Bronze is a metal made by mixing tin and copper to create alloys. Some of the many alloys include aluminium and manganese. Bronze has been the traditional metal for cast art sculpture since it is easy to melt and pour into a mould. Famous sculptors Rodin, Donatello, Cellini, Ghiberti and de Vries all worked in bronze. Sometimes this metal is difficult to spot, however, and testing is required to detect bronze.
Do a visual check of the lustre on the piece. Metals are usually shiny and non-metals have a dull appearance. Older bronze pieces, if left uncleaned, develop a "patina" (warm, rich coating) on the surface. The patina may look rich brown or a warm bluish-green. To see a patina, take a trip to a museum or an art gallery where bronze statuary is on display to view the various shades of patina.
Look for oxidisation. Bronze does not oxidise. There should not be white, crusty film or white dust on any part of the piece. These are sure signs that the piece is not bronze.
Lift the piece. Bronze is heavy, even when cast with a hollow centre.
Look for rust. Bronze does not rust. If rust is present, the metal is something other than bronze.
Do a magnet test. Since magnets are attracted to ferrous (magnetic) metals such as cast iron, place a magnet near the surface. If it sticks, bronze can be ruled out. If nothing happens, it may be bronze. Continue with the other tests to make sure.
Put something hot on one part of the piece and feel a section near the heat source. Bronze will conduct heat rather quickly.
Select an inconspicuous spot on the piece and scrape lightly to determine the colour of the original metal. The bottom of the piece, near the centre, is usually a good test place. Don't scrape a large section; a tiny area will be enough to discover the original colour. Cast iron is a dull cement colour and bronze looks more copper-coloured. New brass is shiny bright, without any hint of the copper.
Take the piece to be chemically tested. This test usually consists of doing a chemical spot test that should not damage the bottom of the piece. It will detect the presence of zinc in the metal. If test results show zinc, this means the piece is a type of brass. There are several types of brass and each one has distinct percentages of zinc that are easy to identify.
Drill a "plug" from the piece or take the piece to an expert to have the item definitively classified. This will damage the piece, so do the other tests first. If it is essential to identify the metal, drill a small hole in the piece. Observe the metal as it is drilled. Bronze pieces drop from the piece in string-type sections, while brass will drop in flakes.
- Use a plastic magnet when doing the test. There is less chance that the piece will be damaged with the plastic than using a metal magnet.