system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do we use bronze on buildings and statues if they're only going to turn green? Do the creators know the work will turn green or is it a sign of a lack of upkeep?

For example, in Philly there's a building with green roofing because the roof is bronze. Did the architect know his work would change color or what? Thanks!

EDIT: Copper.  Not Bronze.
gpt
It is a choice made in the design of a building...

Originally it had to do with longevity and reliability of the materials - if you look at steel, the rust that forms when it is left exposed is detrimental - as it rusts the material degrades and gets thinner and weaker until it fails. So steel needs maintenance and protection to last long term (such as bridges being painted, or having steel encased in something like concrete to protect it). The green of bronze however is actually helpful - initially it is nice and shiny, then degrades to the green colour we commonly see, at which point that green layer has created a barrier around the metal, so it doesn't degrade any further. So once it reaches that point it needs relatively little maintenance and upkeep - great for a feature dome at the top of a roof you don't really want to have to access regularly.

It is also worth remembering that most of these structures are traditional ones, built when technology and materials were far more limited, so the green roof was a small price to pay to be able to actually build the structure that may not have been possible with other materials.

Nowadays it mainly comes down to architectural reasons - it will be used to give a certain look and design to a structure that the architect desires.