There is an old masonry proverb that says, "Stone is simply a way-stop for sand in its progress back to the sea."
While we are inclined to think of stone as a durable, even impervious, material, the geological forces that shape it mainly water and pressure ultimately determine its destination. Also, while you and I may not be around to see the entire life of a stone unfold, we will certainly notice the effects of the elements on it in the lifetime of our home or garden. In as little as a year into the life of a stone structure, you may begin to see any number of changes in the stone, called weathering.
Weathering refers to a variety of effects, ranging from destructive to desirable. Some of these effects will change only the appearance of the stone, while others may change its structural properties ‹ in the worst case causing it to destabilize. The factors that cause many of these effects are present in any environment and can never be entirely mitigated. Inevitably, your stone is going to change over time, and you should understand this as a part of its nature.
What follows is a rough guide to some of the major factors in stone weathering, as well as an explanation of how these factors will affect the appearance of the stone.
One of the most common corrosive substances around is perhaps the most common cause of weathering on stone, especially porous stone. Impermeable stones will show effects of salt crystallization only under special circumstances.
In general, salt crystallization is indicated by the formation of efflorescence, a visible growth or film on the surface of the stone that is usually light in color. The film later dries to form a powder, which will flake off or can be washed off, often taking very small amounts of the stone with it. In regions close to the sea, crystallization effects can be dramatic, going far deeper than the surface of the stone and eventually leading to massive decay of the stonework. The simple solution is a soft brush, water, and some elbow grease. Most efflorescence can and should be removed as part of regular maintenance.
Much of the visible weathering of stonework in industrial areas can be attributed to air pollutants and acid rain. In extreme cases, buildings can be blackened by tarry build-up and the stone can absorb the stain deep into its pores, making cleaning difficult. In residential locations, the effects will be less visible. In areas regularly washed by rain, only a slight darkening over time will be noticed. In some stones, this washing process will actually deteriorate the outer layer of the stone, exposing fresh stone and making it appear new again, though it is at the same time slowly decaying the stone.
As you might expect, the effects of air pollutants are more pronounced on lighter than on darker stones. Limestone and marble can be impacted in areas that have significant acid rain conditions, evidenced by pitting and degrading of honed and 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.
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.
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.
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.
On pavers and steps, one of the most noticeable weathering effects is human traffic. Walking on stone surfaces can scratch the surface when hard materials, such as pebbles in the sole of a shoe, are dragged over it. In outdoor settings these scratches are less noticeable, while in delicate interiors they may be unsightly.
The second main effect of human traffic is the honing, or polish, that feet and wheels leave behind on stone. The process of walking or driving on stone hones the surface, making it less porous and harder at the surface, and causing a waxy, mellow shine called a patina. Though the process that brings about this patina is similar to polishing a stone, nothing but time can duplicate it. Honing involves the deposit of years of dirt and other matter along with the simultaneous progression of other weathering effects. Like all weathering effects, the formation of a patina also indicates the natural aging of the stone.
Weathering is the sign-post of the stone's progression back to sand. A stone's path to that destination is, as I've stated, varied. How you live with weathering has as much to do with its final effect on your stonework as does its cause. Stone is, like us, the temporary evidence of processes greater than itself. By selecting the proper stone, building it in a thoughtful and appropriate way, and maintaining it over the course of your lifetime, you can enjoy the natural process, along with the stone itself.
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