Silica gel and adsorbed pollutants

Will Jeffers <wjeffers [at] mfa__org> writes

>I was wondering if anyone on the DistList has had any experience
>with silica gels carrying pollutants from one installation to
>another?
>
>In the case of desiccating silica gel that can be reconditioned by
>heating, I would assume that any adsorbed pollutants would be driven
>off with the moisture.  Can anyone confirm or refute this?
>
>Then there is the case of Arten and Artsorb buffering gels.  Do
>these materials readily adsorb pollutants?  If so, how can these
>pollutants be desorbed?

It is true that silica gel does adsorb pollutants as well as water
vapor. Getting the pollutants to desorb is a different story.
Normally, such pollutants remain entrained in the pore structure and
typically do not emerge except under extreme heat, and then only in
trace amounts. The chance of recontamination would be slim,
especially if you completely desorbed your gel and reconditioned it
between exhibits. Garden-variety, regular-density beaded silica gel
will sometimes show a discoloration when contaminated, and this
remains even after desorption.

Due to its larger pore size, ARTEN silica gel is one of the few
desiccants that can readily adsorb big hydrocarbons, but the
desorption process requires large volumes of moving air and a pretty
high temperature (over 500F). In general, silica gel does not
transmit entrained contaminants (held as a function of capillary
condensation) as readily as it transmits water vapor (held as a
function of Van der Waal's forces on the surface and pore walls of
the bead). So, it is unlikely that ARTEN silica gel would release
significant amounts of contaminant at ambient temperatures, unless
the initial level of contamination was high.

On a purely practical level, it's probably best to pitch the gel if
you suspect that it's contaminated (taken on a bad smell or turned a
funny color)--there's no sense in courting risk in the interest of
saving a few bucks. Desorbing contaminated gel with heat and
recharging it will reduce the level of contamination, but will
probably not eliminate it entirely. If the environment within your
showcase is bad enough to cause such gross contamination, then you
should probably include some activated charcoal, which has a much
greater affinity for gaseous contaminants. But under normal
conditions and assuming typical levels of exposure to contaminants
in an exhibit case, you can safely re-use and recharge your silica
gel time and time again.

Thanks go to Jim Druzik for his thoughtful and informative comments
on this subject. But at APS, we strive to make no unrealistic claims
about our desiccant product. Although our gel is capable of
adsorbing some contaminants, its principal use is for stabilizing
relative humidity at a fixed level over time. Arten Silica gel
should be installed in a clean, reasonably sealed environment. We
strongly recommend the application of outgassing barriers on all
wood or wood-product surfaces and advocate the use of
conservation-safe synthetic products in any showcase. In the event
that the exhibit itself is the source of contamination due to some
inherent vice, we recommend the application of potassium
permanganate media or activated carbon (depending on the nature of
the contaminant) in addition to our product.

Craig Oleszewski
Art Preservation Services
ARTEN Environmental Products

                                  ***
                  Conservation DistList Instance 12:91
                  Distributed: Wednesday, May 26, 1999
                       Message Id: cdl-12-91-001
                                  ***