Hi.
"Firmly established by the 17th centry, casting has existed in other forms since the 1300s."
What does established means?Formalized?
Validate(d), or simply confirm(ed).
I think the meaning is probably different here- the method used for casting had become accepted and was widely used.
castling?
Context is always important; labelling is rarely important.
Still the same answer. Accepted, confirmed as valid, widely used.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.