Quote:
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Originally Posted by Encyclopedia Brittanica
As the weathered copper ores in given localities were worked out, the harder sulfide ores beneath were mined and smelted. The minerals involved, such as chalcopyrite, a copper-iron sulfide, needed an oxidizing roast to remove sulfur as sulfur dioxide and yield copper oxide. This not only required greater metallurgical skill but also oxidized the intimately associated iron, which, combined with the use of iron oxide fluxes and the stronger reducing conditions produced by improved smelting furnaces, led to higher iron contents in the bronze.
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It seems that there was a convergence between the declining availability and quality of copper/bronze, and the gradually increasing knowledge of iron working, that led to the change. It is just conjecture, but from what I'm finding, it sounds like bronze would not have been supplanted by iron until later in history, had supplies of higher quality ore been stable - this is highlighted by the fact that Egypt continued to use bronze almost exclusively for centuries after iron became more prevalent in other regions of the world.
I think with a little effort, some hard numbers could be found as far as the relative mass/volume between late bronze and early iron, as well as tensile strength etc between the commonly found alloys.