In my travels to Europe I have encountered occasions in which some of what I came to see was covered up by scaffolds and barriers erected for the restoration and preservation of the artifact. I could not view the medieval stained glass at Salisbury and Chartres due to this. In Rome temple façades and even entire temples were covered. In Venice part of the façade of San Marcos Basilica was covered.
Since ancient times buildings have been subjected to natural damage. Earthquakes have collapsed some of them. Parts of the Coliseum in Rome have collapsed in earthquakes. Lightning strikes have damaged buildings. Some, like much of ancient Alexandria are now under water. Fires, a regular occurrence in ancient Rome, have destroyed buildings and art. From earliest times, armies have leveled ancient cities. These include Troy, Carthage and Corinth.
Then, as now, ‘urban renewal’ leveled parts of cities, with new buildings erected on the ruble of older buildings. Not infrequently the good building materials of old buildings was stripped and used in new buildings. The exterior surface of the pyramids and Coliseum were stripped and the underlying stone, brick and concrete exposed.
For centuries ancient site have been robbed and the art removed or, in the case of gold, silver and bronze artifacts melted down for their metal. The Romans regularly raided Greek and Asian sites for their artwork. For centuries beginning in the Renaissance artwork from ancient Greece and Rome were stolen and placed in private royal collections and museums. I have never been to Greece before, so I have never seen the Parthenon. However I have seen the frieze that ran around it. That is in the British Museum in London. The damage to the Parthenon is partly due to the natural elements, but the major structural damage was caused when the Ottoman Turks used it for an ammunition dump and the Venetians shelled it. Good thinking guys.
Also on the Acropolis is the Erchtheion, famous for the Porch of the Caryatids. There are six large statues serving as pillars upholding the roof. The statues there today are copies. Five of the originals are in the Acropolis Museum; the sixth is in the British Museum in London. The Greeks want it back.
In some respects the removal of the original stonework, such as the friezes of the Parthenon and the Temple of Poseidon in Paestum, and major artworks to museums, such as from Herculaneum and Pompeii to the Museum in Naples and from Rome to the Vatican Museum, are good because they are safer from the corrosive effects of the modern industrial atmosphere. Industrial emissions and the exhaust of internal combustion engines is bad for the health not only of humans but buildings and art. Much of the restoration and preservation work seen in England, France, Germany, Italy and Greece are to address the deterioration caused by petroleum fuels and to prevent further damage.