The Lead Bar is the refined form of Lead Ore, requiring three ore per bar. It is a material added in 1.2. It acts like Iron grade tools and can be used as a substitute for it in recipes as well, but armor and tools made with it will be classed as Lead equipment. The player will need a standard Furnace or higher to craft them and an iron/lead anvil to craft items with it.
In the real world, lead is highly toxic. It can cause allergies and your skin to develop rashes, so it is useless as an armor metal in real life, and is also extremely heavy but not as heavy as pure gold.
Prior to 1.2.4, a Lead Bar could be crafted into 4 Metal Fences. The 4 Metal Fences could then be crafted into an Iron Bar. this made it one of the few ways to get iron on a lead world