MadSci Network: Environment
Query:

Re: What are the effects of acid rain on outside artwork?

Date: Tue Sep 12 19:21:30 2000
Posted By: John Christie, Faculty, School of Chemistry, La Trobe University, Bundoora, Victoria, Australia
Area of science: Environment
ID: 968434683.En
Message:

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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