Solvent gels

We are actually working on the cleaning of an outside wall made of
enameled stoneware plates. It was covered with polyurethane varnish
during the 1970's and today the resin is very deteriorated: on some
colors we do remove it mechanically, but on the red enamel the
varnish is only whitened and still very well attached to the
surface.

To be able to remove it mechanically, we are are trying to make it
swell with solvent gel mixtures.

Using the Teas triangle and Horie's diagram of PUR swelling, we've
tried several mixtures, applied directly, or through a fabric, to
the surface. The gel is covered with aluminium foil during different
times (1,2,4,8,12 hours). We have used until now the less toxic
solvents, with mitigated results :

    4% of hydroxypropyl cellulose (Klucel G) in ethanol/acetone
    (10/90 w/w) softens quite well the varnish, but it evaporates
    too fast and the gel sticks on the enamel surface and is
    difficult to remove.

    3% of hydroxypropylmethyl cellulose (HPMC) in acetone/water
    (50/50) was not efficient enough in terms of varnish swelling
    (still too fast evaporation of acetone, and mixture probably out
    of the swelling zone) but makes a gel that is easy to remove.

    3% HPMC in acetone/water/2-Ethoxyethanol (Cellosolve) 45-40-15)
    was better in terms of retention but still not efficient enough
    on the more rough surfaces.

Before trying more toxic solvents we would like to hear about
others' experiences on similar cases. We also would like to know if
someone has an explanation on the fact that the varnish is less
deteriorated on the red enamel plates than on the blue, green,
yellow and black (we do think it's about light absorption and
reflection).

Agnes Gall Ortlik
Ceramic, Glass and Enamel on Metal Conservator


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                  Conservation DistList Instance 18:56
                  Distributed: Saturday, June 4, 2005
                       Message Id: cdl-18-56-029
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