I've seen "Está hecho de ..." used to mean "It's made of ...".
Why is the verb estar and not ser? Isn't this an adjective that's permanent and not going to change?
I can understand phrases like "la puerta está abierta" because that's a temporary thing, but what something's made of isn't going to change. This seems especially odd because I know that "Es de ..." can also mean "It's made of ..."
So, why is estar used in this phrase? Is there a rule that I should know for when to use estar with an adjective and when to use ser?