This is the sort of thing which is easily modded in, but I don't see that it should be a mod. I think it's a part of the game which is a bit out of balance.
Problem one: Bronze currently has strength 75, compared to 66 for copper and 100 for iron.
This is a problem firstly because bronze is actually STRONGER than iron (though not steel), secondly because it's hard to find the tin. I don't wish to make a big reality-based argument here, the problem is in gameplay. Currently bronze's factor is so low that it's hardly worth the trouble of finding and smelting it when you can just use the raw copper (which is often very plentiful). The difference between 66 and 75 is so small a single difference in quality level largely negates it, in fact completely negates it at low levels. As it currently stands it does not seem worth smelting bronze at all. The increase in performance is not worth the fuel it takes to do the smelting, much less the trouble of trying to get a Granite layer and then poking about in it.
Problem two: The formula for bronze is way off. Currently it's 50/50 tin and copper. This is actually what's known as speculum, a highly reflective alloy once used for telescopes. According to the fairly shallow research I've done, the highest tin percentage in manufactured bronze is believed to be about 23%. Anything over 10% tin is considered a "hard" (weapons grade) bronze. 12% tin seems to be a working average. This aggravates problem one. Not only is bronze rated too low to be useful, but it will be hard to make enough of it given the scarcity of tin.
We can't make a realistic formula for bronze, given that the game only seems to track four bars at once, but 3 copper + 1 tin = 4 bronze seems as close as we would get.
I believe that bronze should be a viable, if somewhat complicated substitute for iron, not a faintly cool but basically inferior metal you get in small quantities off traders. Giving it a higher factor and changing the reaction would be a pretty easy fix.