Fill materials for stone

I am trying to identify a dark grey filler used on a fragmented
sandstone.  The date of the repair is unknown but likely to be
mid-20th century.  The filler material is visually very similar to
neat cement but has a distinct metallic smell and is quite crumbly,
heavy and frequently coated by a white efflorescence.  The fill
appears to have stained the adjacent blond sandstone a sulphur
yellow like colour surrounded by a rusty tidemark.

X-ray diffraction analysis of the dense grey fill material
identified quartz, pyrite and rozenite (hydrated iron sulphate).
There was no indication of any mineral cement, suggesting that there
is some type of organic binder.  The absence of fluorescence under
UV light would seem to rule out a natural resin and, so far,
solubility tests have been unsuccessful.  Examination of
efflorescences and decay products in the surrounding stone found
mainly rozenite along with other iron sulphates including
melanterite, copiapite and jarosite.  Some gypsum was also present.

Has anybody come across a similar filler material?  Do you know what
this might be?

Christa Gerdwilker
Stone Conservator
Historic Scotland


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                  Conservation DistList Instance 28:33
                 Distributed: Friday, January 30, 2015
                       Message Id: cdl-28-33-022
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