Thursday, September 5, 2013

Effect of Acid Rain on Sculptures and Buildings

Acid rain is caused by pollution in the atmosphere bonding with water droplets. The pollution itself is normally sulphur or nitrogen dioxide. These compounds combine with water in the atmosphere, fall as acid rain and enter the general water supply. Initially it is more harmful to plants and animals, but this rain can have devastating effects on older buildings and statues made from certain types of stone.

Statues and sculptures

Older statues, especially those made of marble and limestone, are especially vulnerable to acid rain. This is because the rock contains large amounts of calcium carbonate, which reacts with the acids in the rain. This can cause the stone to turn to gypsum and flake off, ruining the detailing. This effect is cleaned off many statues, but can be regularly seen in graveyards where the headstones show the same erosion effect. Bronze statues are also at risk as the acid oxidizes in the metal causing corrosion.

Buildings

Due to what they are made of, buildings are less affected by acid rain. Older buildings with limestone or marble modeling on the outside can suffer from the same problem as statues. In addition, metalwork within the structure, such as copper pipes and wiring, can oxidize if exposed to acid rain.

The more obvious impacts of acid rain can be seen on particular types of stone, such as limestone and marble buildings, monuments, statues and headstones.  The weathering pits and canyons can obliterate the lettering and features of such structures to a brutal degree, depending on the type of stone and other environmental conditions.  Acid rain can also corrode bronze and other metals, such as nickel, zinc, copper, and carbon-steel as evidenced by streaks and discoloration on bridges and other metal structures, such as many commercial buildings. 

Not all buildings or structures suffer the effects of acid rain.  How big of a threat it is can be determined by the chemical makeup and  interactions of a building's materials.  Limestone and marble, which, historically, were used widely because of their availability and workability by artisans, are especially susceptible because they are composed of calcite, or calcium carbonate, which acidic chemicals can dissolve easily.  To observe this first-hand, drop a piece of blackboard chalk into a glass of vinegar.  Drop another piece of chalk into a glass of water.  The next morning, you’ll see the alarming difference.

Modern buildings tend to use granite, which is composed of silicate minerals, such as quartz and feldspar.  Silicate minerals resist acidic attacks from the atmosphere.  Sandstone, another silica material, is also resistant.  Stainless steel and aluminum tend to hold up better.  But all minerals, including those found in paint and road overlay, are affected, to some degree.
Because of the switchover in the use of certain building materials in the post-Industrial Era, historic buildings, more so than modern ones, tend to show the destructive outcome of acid rain since we first began burning fossil fuels for energy.  London’s Westminster Abbey, the Colosseum in Rome, and India’s Taj Mahal all show signs of degradation brought on by atmospheric nitric and sulfuric acids.

Prevention from deterioration

 One way to reduce damage to monuments and statues by acid rain is to use materials that are known to be acid-resistant. Granite is known for its durability and has been traditionally used for flooring and surfaces that are regularly exposed to acidic substances, including kitchen counter tops. It now is increasingly replacing marble and limestone in the manufacturing of monuments and sculptures. The metal manufacturing industry has developed several new materials that prove to be more acid and corrosion resistant than the traditional products. Other protective measures are acid-resistant coatings and resins that protect stones and metals from the penetration of acid rain. In the meantime, the Environmental Protection Agency (EPA) has developed an Acid Rain Program that aims to reduce sulfur dioxide and nitrogen oxides emissions that cause much of the damage to the monuments and sculptures. The program imposes annual emissions limits on industrial plants that produce most of the harmful substances.

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