The term "Acid Rain" was first coined in 1872 by the English scientist Robert Angus Smith who discovered that vegetation could suffer damage due to precipitation with acidic content, and that most the acidic rain would fall in cities and towns with industrial developments. But first 100 years later, acid rain was considered an environmental problem that also could cause significant damage to man-made structures, including monuments and statues.
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In chemical terms, acid rain is categorised as such when the precipitation has a pH value of less than 5.6. The chemicals and compounds contained in the rain water have high amounts of nitric and sulphuric acids, which are formed when fossil fuels are burnt and released into the atmosphere. The sources for the dangerous acids are formed by man-made sources including industrial plants, machinery and cars, but also electrical utilities.
Acid rain effects stone materials traditionally used in the shaping of monuments and statues, including marble and limestone. Most stone materials constitute calcium carbonate that is dissolved by the sulphuric acid contained in acid rain. Other components of the acid rain precipitates onto marble and limestone and build up heavy crystals called gypsum. The crystals break off pieces of the statue or monument. In both cases, the stone will erode and destroy the monument. Most of the historic monuments and statues in the U.S. are built of marble and limestone, including the Capitol in Washington D.C..
Monuments and statues also can be produced in various metals, including bronze, copper, iron or steel which all can deteriorate through the effects of acid rain. When acid comes into contact with metal, an electrochemical reaction occurs where the metal loses electrons and transgresses into a different form where it becomes soluble. This is made visible through corrosion, or rust. The more exposed a statue is to acid rain, the bigger the damage to the surface as the rust will eat its way through the metal and break off pieces.
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 countertops. 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 sulphur 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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