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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