Clearing snow and ice from marble steps

On behalf of Christopher Gray <methistory [at] aol__com>, Steve
Stokowski writes

>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 cross posted Barbara's query on Bullamanka-Pinheads, a listserv
>oriented towards architectural conservation, and Steve Stokowski
><crushstone [at] aol__com made the piquant remarks below (reproduced with
>permission)
>
>   "Apparently she doesn't know as much as she thinks she  knows.
>    Just why shouldn't salt be used on marble steps? No good reason
>    at all, she may have half read and retained less a newspaper
>    article about salt use on roads. The issue with salt, and I am
>    specifically talking about NaCl, is  that it increases the rate
>    of corrosion of steel, specifically steel embedded  in concrete.
>...
>   "So, unless the marble steps are pinned together with ferrous
>    pins, she should go ahead and use NaCl to deice the
>    steps ...

I do not agree entirely with the advice of Mr Stokowski, both in
terms of the impact of halite on marble, and in other comments made
about steel and concrete. I do agree with the general sentiment that
halite does not necessarily cause damage to the degree that people
assume, however I would not then suggest that it is not a serious
threat or that it is less damaging than other salts. Sound
conservation practice demands that we remove all risks, rather than
advise that the problem is not as great as we think it is. There is
ample evidence, probably millions of examples, where halite can be
directly linked to damage, and to suggest that there are other salts
that may be more damaging is irresponsible comment. Certainly the
repeated application of halite and its absorption into the marble
will inevitably lead to impact. This impact will be active during
the thaw months. It is the conservator's role to advise on
completely safe approaches, such as the second advice that ethanol
may be the solution. Warming is another, as is covering when not in
use. I am not proposing these as solutions but each is infinitely
less threatening than the notion that halite is not as bad as some
salts the commentator can name, therefore get over it and get a
life.

The commentator also states that halite "increases the rate of
corrosion of steel, specifically steel in concrete". This may be a
semantic error but is simply not true. Halite will promote the
corrosion of steel in any surroundings provided the pH of the
surroundings are favourable. Fresh concrete inhibits this and it is
only fully carbonatated concrete, where the pH has dropped to around
8, that will no longer inhibit corrosion. Mild steel in concrete
will not corrode faster necessarily than it will embedded into any
other material, although this is a bold statement that can easily be
contradicted too. Having said all of that the relationship to
halite, steel and concrete has little bearing on the impact of
halite on marble.

So my advice is to retain the caution expressed about the
undesirability of halite as a de-icing approach. This is sound
conservation practice and what we are trained to do. The suggestion
that you have half read a newspaper article is somewhat
condescending and I would advise Mr Stokowski to read the other half
of the technical literature on halite damage to marble.

Andrew Thorn
ARTCARE
614-1926-9326
613-9326-9326


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                  Conservation DistList Instance 23:33
                 Distributed: Wednesday, March 3, 2010
                       Message Id: cdl-23-33-004
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