Clearing snow and ice from marble steps

Barbara Appelbaum <aandh [at] mindspring__com> writes

>A client of ours has asked about the problem of clearing snow and
>ice from marble steps.  She knows that salt should not be used.  Is
>it possible to apply coatings in advance that will mitigate the
>possible effects of salt, or is there something else--ash or sand,
>perhaps--that is not harmful to marble?

I have been following this correspondence and I was a surprised that
it has come to an end with no one pointing out what would actually
happen if you throw sodium chloride on the ice.

Marble is porous. Once in solution, the salt would enter the stone
and remain there. Later on, wet and dry periods would cause it to go
in and out of solution but the most damaging conditions would be the
periods of fluctuating atmospheric humidity, when relative humidity
passes 75%. On rising through this level the salt, which is
hygroscopic, would dissolve and on falling it would crystallize. In
the period before crystallization, evaporation would pull the column
of water in a pore, towards the surface, where the salt it holds
would concentrate. If evaporation is rapid, crystallization would
tend to occur under the surface and the pressure exerted by the
formation of crystals inside the stone would break it down. At
certain times of the year these fluctuations may occur regularly and
the sequence could be repeated daily or at even shorter intervals.

The process is described in many publications but Giovanni Amoroso
and Vasco Fassina give the best explanation in 'Stone Decay and
Conservation' published by Elsevier in 1983.

Eric Miller


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                  Conservation DistList Instance 23:34
                 Distributed: Thursday, March 11, 2010
                       Message Id: cdl-23-34-007
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