The ancient Greeks did some marvellous bronze sculptures. But that was long ago and bronze is sadly mortal1; we have virtually none 2of them left. What we do have, and have in abundance, are Roman reproductions in marble.
The problem is that marble (as you know) is not bronze. It doesn't have the same combination of strength and lightness. This is particularly the case with statues, since a bronze statue is hollow and a marble one is solid. The outflung limbs and extended flourishes that look so good in metal will eventually break off of stone. Think Venus de Milo. It's even more of a problem if the statue's legs are bent or far apart...one breakage can send your whole object d'art tumbling down.
To compensate for this, Roman sculptors added struts and supports when they copied the Greek bronzes. Sometimes it was just a beam of marble from a torso to an extended arm, or connecting a body and a piece of drapery.3 A more subtle technique for figures with weak legs was to add a convenient tree stump behind the weight-bearing leg.4 These were definitely additions, not stylistic mannerisms. Original Roman marble statues of the same period show no such supports.
Now that you know, you'll see them everywhere5. And you can point cooly at the Greek god leaning coolly against a knotted tree stump, or braced by a tidy marble strut, and say glibly, "That's a lovely marble copy of a bronze original.6"
- As cbustapeck points out, bronze is also easily recyclable into, for instance, weapons. Marble is rarely re-used once it's been carven into a statue, unless it's turned into fertiliser like most of the Forum Romanum.
- The only one I know of (thanks, The Debutante) is the Riace Warrior. See http://www.people.auckland.ac.nz/Frances/Classical%20Art/Classical%20Free-Stand.%20Sculpture/Riace%20Warrior%20A1.JPG
- A good example of this is the Doryphorus (spear-bearer) by Polykleitos, where the figure's right hand is attached to its thigh by a marble bar. See http://www.uoregon.edu/~arthist/arthist_204/monumentimages/doryphorous.gif for a picture. (Thanks to WolfDave for the link. My original one went 404)
- The most famous statue where this was used is probably the Diskobolos (discus-thrower) by Myron of Eleutherae. See http://www.the-artfile.com/uk/history/greek/discusthrower.htm
- Well, relatively everwhere, if you hang out in museums a lot.
- I once came across a bronze statue in Amsterdam, of a figure (can't remember who) supported by one of those convenient tree stumps. It was a bronze copy of a marble copy of a bronze original.