Clearing snow and ice from marble steps

John Hughes <john.hughes [at] uws__ac__uk> writes

>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?

I read Dr Hughes' comments and while I agree that the porosity of
marble is low, that is only part of the story. Much of this low
porosity is accounted for by small pore size. Stone that has large
diameter pores is affected less by salt than stone with narrow
pores. The finer the pores the sooner they restrict crystal growth.
The pores of marble are microscopic and, although they may well be
fewer in number than you'd find in the average limestone, any salt
introduced would begin to act sooner. Bear in mind that pore walls
do not stop crystal growth and the pressure exerted actually
promotes it. Go back and read Grace Wever's remarks on marble
flooring in cruise ships and granite boulders in the vicinity of the
great Salt Lake in Utah.

Another point I'd mention is that the low porosity quoted is for
freshly quarried stone and doesn't take account of weathering
processes; weathering increases porosity. In the early years the
increase would be relatively slow but the processes acting on the
marble will have a combined effect. And the weathering of this
marble will be more severe than for anywhere else on the building;
soles of shoes will scratch surfaces and these scratches will let in
rain.  Rain becomes weakly acidic--by dissolving atmospheric carbon
dioxide -- but if there are industrial pollutants in the atmosphere,
it will be more strongly acidic. Even weakly acidic rain will etch
these scratches and--more minutely--grain boundaries.
Heating/cooling cycles can also open grain boundaries by thermal
expansion. By itself, this degree of weathering will take many
decades to have an effect but if you apply salt--the most damaging
agent of decay for porous stone--the rate of breakdown would be
greatly speeded up.

Eric Miller


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                  Conservation DistList Instance 23:37
                  Distributed: Monday, March 29, 2010
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