Unlike materials made from leather, paper or a particular metal, it is not possible to recommend generalised storage and display conditions which will satisfy the conservation requirements of all rock and mineral samples. This is because of the potentially differing conditions needed to preserve the range of chemical types that may comprise such a collection.
Unfortunately in a publication of this size it is also not possible to specify all mineral types and their particular conservation requirements. As conditions for storage and display may have to be tailor-made to cater for the differing sensitivities of materials in a collection, custodians of collections will need to refer to specialised publications (Howie 1992) and contact experts in the field for more specific information.
It is obviously very important to identify the minerals and rocks that make up a collection. Unless materials are correctly identified their needs cannot be adequately catered for. Consult experts and textbooks to ensure that correct identifications are made for all specimens in a collection.
General information can be provided however, which will act as a guide for the care of these types of collections. Protect materials against:
- physical damage by abrasion or shock;
- unfavourable environments;
- inappropriate chemical treatments; and
- poor handling.
It is generally accepted that rocks and minerals are best stored in clean, dustproof surroundings in which conditions of low light and stable, moderate temperatures and relative humidity levels prevail.
Ideal conditions have been described as 50 % relative humidity at 15 – 20 °C (Price 1992). These ‘ideal’ conditions however cannot be broadly applied as deterioration mechanisms and the impact of changes in factors such as relative humidity for example, vary from mineral to mineral Apply strategies previously described to minimise damage from overexposure to light or inappropriate conditions of temperature and relative humidity (see the chapter Preventive Conservation: Agents of Decay).
Of these factors, relative humidity control is the most critical as inappropriate relative humidity conditions can lead to corrosion, gain or loss of water from crystal structures, changes from one crystal phase to another and fracturing of crystals by mechanical stresses induced by cycles of moisture gain and loss.
Store specimens prone to corrosion at relative humidity levels between 30 and 60 %. Stability is enhanced as the relative humidity is lowered. Pyrite is one example of a mineral that will oxidise unless the relative humidity is strictly controlled.
Begin the care and conservation of rocks and minerals at their point of collection. From this time ensure that:
- poor handling does not lead to physical damage;
- specimens are not ‘over-cleaned’; and
- environmental fluctuations are taken into account in transporting the specimens from the point of recovery to their new environment.
Some samples such as fluorite, wulfenite, apatites and fibrous zeolites are susceptible to physical damage. Follow the usual guidelines when handling these objects (see the chapter Handling, Packing and Storage). If delicate samples of these types are to be packaged for transport take additional care, as outlined below:
- wrap specimens in acid-free tissue and place in labelled, washed cotton or linen bags to minimise physical damage; and
- seal the wrapped specimens in polythene bags to buffer against environmental changes that may occur in transit.
For transporting particularly vulnerable specimens such as wulfenite and zeolites “open tissue-lined trays and transport by hand is perhaps the only practical method” (King 1992a).
Storage and Display
When designing storage conditions, take into account the needs of the specimens and of the people that care for them. For example, store mercury in sealed containers to prevent loss through evaporation and poisoning by inhalation of toxic vapours. Take similar care when storing radioactive materials. The degree and type of radioactive emission (alpha, beta or gamma radiation) will determine the type of storage facility required.
Use archival quality materials for any materials that are likely to come into contact with the specimens so that their chemical stabilities are not jeopardised.
Materials used in the construction of the storage building and in storage or display areas may also affect specimens. The alkaline nature of concrete for example, is likely to affect metals, sulphides and silicates. A comprehensive list of common construction materials, their likely effects on mineral specimens and recommended actions is provided in Howie (Appendix I, 1992).
Storage options for particularly sensitive specimens include:
- sealing in glass ampoules or tubes;
- refrigerated containers or environments; and
- buffered micro-environments (silica gel, zeolites).
In all cases the chemical stabilities and sensitivities of specimens must be known before undertaking a particular type of storage. Transparent containers are recommended so that the condition of samples can be monitored easily.