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.

According to Science for Conservators, volume 2--Cleaning, page 71,
morpholine O(CH2CH2)2NH is a selective solvent for shellac. In my
experience Nitromors paint stripper works but it might leave a
purple stain behind.  I have never used it on terracotta though.  If
you haven't used it before apply it with a brush in a test
area--preferably under magnification--and remove it after say 20
minutes--and repeat for longer periods until you find what works.
Stop each test with acetone on a cotton wool swab.  Use fume
extraction or wear a mask--dichloromethane is carcinogenic.  The
other active ingredient is caustic soda, so don't get any of it on
you.  Wear gloves and eye protection.

Eric Miller


                                  ***
                  Conservation DistList Instance 21:35
                Distributed: Tuesday, December 18, 2007
                       Message Id: cdl-21-35-008
                                  ***