Dr Caroline Buttler (UK)
It has long been recognised that art and archaeological collections in museums may need specialised conditions and conservation to survive. However, until relatively recently, geological collections have not had the same level of care. Perhaps, it was thought that rocks, minerals and fossils that had already survived millions of years do not need any particular attention. Although geological material may appear strong and durable, there are factors that can lead to the deterioration and even the complete destruction of specimens. The last 20 years have therefore seen a growing interest in storage conditions for geological collections with some museums appointing specialist conservators to care for them.
The museum environment
The museum environment is traditionally a compromise between the need to preserve objects and to provide comfortable conditions for staff and visitors. Unfortunately for specimens, when there is a conflict, human interest often wins. Environmental factors, including temperature, humidity, light and pollution, can be major threats to geological material.
Temperature alone does not usually cause damage to specimens, but it can speed up the rate of deterioration and changes in temperature can affect relative humidity (RH). There are no ideal levels of temperature and relative humidity suitable for all geological material, but the commonly accepted parameters are 20oC plus or minus 2oC, and 50% plus or minus 5% RH, and air-conditioned stores are set at these. However, many specimens do not have the benefit of these conditions and, even those that do, can still degrade and fall apart.
Geological conservators can sometimes be seen sniffing specimens, at first glance a strange thing to do. However, this is a good way to identify one of the most destructive processes they encounter – pyrite decay or sulphide oxidation, which can reduce a specimen to dust, destroy specimen labels and damage storage materials. The decay of pyrite (and marcasite) occurs when the sulphide component oxidises to form ferrous sulphate and sulphuric acid in humid conditions. A wide variety of decay products can develop, depending upon the mineral composition of the fossil, mineral or rock matrix. Pyrite decay can be identified by a sulphurous smell, hence the sniffing conservator, and also by the formation of cracks and efflorescent decay products that are usually yellow or white. Acid damage to packaging materials, which has the appearance of burning, is also a good indication of pyrite decay.
Pyrite decay has been recognised for a long time and there is a history of treatments. The earliest methods involved immersion in fluids to isolate the specimens from the air. In the 1970s, the process was considered to be a bacterial reaction, which can occur underground in mines, and the treatment was to immerse the affected specimen in antiseptic solution. This did not work and it is now known that the reaction in museums is electrochemical. To prevent pyrite decay, material at risk should be stored at low relative humidity levels, ideally below 40%. Small, low humidity enclosures can be made in which to keep pyritised material.
Another method is to store specimens in anoxic microclimates, as the absence of oxygen will stop the oxidation reaction. (Discussed in Oxygen-free storage for museum specimens in this issue). Specimens that have already been affected by pyrite decay may be treated chemically to neutralise the decay products. They can then be packaged in low humidity or anoxic environments to prevent any recurrence.
Too wet and too dry
Some geological specimens are sensitive to both high and low relative humidities, making them particularly difficult for conservators and curators to care for. Subfossil bone (that is fossilised bone that is not completely mineralised) is susceptible to slight changes in environmental conditions. High relative humidity can cause the bone to swell and low levels can lead it to drying and shrinking,
leading to the bone cracking and splitting. This damage often follows a characteristic pattern, for example, mammoth tusks commonly crack in a longitudinal direction and between the conical layers. Teeth, subjected to humidities below 50% RH, tend to be more fragile and susceptible to damage and, below 30% RH, deterioration increases substantially.
Shale is another problematic material. The clays contain a certain amount of ‘bound’ water that remains bonded to the clay molecules and there may also be a large amount of ‘unbound’ water. This can cause swelling and deformation of this type of rock as the water is taken up in wet conditions and lost in drier conditions, causing cracking and splitting. Pyrite is relatively common within shale and this can make matters worse. It is difficult for curators and conservators to decide what the correct environmental storage conditions should be in this case. Relative humidity below 30% could cause irreparable and irreversible damage through shrinkage, causing delamination and cracking, whereas above 60% could well promote oxidation of the pyrite within susceptible material.
Cold and damp are not always bad
Some geological specimens cannot survive in air-conditioned stores. This is because they undergo chemical reactions within these temperature and humidity limits. An example of this is the mineral melanterite, a hydrated iron sulphate that commonly forms as a post-mining phase in old coal and metalliferous mine workings as a result of the oxidation of pyrite. At relative humidities below 58% at room temperature, melanterite will effloresce, changing from blue/green crystals to a white powder, due to the loss of water of crystallisation. This mineral is more likely to survive in a damp, cold basement, where the conditions are similar to the mine from where it is collected, rather than a dry, air-conditioned store.
Salts can often be observed crystallising on surfaces of buildings, rock structures and monuments. This it is commonly associated with the movement of water and the soluble salts in it. When the salt solutions reach the surface of the rock, water evaporates and the salts crystallise on or near the surface. When crystallisation occurs below the surface (sub-florescence), the forces generated can cause sub-surface cracking and spalling. In geological collections, the cause of efflorescence can be due to ambient high relative humidity causing soluble salts present in the specimen to dissolve in their own absorbed water. The solution moves towards the surface and the salts crystallise back out. It is therefore important to wash carefully any material collected from coastal areas.
Light is well known to affect sensitive watercolours and textiles, but it can also be a problem for some geological specimens. Ultra-violet radiation has sufficient energy to cause chemical changes in susceptible material and this can be seen by the fading, colour change and deterioration of some minerals. Certain minerals, for example realgar (AsS), need to be stored permanently in the dark to preserve them. The specimens are not the only thing that can be damaged – the ink on labels on display in bright sunlight can fade completely leading to the loss of valuable information.
Ultra-violet radiation can easily be eliminated by the means of filters or the use of non-UV emitting bulbs. Light levels should be as low as acceptable while the rooms are occupied and dark when empty, with very sensitive material stored in sealed boxes. If sensitive material is put on display, it must be accepted that light may change the appearance or chemical nature.
Pollution is commonly in the form of dust, smoke, dirt or gas. Gaseous pollution can arise from outside sources, such as vehicle exhausts and local industry or from heating systems, machinery and indoor construction materials, such as wood, surface finishes, fabrics. Some minerals will also emit pollutants, for example, sulphur dioxide from pyrite decay. Dirt and dust are both disfiguring and damaging.
Delicate crystal masses and fragile fossils are easily damaged during the removal of dust. Many dusts readily absorb water from the air and are chemically active. Cement particles from drying concrete and plaster are highly alkaline, while combustion soot is often acidic. Control of external pollution, both particulate and gaseous, is best carried out by sealing and draught-proofing all doors and windows. Good housekeeping should ensure that all surfaces are kept clean and dust free. Internal pollution is controlled by the correct choice of storage and display materials, good design and good maintenance.
The successful preservation of geological materials, first and foremost, depends on an awareness of the mechanisms of decay in geological specimens. Once this is known, stable and appropriate environments for collections can be produced in well-designed storage, packaging and display systems.