Charcoal cloth

Alexandra Zappa <amzappa [at] gmail__com> writes

>I have been looking into the use of activated charcoal cloth to help
>prevent further corrosion of a number of lead coins in an unsealed
>display case with some wooden components but have had trouble
>locating much information that would help to determine the lifetime
>of charcoal cloth.
>
>Does anyone have any experience or information regarding how
>frequently charcoal cloth should be replaced based on the type and
>size of the showcase and pollutant levels? Alternatively, has anyone
>found any reliable indicators that can be placed in a display case
>to help determine when charcoal cloth should be changed?

You may be interested in the paper D. Thickett and K. Short-Traxlet,
Practical Application of Sorbents, proceedings of Metal 2010,
Charleston, 2011, 414-420 which deals with exactly this question. If
you have the same brand of charcoal cloth as tested, know the air
exchange rates of your cases and emission rates of your wooden
components then you could estimate the likely lifetime from the 10
years determined in that study.

At present there is no easy off the shelf indicator (although the
MEMORI project is develping a dosimeter and reader that would be
ideal for this, add website address).  As you have lead, the easiest
way to do it is probably to use cleaned lead pieces (see Robinet
Studies in Conservation 2003 for cleaning method) and watch for any
signs of white corrosion on the surface.  Big question is whether
the lead would corrode before your coins which depends on the exact
composition, purer lead generally is more susceptible to acetic acid
induced corrosion, trace elements can make the lead much more
susceptible, but generally a clean surface is more susceptible than
archaeological lead.  This is used quite frequently at English
Heritage and was used in the British Museum and don't think I've
ever seen an object historical or archaeological (probably over 600
in total) corrode before lead coupons of the purity and cleaned as
described in the Robinet paper.  The other method is to use
diffusion tubes (as in Metal 2010 paper) but they are quite
expensive for acetic acid as it's a method prone to contamination.

David Thickett
Senior Conservation Scientist
Rangers House
Chesterfield Walk,
London SE10 8QX


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                  Conservation DistList Instance 25:38
                 Distributed: Monday, February 20, 2012
                       Message Id: cdl-25-38-011
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