That's solid bronze plate, AFAIK. Not exactly the easiest thing to get anything through.

Quote Originally Posted by geala
When it went out of use after 1635 it was more a matter of logistics and changes in the strategic demands than its inability to protect its wearer.
A little known fact is that breastplates were in use among shock cavalry into the early 1700s. The fashion was to wear them under the uniform coat so they don't normally show in pictorial sources, but I've seen enough literary references as well as the occasional officer portrait with the uniform open and the armour showing. They apparently dropped out of use after that for a while though, until Frederik the Great of Prussia reintroduced them to a limited degree in the "Napoleonic"-pattern cuirassieurs towards the end of the century.