White lead

Long story, short problem: I have been cleaning a shop sign. The
painting is on slate, the layer I want to retrieve is the original
layer, painted between circa 1780 and 1812 straight onto the slate,
i.e. no ground layer. It is oil based and probably cerulean or
Prussian blue, no ultramarine or cobalt, (this is what the colour
looks like my museum cannot afford analysis). On top of the blue
layer and very coherent with it, (it would take a very long time to
remove it mechanically), lays the problem: a layer of white lead
pigment (possibly mixed with kaolin and casein) painted
between around 1800 and 1880.  I tried almost every solvent in the book,
pure and mixed with each other: acetone, xylene, IMS, white spirit,
turpentine, toluene, shellsol, trichloro and dichloro ethane,
ammonia, nitric and Hydrochloric acid mixed with acetone. and
Nitromors. The Hydrochloric + acetone, would work but discolour the
blue layer. Nitromors works but I am taking a fearful risk. The sign
hung in the open, high on a wall, since its creation until around
1930. The white layer feels like thin ceramic. The blue layer is
relatively solid, i.e. acetone takes a while to soften it. Can
anybody help?

Dominique Rogers
Ganapati Kumari, Pinmill, Ipswich, IP9 1JW, UK

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                  Conservation DistList Instance 10:22
                Distributed: Wednesday, August 28, 1996
                       Message Id: cdl-10-22-003
                                  ***