Natural history collections
Some specimens in mineralogy and paleontology collections are persistently troublesome when it comes to preservation. For example, minerals in the iron (II) sulfide family will spontaneously oxidize when exposed to humidity and oxygen. This oxidation mainly involves the sulfur, which changes from a reduced form of sulfide, with an oxidation number of -2 or ‑1 depending on the mineral, to an oxidized form of sulfate with an oxidation number of +6. Underlying this is a complex series of chemical reactions during the course of which seven or eight electrons are exchanged.
One of the ferrous sulfides commonly found in many fossiliferous sites is pyrite. Fossils obtained from such sites are said to be “pyritic”. Buried in the earth, generally in reducing conditions, they can be preserved for millions of years. From the moment that they are extracted, however, they tend to oxidize, leading to efflorescence that can obscure the fossil print and weaken the matrix. The degradation can be quite spectacular and even result in the total disintegration of the specimen. It involves even more complex mechanisms than for the minerals themselves since the fossil is made up of composite and porous material. The way that it unfolds depends on many and varied factors: porosity, size of the pyrite crystals, but also the presence of organic sulfur or even bacteria.
Ultimately, the damage suffered by pyritic fossils affects, indirectly, the cellulosic materials used for storing them (such as boxes and labels). Indeed, the iron and sulfates released as the pyrite deteriorates could migrate through the cellulosic support and cause similar degradation to that observed in documents written in iron gall ink.
Our work aims to arrive at a better understanding of the mechanisms involved in the decay of fossils and fossil storage materials, and to ascertain which parameters bring the most influence to bear. It is therefore important to recreate the observed degradation in laboratory conditions so as to better study it. Artificial aging tests are thus conducted so as to reproduce deterioration phenomena on freshly extracted material that are similar to those observed on collection specimens.
The long-term objective of this work is to establish preservation protocols, not only for preventive solutions (enabling an assessment of potential risks and optimal preservation conditions), but also for curative procedures (development of treatment methods capable of limiting or halting damage).
1- Origin of Deterioration observed in Carbon-bearing Argillites rich in Sulfides and Organic Matter
This project deals with argillites from the Autun basin. Some were extracted directly from the site (as is the case, for example, with the Muse site, currently under excavation), while others are already part of MNHN holdings (the Flouest collection and the Oudard collection). These materials contain organic and mineral sulfides. The research aims to identify which of the various different phases is most vulnerable to degradation. The goal is not only to better understand the damage observed in the collections, but also to determine which areas are potentially more reactive for any new specimens entering the collections.
2- Behavior of Lignitized Wood upon Drying
This project is concerned with lignitized wood obtained, chiefly, from two sites: Rivecourt (65 mya, Oise, France), nowadays underwater, and Angeac (130 mya, Charente, France), which is currently being excavated and the name of which is familiar to the general public on account of the sauropod remains discovered there. These woods contain organic sulfides embedded in the lignite, but also inorganic sulfide inclusions made exclusively from pyrite. These inclusions range in size from macroscopic to microscopic and upon oxidation crystalline sulfide efflorescence forms. When excavated, these woods are completely waterlogged. Drying the wood is a delicate process: if performed too slowly, crystalline efflorescence will result; too fast, and mechanical constraints lead to multiple fractures. Research is therefore under way so as to shed light on the degradation mechanisms at work during the drying process, as well as during “cleaning” of these woods, which very often remain submerged in situ between one excavation dig and the next.
3- Degradation of Cellulosic Materials by Ferrous Ions in an Acid Environment
The deterioration of cellulosic materials is often observed when they are in contact with pyritic substances that are themselves deteriorating. We are working to better understand the physical-chemical mechanisms behind this degradation triggered mainly by the presence of both iron and sulfates within the paper. The degradation of paper in the presence of iron is generally attributed to free radical oxidation reactions – the Fenton reaction – and/or acid hydrolysis mechanisms. This research, which uses a relatively fundamental approach, seeks to pinpoint the dominant mechanism underlying the deterioration, with a view to finding more effective remedial strategies.
- Véronique Rouchon | Heritage scientist
- Oulfa Belhadj | Heritage scientist
- Sophie Cersoy | Heritage scientist