How do you get bronze ore in RuneScape?

You take the metal ores - in the case of bronze, you need 1 copper + 1 tin for every bronze bar - to a furnace. Alternately, you can use the "Superheat Item" magic spell. This requires Magic level 43, and costs a Nature Rune each. Because of the cost of the Nature Rune, it isn't worth-while for most low-level players.