Silica gel

In Instance 11:60, Gillian Noseworthy asks about the absorption
isotherm for grade 44 silica gel.

The absorption isotherms for ordinary (microporous) silica gel are
very similar, and, given the assumptions that are built into the
calculations, you will not go very wrong if you use the standard
isotherm.

Garry Thomson recommended the use of 20kg of silica gel per cubic
metre of show case (The Museum Environment, 2nd edn, 237) in order
to stabilise the RH inside the case as far as possible.  Depending
on the fluctuation of RH in the gallery, and the mean RH you are
trying to achieve, this should be adequate, although in my
experience there is no harm in using more--eg 50kg per cubic metre.

It is indeed difficult to tell when activated carbon is exhausted.
Depending on the pollutant(s) you are trying to absorb you could use
diffusion tubes or simple polished metal coupons to tell you when
the case atmosphere starts to become corrosive.

Barry Knight
English Heritage
London

                                  ***
                  Conservation DistList Instance 11:61
                Distributed: Wednesday, January 14, 1998
                       Message Id: cdl-11-61-002
                                  ***