As an example, Giambologna was a prolific artist and the sculptures that we know for sure as made during his lifetime are now exhibited in the Bargello Museum in Florence. Still, Giambologna required the help of founders, notably Antonio Susini who was succeeded by his nephew Francesco.
Antonio Susini, who died at a respectable age in 1624, worked for Giambologna but also produced numerous copies after his works. Today, when falls across a Giambologna piece, it remains quite hard piece to ascertain whether it was cast before or after the death of that artist or whether it was made around the 1650s or even later.
Giambologna's bronzes were so popular that not only the Susinis copied them in Europe but once again, when well chased bronzes with great patinas appear on the market they usually fetch tremendous prices even though these were not produced by the artist himself. Most sculptures made before the 19th Century, which art historians consider as authentic are in fact marble, terracotta or wax pieces while bronze works were mostly copies made from plaster models designed by the artists themselves.