Non-aqueous mortar

Re: Ellen Pearlstein's request regarding fill for Egyptian salty
limestone

When gap-filling we always apply a protective barrier layer first.
This helps to isolate the object from the gap-fill material and
facilitates any later removal of the gap-fill. Our usual choice for
the barrier layer is a viscous solution of Paraloid B72 in acetone.
This is applied quite thickly and allowed to dry thoroughly.

Suitable gap-fills for Egyptian limestone have included:

    Interior Grade Polyfilla (the dry powder, not the premixed
    paste)

    Polyester resin mixed with vermiculite (expanded mica) for large
    gaps which require light weight fill

    High-fired bricks with white cement for gaps which required
    structural support.

When Polyfilla alone is used, it is mixed with the cleanest water
available (in Egypt the "distilled" water used for car batteries) to
the required consistency. Unlike plaster it responds well to being
mixed to a paste or even a stiff dough. It can be smoothed to the
approximate shape whilst soft and hardens more slowly than plaster.
It can be filed, sanded or carved when hard, then painted with
acrylic paints. It is even possible to burnish it.

When the coarser gap-fills are required (especially when gap-filling
monumental statuary or masonry), they are applied to within an inch
or two of the surface. After hardening they are then covered with
Polyfilla to provide a suitable outer surface.

A word of warning: in Egypt, low-fired bricks and green cement are
both salty and cause great problems. We have used high-fired bricks
and the local white cement, cement abiyad, with good results. For
confirmation, check the duad statue outside the Small Temple at
Medinet Habu for Polyfilla gap-fill and the 18th dynasty chapels
there for brick/cement gap-fill.

We would not recommend using limestone chunks for several reasons, chiefly
because they may contain other salts which can cause violent efflorescence and
 because future conservators will never be able to prove that those chunks
were not associated with the statue and will have to retain them. Local
restorers have often used chunks of stone or pottery which lay beside the
monuments as gap-fill or ground up as a pigment for gap-fill.

Richard and Helena Jaeschke

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                  Conservation DistList Instance 10:28
                Distributed: Tuesday, September 17, 1996
                       Message Id: cdl-10-28-004
                                  ***