Oxygen-free storage for pyrite speciments

Dr Caroline Buttler (UK)

Oxygen is responsible for the majority of chemical reactions that lead to the decay and degradation of museum specimens; the corrosion of iron and the fading of many pigments when exposed to ultraviolet light could not occur without the presence of oxygen. It is also essential for the life forms responsible for biological decay such as insects, fungi and bacteria.

The most common oxidation reaction affecting geological specimens is pyrite decay, which damages specimens containing pyrite or marcasite.  Pyrite decay occurs when the sulphide component in these minerals oxidises to form ferrous sulphate and sulphur dioxide, and can result in the complete destruction of the specimen and the associated labels and packaging materials. If pyrite specimens can be stored without oxygen then deterioration could be prevented.

The technology to produce oxygen-free environments to museum standards has burgeoned in the last few years. Nitrogen and other inert gases such as argon and helium have been successfully used to display specimens without oxygen, but it is costly and only used for rare or valuable objects. For example the American Charters of Freedom, which include the Declaration of Independence, the Constitution and the Bill of Rights, in the Rotunda of the National Archives Building in Washington, DC, are displayed in cases inside which an anoxic environment has been created containing a humidified argon atmosphere at 19°C.

Anoxic storage can also now be achieved relatively cheaply and efficiently with the use of barrier films and oxygen scavengers. This means that it can be used for more routine storage of geological specimens.

Barrier films are multi-layered laminates, which include a gas and water vapour barrier film and a heat sealable layer. Opaque foils, with an aluminium layer, are an excellent barrier to the transmission of water and gas. They have been used for creating microenvironments and lining display cases to protect objects from emissions from wood and fibreboard.

However, for the long term storage of museum specimens transparent films are desirable, so conditions can be monitored without opening the enclosures. Transparent barrier films can be composed of a variety of materials and have a huge range of water and gas transmission rates. Until relatively recently these rates had not been good enough for long-term storage, but some of the latest generation of transparent multi-laminar films are proving suitable for museum specimens. They include a middle layer of a vacuum-deposited ceramic on a PVA substrate. The barrier films can be cut to the size required and formed into enclosures by heat-sealing the seams. 

Oxygen scavengers are chemical or physical systems used to absorb oxygen from the air leaving an inert environment. Effective oxygen absorbers designed for the food industry have been available for over ten years. These have been used in museums for pest control, low humidity microclimates, anoxic storage and display. Ageless™ oxygen absorbers (Mitsubishi Gas Chemical Company) were one of the first readily available oxygen absorbers. They take the form of sachets containing an iron-powder based system. The first varieties worked at high relative humidities within the enclosure.

While oxidation reactions would be halted, there was concern about subjecting specimens to such high relative humidities, especially if the enclosure failed. Microclimates in museums are commonly used for specimens such as pyrite or marcasite that need low or specific relative humidity conditions, so these original oxygen absorbers were not considered suitable. The development of the RP System™ (Mitsubishi Gas Chemical Company) solved this problem.

This oxygen absorber comes in two varieties: one has incorporated an inorganic desiccant so is ideal for objects requiring very low humidity conditions, the other is moisture neutral. As well as oxygen, the RP system™ also removes corrosive gases, for example sulphur dioxide, hydrogen sulphide, hydrogen chloride and ammonia. Specimens containing pyrite can be packed in micro-environments with an oxygen absorber and prevent the oxidation reaction occurring. The enclosures have greater longevity if they are composed of a double wrapping of barrier film.

Monitoring the level of oxygen in the enclosure is important. Oxygen indicating tablets can be used to show the initial integrity of the enclosure, but they do not have as long a life as the Ageless oxygen scavenger, and their colour will fade with time, so they cannot be relied upon in the long term. There are other methods of monitoring the enclosures but many of these are very expensive.

Barrier films and oxygen absorbers are not cheap but can be affordable for museums, a cost worth expending if it will prolong the life of valuable geological specimens.