Adhesive for exterior woodwork

I am investigating adhesives to be used in the treatment of four
exterior ionic capitals (attr. to Bernard and Jugiez, ca. 1765),
carved in vertical grained southern yellow pine. Treatment of the
capitals will include adhering the split parts, priming and painting
and reinstallation at Mount Pleasant.

We have a fairly good, though basic, idea of which adhesives we
would like to test, but are still open to suggestions. If you have a
good idea about suitable adhesives, companies, tests or other
related topics, we would be very grateful to hear from you.

The following is a summary of our research so far.

A list of desired properties of the adhesive was made:

    *    Resistant to weathering (moisture, cold, heat)
    *    Good elasticity (to enable wood to expand and contract)
    *    Reversible/ retreatable
    *    Gap filling (up to 1 mm)
    *    Good ageing qualities (10-50 years?)
    *    Not inhibiting adhesion of alkyd primer and paint
        (Benjamin Moore)
    *    Good working properties as the fills need to be carved

After a rather extensive search in literature we came up with four
different categories for exterior wood adhesives that can be
suitable for conservation purposes. The categories are based on the
binder:

    1.  Thermoplastic resin, like Butvar B98 (Tg of 144-154 oF or
        62-68 oC) with a filler like granulated cork

    2.  Flexible Epoxy with or without a filler

    3.  Silicone rubber with or without a filler like phenolic
        microballoons or chalk

    4.  Polyurethane Marine Adhesive

For category 2, 3 and 4 the wood would need to be sealed with a
product like Butvar B98 to avoid penetration in the wood.

We are considering to test adhesives on the following properties,
possibly before and after (accelerated) weathering:

    *   Elasticity of the joint: tensile strength testing machine,
        tested under (cyclical) loads of compression and tension.

    *   Adhesive qualities to wood: visual examination of the bond
        to the wood before and after each test, including
        weathering, and tensile strength tests.

    *   Compatibility of combinations of a sealer and a filler can
        be visually examined as well.

    *   Compatibility with paint: preparing a second set of the same
        samples, primed and painted and running them through the
        same tests.

Obviously the previous is a condensed version of our findings up to
now. Please feel free to ask for more information.

Rian Deurenberg
Andrew W. Mellon Fellow in Conservation of Furniture and Woodwork.
Philadelphia Museum of Art
Box 7646
Philadelphia, PA, 19101-7646
United States of America
215-684-7556


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                  Conservation DistList Instance 16:73
                  Distributed: Thursday, May 22, 2003
                       Message Id: cdl-16-73-013
                                  ***