Shellac stains in terracotta

Susan White <smwhitewhite [at] hotmail__com> writes

>I am currently treating a group of unglazed Greek terracotta vessels
>for a small museum.  Several of the pieces have been poorly restored
>using shellac as an adhesive.  Unfortunately the shellac has badly
>stained some of the pieces along the join lines.  I've used ethanol
>poultices and have had little luck in removing them.  Does anyone
>have any good ideas for safely removing old shellac stains from
>terracotta?  I've heard methanol can sometimes work, but wanted to
>check with others first before trying it.

Assuming that your staining is pinkish/purple in color, I have had
similar problems with a Greek ceramic that developed stains while
reversing aged shellac along the break joins.  We determined that
the staining was caused by lac dye present within the unrefined
shellac. We've been researching this staining (how it is created and
why it is so irreversible) and will publish our results soon.

The next phase of this project is reversal of the stains which is a
more complex issue. Although I do not have the best answer for you
at the moment, I can say that after trying almost everything under
the sun when treating the Greek ceramic, one practical method that
somewhat worked was poulticing with Fuller's Earth and methanol (a
1:1 mixture of acetone/ethanol worked also).  The stained area was
moistened with solvent, the poultice mixture applied and covered
with plastic wrap to prevent evaporation. After a while small holes
were punctured in the top of the plastic to direct the evaporation
away from the ceramic and draw the stain out.

This technique required several attempts and reduced the stain in
most cases.  Do let me know if you try this and if it works. In the
meantime our project continues and hopefully we will be reporting on
the best methods for reducing this type of staining soon.

Marie Svoboda
Associate Conservator
Antiquities Conservation
J. Paul Getty Museum
310-440-7052


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                  Conservation DistList Instance 21:35
                Distributed: Tuesday, December 18, 2007
                       Message Id: cdl-21-35-007
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