Demo One Why Patina on Bronze
is a good thing!
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Patina is a film on the surface of bronze or similar metals (produced by
oxidation over a long period); a sheen on wooden furniture produced by
age, wear, and polishing; or any such acquired change of a surface
through age and exposure.  On metal, patina is a coating of various
chemical compounds such as oxides or carbonates formed on the surface
during exposure to the elements (weathering).  Patina also refers to
accumulated changes in surface texture and colour that result from
normal use of an object such as a coin or a piece of furniture over time

Patinas are restricted to exposed surfaces and are fragile (that is, they
can flake off).  One reason bronze is so highly valued in statuary is that
its patina protects or passivates it against further corrosion. This natural
patina is solid and seldom shows a tendency to flake.  Brass is also
resistant to corrosion, but it is, in the long run, not as attractive since
local pitting shows against the shiny background.

The green patina that forms naturally on copper and bronze is often
mis-named verdigris and usually consists of a mixture of chlorides,
sulphides and carbonates.  Copper carbonate or copper chloride.  
Atacamite is another name for the patina compounds. Verdigris can be
produced on copper by addition of vinegar (acetic acid) - such a
verdigris is water-soluble and will not last on the outside of a building
like a "true" patina.

One example of a patina is a green surface texture created by slow
chemical alteration of copper, producing a basic carbonate.  It can form
on pure copper objects as well as alloys which contain copper, such as
bronze or brass.  The Statue of Liberty gets its green color from the
natural patina formed on its copper surface.

A Patina layer takes many years to develop under natural weathering. A
copper roof will patinate faster than a copper facade, due to the longer
dwell time of water on the surface. Buildings in coastal / marine locations
will weather and develop a patina layer faster than ones in inland areas.
For example, a new copper facade in central London will most likely not
develop a "typical" green patina until after 50 years!
The Statue of Liberty gets
its green color from the
natural patina formed on
its copper surface.
Facade cladding (copper cladding) with
alloys of copper, eg Brass or Bronze, will
weather differently to "pure" copper
cladding. Even a lasting gold colour is
possible with copper-alloy cladding. Look at
Colston Hall in Bristol, or the Novotel at
Paddington Central, London. There you can
see some colours that one might not have
expected from copper / copper-alloy
cladding.
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