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.

I have had the most success in shellac removal with successive
soakings in 50:50 acetone:ethanol.  Owing to the nature of most
shellac adhesive applications, there may parts of the shellac that
have literal;y been "burned" into the clay body.  One of the reasons
shellac was so popular was the fact that it is a thermoplastic resin
and after application it could be re-heated to move pieces around
and correct mis-aligned joins. Unfortunately, the common heat source
for this was an alcohol lamp or a bunsen burner (both of which I
have seen used for this task). Thus the burned shellac, which is
then a virtually a permanent stain.

Mix the acetone/alcohol together before immersing the pottery
fragments. Immersion is necessary, for at least 1 hour, and
poulticing does not give the same result.  Brush off softened
residues, re-soak in clean acetone:ethanol.  You may need to do some
careful cleaning under a microscope using a bamboo skewer or scalpel
to remove stubborn residues. And you may still have some very dark,
intractable stains, but they should be minimal.

I have never seen these solvents damage a ceramic, nor have any
deleterious effect on slips/glazes.  Always wear gloves when
handling this solvent mixture.  It is much harsher on your hands
than either solvent by itself.

Stephen Koob
Conservator
The Corning Museum of Glass
One Museum Way
Corning, NY 14830
607-974-8228
Fax: 607-974-8470


                                  ***
                  Conservation DistList Instance 21:35
                Distributed: Tuesday, December 18, 2007
                       Message Id: cdl-21-35-006
                                  ***