Gardner, What is Art History?

Some reflections on the lectures and readings so far:
The heads of the 13th century madonnas by Cimabue and Duccio shown in the Powerpoint illustrations of 29 August might have been xerox copies: both
heads tilted at the same angle, both faces virtually expressionless. By the 15th century, Fra Filippo Lippi and Bellini are drawing figures that are recognizably human, flesh and blood creatures. I'd like to understand better what led these artists to choose such different ways of representing essentially the same devotional subject.
The Head of a warrier (figure 1-17 on page 11 of Gardner) I found interesting as an early example of casting large objects in bronze. Of course bronze had
a military application in those days and thus there was some motivation to develop the art of working it. I was puzzled by the information that such a large object was "welded" together (and presumably still intact). What sort of welding shop would be in operation in 500 BC, without acetylene or arcs?
But this is bronze, of course, and bronze is "welded" by the process of  brazing, a sort of low temperature form of welding, which could presumably be
done by a charcoal flare and bellows. An explanation of the process the Greeks of this period used to cast bronze statues, can be found at this ThinkQuest site