Although we conservation professionals use industrial adsorbents to remove a specific vapour or gas from showcases or storage areas, their chemical and physical structures enable them to adsorb families rather than specific pollutants. You are using silica gel to adsorb water vapour but it is also an effective adsorbent for formaldehyde and acetic acid. Likewise, activated charcoal has a surface area of up to 1500 m2/g with pores between 10 and 30 Å in diameter). Molecules are held within pores by weak physical bonds. Activated carbon is non-polar and therefore has a high affinity for organic molecules. Although it has a low affinity for water, its ability to absorb liquid water was determined and published as early as 1949 http://pubs.acs.org/doi/abs/10.1021/ja01170a051 I suspect that the activated charcoal is contributing to the reduced RH in your showcase. Removing it and observing the resulting RH will confirm or refute this suggestion.
Yvonne Shashoua Senior Researcher National Museum of Denmark email: yvonne.shashoua-at-natmus.dk ****** Unsubscribe by sending a message to [email protected] Searchable archives: http://cool.conservation-us.org/byform/mailing-lists/cdl/
