As mentioned above, adequate preservation of archaeological specimens requires storage under conditions that minimize the rate of processes which might cause them to deteriorate. Consequently, it is important to maintain a stable, moderate environment, to minimize mechanical impacts, and to monitor collections regularly to ensure that they are not deteriorating.
Two of the environmental factors that need to be controlled and monitored are temperature and humidity. Higher temperatures accelerate chemical reactions, such as oxidation, so artifact storage should be in a climate-controlled space with temperature stabilized around 20 °C or 70 °F. High humidity is particularly damaging to metal artifacts, but frequent fluctuations in humidity are damaging to most kinds of archaeological specimens. Perforating any polyethylene bags that are used to contain artifacts prevents moisture from condensing inside. Specimens that require particularly stringent protection from humidity should be in sealed polyethylene boxes equipped with silica-gel, which absorbs atmospheric moisture until its micropores become saturated. It is necessary to reset the silica-gel every few months by heating it in a lab oven at 105 °C (220 °F) for about ten hours to drive off the accumulated moisture, then returning it to the sealed container after it cools. An indicator strip in the box will signal when the silica-gel needs replacement, while putting a hygrometer in the container or sealed cabinet makes it easier to monitor humidity and make sure it stays within a safe range. For iron artifacts, for example, it is necessary to use enough fresh silica-gel to keep their relative humidity between 10% and 20%.
However thorough the attempts to provide artifacts with acceptable environments, deterioration of their condition is always a risk. Condition monitoring is our tool for alerting us of such risks. It involves regular checks on archaeological collections, recording any change in condition that artifacts may have experienced, changing silica-gel when needed, and ensuring that the artifacts’ environments are within acceptable ranges. The record that results is called a condition census. Note that having clear lids on any boxes used for artifact storage facilitates monitoring.
It is important to note that archaeological laboratories must not only preserve their collections of artifacts and other specimens; they must also preserve the documentation of excavations and analyses, including the records of conservation itself. In addition to paper hard copies, such as archaeological field notes kept in binders and maps and plans kept in map cabinets or tubes, archaeological laboratories typically have substantial collections of slides, negatives, and digital data and images. Archiving these safely, including making sure that old digital files are transferred to new formats as these change, is very important.
One important component of lab documentation is a ‘leave log’ to track any specimens or documents that anyone has removed from the collections for outside analysis, photography, copying, or teaching. Those borrowing specimens should sign them out and sign them back in.
World History
