J. P. Brown
Low-humidity microenvironments using polyethylene (PE) and, latterly, polypropylene (PP) boxes with ca. 80 kg/m3 of desiccated silica gel are widely used in preventive conservation for the storage of unstable archaeological metals from terrestrial contexts (Cronyn 1990, Logan and Selwyn 2007, Scott and Eggert 2009). One notable disadvantage of this method is that, because PE and PP are poor moisture barriers, the silica gel must be regenerated or replaced annually. The Field Museum was found to have 4,500 unstable archaeological metal objects distributed through its stored collections, requiring an estimated 400 kg of silica gel if PE or PP boxes were used. Instead, we opted to use enclosures made of Escal, a transparent plastic laminate primarily used for anoxic storage because of its low oxygen transmission rate, but which also is also suitable for low-humidity applications by virtue of its extremely low water vapor transmission rate (comparable to aluminized plastic barrier films). Using Escal has allowed us to reduce the amount of silica gel used to 5 kg/m3 with a predicted regeneration interval of at least ten years for iron objects and at least forty years for copper alloy objects. In other words, the use of Escal reduces the quantity of desiccant by a factor of sixteen and extends the interval between regenerations by a factor of ten. In this paper we discuss the theoretical considerations that led to our choice of Escal as a barrier film, the practical details of implementation, and report on the progress of the program after the first six years.