Cleaning wood sections

Sally Shelton <libsdnhm [at] class__org> writes

>We have a section of a redwood tree that has been on exhibit A Long
>Time. The surface coating is very thick and deteriorated. I  suspect
>that it started out as an annual application of shellac and then got
>into fully synthetic polymers.

I would suggest that you try a cotton poultice on a small area using
50/50 acetone/ethanol.  Cover the poultice with a piece of
polyethylene and leave it for 20-40 minutes (perhaps check it after
20, and re-wet if necessary). This should solubilize shellac, or at
least swell it so that it can be brushed off.  It will also
solubilize most early coating materials (cellulose nitrate, PVA,
some of the cellulose ethers) and will also swell polyester resin.
The solvent mixture needs to be in prolonged contact to work on
shellac, so simple swabbing will not work.

The biggest concern is the condition of the wood underneath all
these coatings.  Is it so badly deteriorated that these were used to
"consolidate" as well as enhance the grain?  Solvents may have a
harsh drying effect, but acetone and alcohol would not be as bad as
methylene chloride (as a solvent or water-based stripper).

Stephen Koob
Department of Conservation and Scientific Research
Freer Gallery of Art
Smithsonian Institution
Washington DC 20560
202-357-4880 x276

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                  Conservation DistList Instance 9:73
                    Distributed: Sunday, May 5, 1996
                        Message Id: cdl-9-73-002
                                  ***