MadSci Network: Environment |
The important thing here is the material from which the outside artworks are made. Many of the world's great outdoor artworks are made of bronze or copper. Acids react with many metals, and will gradually etch them away. When copper or bronze weathers in normal outdoor conditions a grey-green patina of basic copper salts forms on the surface (verdigris). This gives some protection, preventing further erosion of the underlying metal. But acid rain can dissolve some of the protective patina and allow deeper corrosion. Even so, bronze and copper artifacts stand up to acid rain fairly well. With marble, it is a much sadder story. Many of the world's most beautiful and famous buildings and outdoor artworks are made of marble: the buildings of the Acropolis in Athens (Greece), the cathedral and leaning tower in Pisa (Italy), and many of the world's outdoor sculptures. Marble has the chemical composition of calcium carbonate, and all carbonates are quite readily attacked by acids, producing soluble salts and carbon dioxide. Marble erodes very badly in acid rain. Athens, in particular, has a serious acid rain problem. There are several relief sculptures on friezes of the buildings of the Acropolis, dating back around 2500 years. Most of the detail has been completely etched away. The surfaces of the columns of the buildings are deeply pitted as well. But some of these sculptures were taken to England in 1806 by Lord Elgin, and housed indoors (The Elgin Marbles). The fine detail of these sculptures has been well preserved, and can be seen today. They are a matter of considerable controversy: Lord Elgin's action can be seen as anything from a high-handed act of cultural arrogance to an enlightened and necessary step to prevent the destruction of the sculptures by the then Turkish rulers of Greece. (The danger from the Turks was real: 1. A bit of vandalism to their most precious cultural icons helps keep a subjugated people in their place. 2. The marbles contained realistic and lifelike images, and pagan ones at that: definitely unacceptable to the Moslem culture of the Turkish rulers. Many of Britain's own artworks had much earlier been destroyed for very similar reasons in a rampage of destruction of images in the churches to try to break down any residual attachments to the Roman church). There is probably some truth at both ends of this spectrum! For several decades, the Greeks have been arguing that now that they are in charge of their own country, the sculptures should be returned to them. It is hard to see the British refusal as anything but cultural arrogance. Nevertheless, if the marbles had been returned to their original outdoor setting in Athens, it is almost certain that these sculptures would have been irreparably damaged by now. Some of the world's great outdoor artworks are made of other materials: Wood is not particularly affected by acid rain, but does not weather particularly well anyway. Granite and obsidian (volcanic glass), sandstone and quartzite, are unaffected by acid rain. An interesting case is gold. Many of the spectacular buildings of Eastern Europe and Western Asia have domes covered with gold leaf. The gold leaf on the dome of the cathedral in Sofia, Bulgaria, is being damaged by acid rain. But acids certainly do not react with gold! I think that what is happening is that acid rain is getting in through flaws in the leaf cover to the underlying dome material, and corroding that. This affects the adhesion of the gold leaf, causing it to lift, making a bigger flaw for the acid rain to get in, and so on in a vicious cycle.
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