Masonry Magazine August 2005 Page. 24
NATURAL STONE
polished finishes. In general, the effects will be more pronounced on areas that are seldom touched by rain or water run-off, such as the area directly below a crown molding, resulting in uneven discoloration of the surface.
There is a great deal of research, as well as debate, on the use of chemical sealers and coatings to combat the corrosive effects of air- and water-born pollutants. Completing research on a case-by-case basis on possible long-term solutions is warranted.
Freezing
MORE OF A CONCERN in areas with heavy freezes during the winter, freezing can have surprising effects on stone. These effects occur only when stone is frozen while wet. This is most likely to occur in parts of the structure where water can accumulate without running or drying, such as the tops of stone steps.
When water is cooled along the surface of the stone, crystallization can cause parts of the stone, usually in flakes, to break off and wash away when the ice melts, known in the industry as spalling. The detached piece is often thin, but the shape can be dramatic and noticeable, as the newly exposed stone beneath can be of a different color than the remaining stone.
In rare cases, a stone can crack all the way through during the thawing process, rendering it unstable. The best cure for this condition is selecting the proper stone, and paying close attention to proper installation details and techniques that assure water can drain out of and away from the stone.
Plant Growth
THOSE IN THE NORTH are probably familiar with the sight of ivy climbing a stone or brick wall. If left unchecked, climbing plants such as ivy can root in the joints of stonework and cause structural problems. If they stay rooted in the ground, they are mainly harmless and can be cleared off easily.
Stone is, like us, the temporary evidence of processes greater than itself.
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Other types of growth, like algae and lichen, can have more permanent effects on the appearance of your stone. Because algae and lichen do not require soil for nourishment, they can spread over the entire surface of the stone, forming large patches. This effect can be pleasing in a garden, unless the growth becomes a magnet for dirt, which may cause the area to appear black and dingy.
In the short run, plants and organic growth may not seriously damage stone; however, restoration of stone structures involves careful cleaning and removal of both organics and pollutants. Personally, I seek a balance in what I build in stone. In the garden I use thick and massive stone that resists the effects of surface growth, so owners can enjoy the beauty of moss and lichen on the garden walls. On buildings I am careful to make sure that climbing plants do not have aggressive tendril growth that can damage the stone veneer over time, and I recommend cleaning procedures in two- to five-year intervals as required by the stone selected.
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