Silica gel passive buffers
An extension of the self-buffering concept is to provide a purpose-made buffering material that can be added to the showcase. A large number of organic materials could be used; over 1700 pounds of canvas hose was proposed for The Orangery of Hampton Court Palace in 1934 . Inorganic silica gel offered many benefits, including seemingly infinite capacity for reuse. It was originally developed at the end of the First World War as a desiccant. In general use outside museums silica gel is usually first heated to remove moisture and is then used to capture and sequester humidity. However
museum microclimate buffering uses silica gel’s capacity to both retain and easily release moisture. This application uses a very small range of its moisture holding capacity, and regular silica gel is not very efficient as a buffering material. By varying the microscopic attributes of this material, silica gel can be tuned to form different grades, which provides more effective buffering in the range of normal museum storage humidities. In display cases where leakage is well controlled and in an ambient environment where the average of year round humidities is close to the desired relative humidity,
passive buffering can be very effective. A buffer in a case can absorb excess moisture as humid air leaks into the case and release it later when dry air leaks in. In theory, some buffers may never have to be changed. In environments where humidity conditions are consistently outside the desired target, larger quantities of buffering materials, frequent replacement, or tighter cases are needed.
By the nineteen seventies, silica gel had become a standard solution for case buffering and display cases could be ordered complete with drawers to hold a supply of silica gel for buffering While passive buffering using silica gel can be a vast improvement over cases with essentially no microclimate control, there are still significant areas where silica gel buffering proves ineffective. In some installations an inadequate transfer of moisture to and from the buffering compound into the case air results in stratification . Air leakage through the case, and inadequate quantities of buffering materials can overwhelm the buffering capacity. Large cases can be especially vulnerable to these effects. Monitoring and maintaining buffers can easily be overlooked, and is often neglected.