Whether you have just a few specimens or many thousands, creating a suitable storage environment is one of the most effective ways to prevent deterioration and damage. This can be done at the macro level (i.e., for an entire building or room where collections are housed) or at the micro-climate level (a display case, storage cabinet or box).
High temperatures are generally bad for collections because they promote physical ageing and deterioration. On the other hand, temperatures that are too cold can promote thermal shock, in which a specimen could become brittle and crack or shatter; they can also be uncomfortable for people working in the collections. Widely fluctuating temperatures can cause a variety of kinds of damage to specimens. Incorrect temperatures also can lead to changes in the crystalline structure of minerals; soften adhesives that are used in fossil preparation, resulting in slumping; and, when combined with high relative humidity can lead to mold growth on specimens, labels, and storage containers. Generally speaking, a steady temperature of 68-70 degrees Fahrenheit represents the best compromise for good collections care and human comfort. For paleontological collections the National Park Service Museum Handbook recommends temperatures of 59-77 degrees Fahrenheit.
Relative humidity (RH) is closely linked to temperature; warm air holds more moisture than cool air. For the best storage environment, the goal is to keep RH levels moderate and stable. That means keeping RH generally close to 50% and attempting to minimize short-term variation to between 45-55% even if broad seasonal trends are hard to avoid. Large shifts in RH may result in physical damage to specimens; fractures and crumbling can occur as the specimen alternately absorbs or releases moisture, causing swelling or shrinkage. High RH can also promote oxidation and corrosion of certain minerals, such as iron pyrite (see Pyrite Disease below).
A variety of different environmental control strategies can be used depending on the situation – at the macro level this includes centralized control of air temperature within a building (with or without active relative humidity control); room level temperature control (e.g., radiators with window mounted a/c units); or entirely passive control based on the buffering effects provided by the building itself (which can be improved by additions such as door and window seals).
Controlling RH at the building or room level can be very expensive, and if not done properly may actually end up causing structural damage to the building. However, RH can sometimes be very effectively, and relatively cheaply controlled by creating a micro-environment around the specimen, using a combination of well-sealed storage cabinets or exhibit cases and buffering materials, such as acid-free tissue, wooden drawers, and silica gel.
Even if you have limited ability to control the environment or your collection buildings or rooms, it is a good idea to know what conditions your specimens are being subjected to, so that you can anticipate problems and come up with micro-climate level solutions when possible. Monitoring environmental conditions can be done using equipment such as Heating Ventilation and Air Conditioning (HVAC) and/or Building Management Systems (BMS); if you don’t have access to these, reasonably priced electronic data-loggers or recording hygrothermographs are available and can be used very effectively.
Pyrite (or iron persulfide: FeS2), also known as “fool’s gold,” is a common sulfide mineral that is often found in sedimentary rock. In some fossil deposits, pyrite gets incorporated into bone, invertebrate shell, and plant fossils during the process of fossilization. If these fossils are exposed to conditions of high humidity, “pyrite disease” (also known as “pyrite rot”) can occur. The mineral oxidizes and forms iron sulphate (FeSO4); this oxidation product is several times the volume of the original mineral and the resulting crystal growth and expansion causes the specimen to fracture and crumble. Keeping fossils in dry conditions – under 45% RH - is the only way to prevent this deterioration. Once the damage begins it is irreversible.
For an example of what can happen to fossils with pyrite disease click here to visit the website of the Triceratops project at the Smithsonian Institution’s National Museum of Natural History where pyrite oxidation was one of the reasons that the fossil skeleton was removed from display.
For more on this subject see:
- Sections U:7 and U8 of the National Park Service Museum Handbook Part I.
- Pyrite oxidation: Review and prevention by Akiko Shinya and Lisa Bergwall a poster presented at the 2007 Society of Vertebrate Paleontology annual meeting and available for viewing at practices
- Pyrite Preservation by Sally Shelton in the Knoxville Gem and Mineral Society KGeMS Volume XXXII, Issue 2 from February 2001 p. 8
- Collins, Chris. 1995. Care and Conservation of Paleontological Material. Boston: Butterworth-Heinemann.
The most common and problematic contaminant for fossil collections is the dust and dirt that builds up in storage areas without proper housekeeping. Dust can be abrasive and attract pests, and removing it can cause damage to fragile specimens. Well-sealed cabinetry greatly reduces this problem, but can create other problems if the materials of the cabinetry, or the storage materials contained inside, off-gas, leading to a build-up of harmful gaseous pollutants.
Contaminants can also come in the form of chemicals used in the preparation of specimens (e.g., acids or salts not rinsed away after treatment) or materials used in treatment such as adhesives and consolidants. Specimens suffering from pyrite disease (see box) emit sulfuric acid, which will contaminate storage materials and damage other specimens nearby. Any specimens suffering from pyrite disease should be isolated from the rest of the collection.
Tips to reduce dust and grime
- Make sure circulating air in the collection is as clean as possible by using filters on your A/C or HVAC system and making sure that they are changed regularly
- Keep windows closed in storage areas
- Keep specimen cabinet doors closed
- Use dust covers on open shelving
Most fossil specimens are not directly affected by either visible or ultraviolet light, but other mineral components of a collection can change color, change phase, or decompose in response to high light levels. A bigger concern for paleontology collections is the ability of light to affect adhesives used in the preparation or preservation of a specimen, as well as its effect on other collection housing materials. Light damage is cumulative and irreversible. As a result it’s a good idea to limit light exposure if possible.