There is a very comprehensive Wikipedia article on the Elgin marbles (see related link)
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The Elgin marbles were made for the Parthenon in Greece
The controversy derives from the different Greek and British views as to who should own the Elgin Marbles. From the Greek perspective, the marbles used to be part of the Parthenon in Athens, Greece and Greece maintains they should be returned there since they are a part of Greek cultural heritage. Britain says Lord Elgin was allowed to remove these from the Acropolis by the Ottoman Turkish government that was in power at the time that Elgin made the request. Therefore, the British legally removed them, even though the Greeks at that time were not consulted. Also these marble reliefs would have been destroyed by now if they had been left in their original place because of the various wars that were fought in Athens since Elgin took the marbles out of Greece. So Britain has preserved the Elgin marbles for 200 years and ought to keep them.
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Under the instruction of Lord Duveen, the Elgin Marbles undergo regular cleaning. This 'cleaning' maybe the cause of the damage as it is under a great deal of controversy. It may be that the tools they use to clean them with are the cause of the damage.
They are reliefs cut out of blocks of marble in the 430s BC.