Resin for bronze

    **** Moderator's comments: This item required considerable
    editing to make it understandable. I hope I have not introduced
    any technical errors and will welcome corrections if I have

I have been asked to find a soft, flexible, clear resin with
long-term stability that can be used as a kind of armature to hold
tiny archaeological bronze fragments in precise positions. The
fragments size will range from one millimeter to one centimeter in
size.  The resin must allow rapid removal and then re-fixing the
fragments safely.

We have reviewed various epoxy, polyurethane and silicone systems,
but none meet our expectations. We would very much appreciate it if
someone could help us to locate an appropriate polymer and thus,
ultimately, a commercial product.

Our requirements are that the resin be clear/transparent, though we
could accept some small amount of tint since the fragments are tiny;
but we want to avoid a completely opaque resin.

We're hoping the casting can be done at the bench without too much
complicated technology, if possible.

The major problem we are facing is the migration of copper ions into
the amine/epoxies and anhydride/polyurethane resin we have now
tested. Some react after only few weeks, with copper ions even
visible by naked eye in the transparent resin. The silicones are
more acceptable but seem to release oils in contact with any porous
items. Neoprenes are unfortunately not transparent enough and
PMMA/PCs are really not flexible enough for us.

We understand we will need to compromise, but from your experience,
what might be the best best way to design appropriate selection
criteria for our requirements?

The resin has to support small freshly excavated archaeological
copper fragments that are chemically quite reactive. It has to be
soft and rubbery and capable of gently gripping the fragment. And it
also has to be chemically stable over a period of several years. But
we are primarily interested in a material that is soft and perhaps
slightly tacky.

At this point, we do not know where else to look to meet these
requirements and we would greatly appreciate any input and
experience in this matter, both good and bad. We are not yet under
time constraints, and could afford some delay in response if that is
more convenient.

David Cottier-Angeli
Associated Member of the Swiss Chamber of Technical and Scientific
    Forensic Experts
5C Route des Jeunes
CH-1227 Geneva
+41 22 300 19 55
Mobile: +41 79 319 319 0
Skype: cottiermetal


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                  Conservation DistList Instance 24:10
                   Distributed: Friday, July 30, 2010
                       Message Id: cdl-24-10-007
                                  ***