Salt weathering is a chemical process that happens to stone structures over time. Often, this will not harm the structural integrity of building, but it will mar its appearance.
If you’re unfamiliar with salt weathering, it might be easy to compare it to erosion – but the truth is that the two are quite distinct. Erosion occurs when some natural agent, such as water, ice or wind, moves past the stone object, changing its shape or position over time. Extreme examples of erosion would be the Grand Canyon or the stone structures in Monument Valley. Weathering, on the other hand, involves no movement at all. As the name suggests, the damage typically occurs as the result of weather events over a long period of time.
Salt weathering in particular is the result of naturally created salts having a reaction with the chemical makeup of the stone structure. More on that further down. First, it’s important to know where the salt comes from and how it reacts with stone.
No salt shakers required
Although some salt weather is indeed caused by same kind of salt you might find on the dinner table, there are actually a number of different chemicals that can be considered a salt. That might sound complicated, but for the layman, a salt can be thought of as a crystalline chemical formed from the reaction of elements – it is in part the structure of salts that cause weathering damage.
An article featured in the Quarterly Journal of Engineering Geology and Hydrogeology reported that salt weathering has been a documented problem in desert regions of the U.S. since at least the early 20th century. Of course the chemical process itself has been around forever, but it is only within the past 100 years that people have aimed to prevent or halt its effects.
So where does the salt come from? The primary sources of the salt are the oceans, contaminated groundwater and the building stones themselves. That last source might seem confusing – to understand, think back to the concept of erosion. As stones such as granite, a common building material, are eroded by a mechanical source such as the wind, they break down into various chemical components. A block of granite isn’t just made of a single “granite-chemical” but rather dozens of other chemicals, some of which are salts. Therefore, when the mechanical erosion occurs, these salts could become separated and cause weathering damage.
Some more quick chemistry
Another common chemical reaction that plagues facility managers of old buildings is the oxidization of copper. Where there was once shiny, penny-colored copperwork, there now sits a green material that in no way resembled the former luster of the piece. Perhaps the greatest example of copper oxidation is the color of the Statue of Liberty. When the giant gift from the French people was first put in place, its color resembled that of a brass pot. Over time, the copper absorbs and binds with oxygen molecules, turning the metal blue-green.
In the case of the Statue of Liberty, this color change was intended. However, like salt weathering, the same effect on older commercial buildings was not. Luckily, fixing either problem shouldn’t require a degree in chemistry.
“Sealing exposed cracks is a key way to prevent salt weathering.”
Preventing harmful chemical reactions
One key way of preventing salt weathering from the start is to keep moisture away from the building. In humid environments, this is especially important. Managers with an older building under their control may want to consider sealing the outside of their building to prevent moisture from keeping into exposed cracks and crevices. Broken stones may need to be replaced, but often a professional can repair them or significantly delay replacement. Regular cleaning of the building is another way to stave off the effects of salt weathering.
Seeping ground water should be dealt with as soon as possible. It may have come in contact with salts underground and transfer them to the building. If a building becomes covered in part with ground water, getting it cleaned soon is very important.
But what about buildings that already have extensive salt weathering damage? In those cases the salt is visible in crust-like formations. Because scraping or chiseling may damage the underlying stone, other methods must be employed. Desalination is one method. Another is to use a highly absorbent poultice to draw the salt off the building. The second method is time consuming, but quite gentle on the stone.
As for copper oxidation, the repair method is usually a simple matter of polishing and applying a clear coat to keep the elements away. Preventing oxidation and salt weathering are two indispensable methods of keeping old buildings looking like new for the years, decades and centuries to come.