I have been playing a crafter now for a little bit, and this is where I think a lot can be done to give the game more flair and make more use of items already in the game.
1). Currently for the crafting of Armor and Weapons we have the following metals available
A) Copper
B) Bronze
C) Iron
D) Steel.
There are other metals that can be mined, but if you are being true to chemistry and physics, then you wouldn't
use Zinc, Mercury,Silver or Gold except as ornamentation.
2). As it stands right now, unless changed, only steel will be used for weapons and iron for shields.
This means that we have at least two metals that even if you added the rules for them wouldn't be used on shields and three on weapons, because steel weapons and iron shields (steel if you allowed it) are the best. And who is going to make inferior weapons when all of his competitors can also make steel. So if you want to see some more variance in the composition of weapons then you need to structure it so that it is the less effective weapons that can be made first. The Copper, the Bronze, the Iron swords and clubs. (not to sure about the copper but I know that bronze was in use long before iron or steel.). This is not to say that your current system doesn't work, just that there is no justification for making bronze. You have an alloy in the rules that wouldn't be used to make weapons at all since steel exists.
So here is my proposition as it were.
Metals used
A) Copper
B) Bronze (alloy of copper and tin)
C) Brass (alloy of copper and zinc)
D) Iron
C) Steel (alloy of Iron and Carbon aka Coal)
Weapons and Shields
A) Low levels use copper.
B) Mid Low levels use bronze
C) Mid levels use brass
D) Mid High levels use iron
E) High levels use steel
Metallurgy From Low to High
A) Melting Copper, Copper ingots
B) Melting Tin, Tin Ingots. Copper Stocks
C) Melting Zinc, Zinc Ingots, Tin Stocks, Making Bronze, Bronze Ingots
D) Making Brass, Brass Ingots, Zinc Stock, Bronze Stock
E) Melting Iron, Iron Ingots, Brass Stocks
F) Melting Silver,Mercury. Silver Ingots, Mercury ingots, Iron Stocks
G) Making Steel, Steel ingots, Silver Stock Mercury Stock
H) Melting Gold, Gold ingots, Steel Stock
I) Gold Stock.
As you can see from my list it would cause more variety of crafted items to enter the game, It would extend the length of time needed to master your craft.
What I don't know is because this radically alters the availability of the various metals what you would have to do about all the other items currently running around in the game. It would certainly force the creation of a greater variety of weapons, shields and armor.