Clearing snow and ice from marble steps

Eric Miller comments that marble is porous. Well, yes, but it is a
matter of degree. It is not, relatively speaking, very porous. Being
a mostly granoblastic product of metamorphism, the typical grain
structure is highly interlocked, eliminating almost all porosity. A
typical quote for porosity levels in granite is between 0-2-2%,
though this is unsupported by direct measurement as far as I could
see from a very brief Internet search, except for one source from
Slovenia (Sarpun et al 2009) that quotes measurements on 8 stones,
values given between 0-2-0.5. They did choose the samples for this
range however. Does anyone have better values?

Work by Malaga-Starzec in Sweden relates thermal stress in marble
used in buildings to an increase in porosity over time, mainly due
to intergranular 'weakening'. Of course the porosity of the stone
and its behaviour in response to stress will be determined by the
composition too and how isotropic it is.  The relatively low
porosity of marble would suggest that the use of salt on steps would
not be problematical in the same way as for other materials from
sandstone to concrete.  I'd be more worried about the spread of the
salt to other areas (as K. Dotter pointed out correctly), and
anything that increased the dissolution of carbonate, rather than a
potential source of physical weathering.

Dr. John Hughes
Research Lecturer
School of Engineering
University of the West of Scotland
Paisley Campus
PA1 2BE
Scotland
+44 141 848 3268


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                  Conservation DistList Instance 23:36
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