Cleaning ancient Egyptian wall plaster

At the moment we are working in Egypt in a New Kingdom rock-cut
tomb. The internal walls are plastered in layers, which are made up
of clays, sand, limestone and straw temper, and various ochres for
colour. There is a final top coating  of gypsum and water. This
final white layer forms the background for the incised reliefs.
Pigments here are reasonably robust, especially the natural ochre
ones--reds and yellows. The whole environment is dry.

My problem is this: at some time in the not-too-distant past, very
fine mud, heavy with carbonates, has flowed from a ceiling fissure
down across an area of wall (about 1 metre square) finely coating
scenes, hieroglyphs and  the white background. It is unsightly and
obscures the inscriptions ....and we are under pressure to improve
the area as much as possible. Its cement-like hardness has defied
removal with brushes, scalpels, probes etc. I hesitate to use the
wet cleaning methods that I know, as any low pH solutions that would
soften/remove the mud layer would also attack the fabric of the
plaster itself.

Does anyone have any suggestions? Any advice as to preparations
and/or methods? Any or all suggestions will be very greatly
received,

Robyn Luhrs
Field Conservator, Theban Tomb Project
Luxor, Egypt


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                  Conservation DistList Instance 13:41
                Distributed: Thursday, January 27, 2000
                       Message Id: cdl-13-41-016
                                  ***