Fill materials for stone

Christa Gerdwilker <christa.gerdwilker<-at->scotland<.>gsi<.>gov<.>uk> writes

>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).

>From your description of the colour, smell, density and
decomposition products, I'm sure you're right in thinking that the
main component of this mixture is pyrite, FeS2.  I am not aware of
any conservation material based on pyrite, but I did come across US
Patent 343198, "Composition for making artificial stone", granted to
Herman Pools of Buffalo NY on 8 June 1886.  This composition
consists of 1 part sulfur, 1 part pyrite and 1 1/2 parts ground
stone; the sulfur is melted, the pyrite is stirred in, and then the
ground stone is added.  The mass can then be moulded into any shape,
and becomes hard and durable when cool. See:

    <URL:http://www.google.com/patents/US343198>

It seems possible that this mixture has been used as a stone repair
material in this case.  Try heating a small piece and see whether it
softens.

Barry Knight
St Albans
England


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                  Conservation DistList Instance 28:34
                 Distributed: Tuesday, February 3, 2015
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