Desiccated Microclimates for Heritage Metals: Refining Procedures for Use of Silica Gel

Desiccated microclimates offer an effective method of managing safe storage of archaeological metals. They utilise simple hardware that can produce low relative humidity (RH) environments on a small scale to control post-excavation change in objects. Previous studies have highlighted the complexity of decision-making when setting up desiccated microclimates involving many factors that can impact on their performance. These include the design of the container used to house the microclimate, the ambient external atmosphere, the target internal RH, the amount of silica gel used and its regeneration schedule. This paper builds on that understanding by replicating reported sector-wide variations in how silica gel is used within desiccated microclimates. The desiccation efficacy, rate of change in RH and response to short-term fluctuations have been examined by monitoring the RH in polypropylene containers when silica gel is used loose, in polythene bags and Tyvek® bags. The effect of variables in the use of polythene bags to hold silica gel, including the distribution of bags and the number and size of holes in bags has also been investigated. Results indicate that these variables impact rates of change in RH and how effective the desiccated microclimate is at buffering external RH fluctuations. All tests reinforce the importance of airflow between the silica gel and the microclimate. Where airflow is restricted, the ability of the microclimate to desiccate the environment below lowest known corrosion thresholds (15% RH) is compromised. The practical implications of the results have been discussed to support decision-making and guidance is offered on best practice.