When most people think of ancient Greek statues, they think of flawless figures carved from white marble. But the Greeks didn't sculpt in marble—they used bronze. Why do we associate marble with classical Greek art? Because the Romans were some of history's best copycats.
Roman Copy of the Greek Original
Many Greek statues in museums are actually Roman copies of the Greek originals. When the Romans conquered Greece, they went crazy for Greek culture. Wealthy Romans wanted copies of famous works of art. In Greece, artists made plaster casts of the Greek bronzes. Then they shipped the casts to studios around the Roman Empire. Roman sculptors carefully measured the casts and carved faithful reproductions, congruent to the original sculptures.
Well, not completely congruent, in fact. Marble is a heavier material than bronze, and many of the copied statues couldn't support their own weight. Roman sculptors had to add supports to their copies without ruining the look of the statues. That's why Roman copies often have strategically placed columns and tree trunks that weren't a part of the original sculptures.
Today, the Roman copies are often all that remain of the Greek originals. As time passed, invaders melted down the Greek bronzes to make weapons and tools. Meanwhile, the marbles endured. We can look at these practically congruent copies to get an idea of what the original bronze sculptures looked like. And we can understand why the Romans were so crazy about copying Greek art!
See for yourself: http://www.youtube.com/watch?v=88gXWW3qN7o
Check out the following links to learn more about the art of ancient Greece and Rome.