There are several circumstances when a metal sculpture owner may need to determine exactly what material a piece is made of. For example, perhaps they need to write a description of the item to sell or insure. In other cases, a piece has been damaged, and when inquiring about getting it repaired, the restorer/conservator needs to know the material in order to give a quick estimate.
First of all, look at the surface of the piece. If exposed to the weather without proper finishing and waxing, the piece may show corrosion. Corrosion will have a different appearance on different metals. Secondly, if a part is broken from the sculpture, this is an opportunity to look at the unfinished, raw material, which can be very helpful. There are a couple of other tips for different metals, which I will discuss under each topic.
Most of the older European bronzes were poured in what is called “red brass,” which is 85% copper, 5% tin, 5% lead, and 5% zinc. This combination of metals was used to improve the performance of the metal during the pouring, finishing, and patination process. A corroded bronze sculpture will have a slight greenish cast and possibly a patchwork of irregular black and green areas.
At a break, the metal will have a slightly reddish, golden quality to it. When you try to scratch the inner part with a sharp object, it may leave a slight scratch but will be somewhat hard and will not leave a burr on the edges of the cut.
A very common metal for the less expensive sculptures but can still be highly prized. The casting process requires a reduction in heat as the melting temperature is lower. Zinc is a metallic element and is normally a gray-silver color. Corrosion on zinc will be white. Many of the fine zinc pieces are copper plated, then patinated and sealed. Over time, both the patina and plating can become worn and disappear altogether.
Very few sculptures are made from pure gold, with the exception of many Egyptian and pre-Columbian pieces. More widespread use of gold is to plate bronze sculptures with a coating of gold. Gold does not tarnish with time, but it can look discolored from dust and grime if not kept clean.
There are a few sculptures out there made with pure silver, but a more common use is to plate bronze or zinc sculptures. Just like silver jewelry or tableware, it will tarnish to black when not kept polished.
Place a small magnet near the surface of your piece. If it attracts, then your piece is probably cast iron.
Obviously, it would be most useful to have sculptures of different materials to compare with one another to illustrate the differences described in this article. If you are so inclined and have the time and interest, I recommend visiting an art museum or gallery to compare what you see with your piece and perhaps chat with the conservator or expert at the facility. In the end, you may still require to contact a professional to help you with the process. However, with a little observation and investigation on your own using the above information, you will at least be well informed and ready to discuss the conditions in a way that will be useful and helpful.