Caulking outdoor sculpture

Joel Welter <woodyna [at] pacbell__net> writes

>Does anyone have experience or know of a suitable caulking for an
>outdoor sculpture?  The surfaces to be caulked would be metal to
>ceramic and metal to metal.  There is already a black flexible
>caulking which appears to be silicone.  It is in good condition but
>does not adhere well to the surfaces.

Presumably the caulking used on the outdoor sculpture in your care
is a contemporary building material--probably a black silicone
(possibly sulfurous).  Considering it's structure and performance
characteristics (weathering, expansion/contraction, UV degradation),
it is not uncommon for such an organic, usually silicone, caulk to
become separated from the surface(s) it is supposed to be joining.
Most of use have first-hand experience in our bathtubs, where mold
and other goodies nestle in the space between caulk and ceramic.
Hence, it may be safe to say the caulk is not in good
condition--they do have relatively short life-spans.

After all, a lot is expected of a caulk: vertical water repellent;
barrier to water and other molecules at the caulk-to-material
contact points; structural integrity of the caulk bead itself
(caulks do degrade, potentially forming acids); flexibility
parameters to compensate for movement within the building or
sculpture; and, god forbid, structural adhesion (it is possible that
the black "caulk" used was originally intended as a structural
adhesive). Ideally, a caulk should not have any structural
requirements--it should just serve as part of the cladding or
exterior envelope system--but, sometimes it does serve a structural
function, either by design or by the circumstances of aging.  You
need to determine this first--then decide if and how that issue
would be corrected.

Otherwise, I have a number of questions about the size/shape of the
sculpture; it's siting and weather exposure; internal structure;
overall condition; expansion joints; maintenance expectations
(who/how/how often caulk removed and replaced now and in future);
vulnerability of the ceramic and metal surfaces (to raking out the
old caulk); color and other expectations of the artist or
commissioning agency; accessibility to the sculpture; etc.--before
selecting a replacement material.

Have you found an article written by Scott Carroll (and another
conservator) about the materials and performance of commercially
available caulks?  Their research may have been on interior uses
only. Scott is a conservator for the Museum of the American Indian
(Smithsonian) currently located in New York City but moving to DC
soon.

Also, I understand that the AIC RATS (research and technical
studies) group and the National Center for Preservation
(Natchitoches, LA) are gathering/creating similar data on commercial
building materials (expanded Oddy-type testing).  Of course, there
is always ASTM if you want to review the testing parameters used to
standardize these materials.

Molly Lambert
Architectural Conservation
1334 Derby Street
Berkeley, CA 94702
510-849-3811
Fax: 510-849-3812 fax

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                  Conservation DistList Instance 12:52
               Distributed: Wednesday, December 16, 1998
                       Message Id: cdl-12-52-011
                                  ***