Comment 75196

By GerryM (registered) | Posted March 13, 2012 at 08:59:20

"Black Soiling" has been extensively investigated in Europe, but very little in North America -- I do not know of any publications originating in Canada. The most relevant research for Canada is that done in Scotland, where most of the buildings are constructed from sandstone and (I believe) the term was first used. There is a useful guide produced by the Government on cleaning (pdf)

The whole subject is very complex, because many different factors may contribute to soiling. Our examples are quite like the Scottish one, but the Whirlpool sandstone is a quartz sandstone, cemented by quartz with a very low porosity, so not quite like the Scottish sandstones. In Scotland they have had bad experiences with a whole range of different cleaning techniques, and recommend that cleaning should be avoided altogether!

It is interesting (and unexplained) that most local dolomites are not blackened, though this is common on limestones in Europe. Part of the explanation may be that acid rain erodes the surface of carbonates stones, but does not much affect non-porous sandstones like the Whirlpool. The blackening is caused by soot particles adhering to the surface (followed by a lot of complex chemistry and biology!). But why should limestones blacken, but dolomites do not?

Comment edited by GerryM on 2012-03-13 09:01:30

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