Bronze Ingots are made by placing an Ingot Mold, Copper Dust, and Tin Ore in a Primitive Forge, with Charcoal used as fuel. 3 Copper Dust and 1 Tin Ore crafts 1 Bronze Ingot. 

They are the earliest attainable ingots to be considered Sturdy Ingot

Bronze Ingots cannot be made using Copper Ingots and Tin Ingots, only Copper Dust and Tin Ore.