approach when dealing with a collection that is meant for scientific research, where all restoration steps should be visible for everyone who would like to do research on the specimens. The pigment is fixed on the bone with Paraloid b72™.

Fig. 6. Filling in the desiccation cracks with calcite paste.

Step 5. The last step is to reassemble the bones on the metal frame, without damaging them. In the case of the Dendermonde Mammoth, we used metal wire to secure the position of each bone. After positioning, there is the possibility to fix the last scratches with calcite paste and pigment.


Pyrite decay is a severe problem in many paleontological collections and can spread itself from one specimen to another, causing a lot of damage to a (scientific) collection. External factors, such as variation in relative humidity and temperature, can play an important role as accelerants within the chemical reaction. When asked how quick pyrite decay can affect a collection, there is no standard answer. Within a confined space, where a collection is stored, the sulphuric smell might be the first indication that the reaction is going on. The reaction itself takes place on a microscopic level and, based on research, seems to occur often when dealing with framboïdal pyrite (this is pyrite with a mineral structure looking like a raspberry). It is necessary to monitor and react in time by all possible means, when faced with a visual sign of pyrite decay.

Finally, treatment should never be seen as a permanent cure for pyrite decay, but should be repeated on a regular basis, and certainly when faced with a visual outburst of pyrite decay on fossils. Only then is there a guarantee that a collection can last for several generations.

Successful Collaboration

As a result of this successful collaboration, visitors of the museum are once again able to meet the ‘oldest’ resident of the city of Dendermonde restored to its former glory. This project is the perfect illustration of how federal institutions, local government, and scientific organisations took action and worked together for the preservation of national scientific and paleontological heritage in times when the financial means for scientific research are scarce. We can apply an important lesson to our own collection based on this restoration project. In the first place, we should acknowledge the importance of local paleontological history, and try to contribute to its preservation and scientific study by maintaining an open collection for scientific research, and invest in descent preservation of your collection.

Fig-5-SEM-micrograph-of-framboidal-pyrite-from-the-sediment-layer-0-2-cm-below-the copy
Fig. 8. Framboïdal pyrite – under a scanning electric Fig. 7. Mounting and colouring the bones. microscope (Lojen, 1999).

About the author

Anthonie Hellemond is president of the Belgian Paleontological Association (BVP) and Vice-President Council for Earth Sciences (RAW-CST). He can be contacted at:

Further reading

Germonpré, M. (1993). Taphonomy of Pleistocene mammal assemblages of the Flemish Valley, Belgium: Bulletin.

Koningklijk Belgisch. Instituut voor Natuurwetenschappen, 63, p 271-309.

Hellemond, A. (2017). De mammoet van Dendermonde, de restauratie van een verborgen paleontologische parel. Spirifer – Belgische Vereniging voor Paleontologie – Brussel 2017 (41) nr. 2 p. 2-8.

Lojen, S., Ogrinc, N., Dolenec, T. (1999). Decomposition of sedimentary organic matter and methane formation in the recent sediment of Lake Bled (Slovenia). Chemical Geology 159(1-4):223-240 · July 1999.

Mourlon, M. (1909). Découverte d’un dépôt quaternaire campinien avec faune du mammouth et débris végétaux, dans les déblais profonds à Hofstade, à l’est de Sempst. Bulletin de l’Academie royal de Belgique, Classes des Sciences 2: 427-434.

Shinya, A., Bergwall, L. (2007). Pyrite Oxidation: Review and Prevention Practices. The Field Museum, Chicago IL (Poster).

Vanhoorne, R., Van Strydonck, M., Dubois, A.D. (1978). Antwerp University Radiocarbon Dates III. Radiocarbon vol. 20 No: 2 p 192-199.


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