"And them that take the sword, shall perish by the sword."
-The Bible, Matthew 26:52
Boosts production and naval power.
Metal Casting allows you to build Forges and enables your Workers to build Workshop improvements, both of which increase city production. Metal Casting also allows Triremes to be trained and enables the Colossus wonder.
Metal casting is a way to fabricate metal objects. When subjected to extreme heat, metal melts into a liquid. The molten metal is poured into a mold, where it cools and returns to a solid state, holding the shape of the space inside the mold. Once the object is removed, the mold can be reused to make an identical object.
Metal casting is a complex process. First the mold is created. The "lost wax" process is an effective way to create a mold which has been in use since the Bronze Age. In this process the artisan creates in wax a replica, or "positive" copy, of the object to be cast. Then the artisan covers the wax object in clay. When fired in a kiln, the clay hardens around the wax, creating a "negative" image of the object. This is the mold. (The original wax positive melts during the process thus its title.) Molds are often created in two or more pieces so that the cast metal can be removed without destroying the mold.
Metal casting has some important advantages over earlier methods of working metals like blacksmithing. In blacksmithing, a smith heats and pounds metal until it achieves the required shape. This is a difficult process which requires a lot of skill. It's also quite time-consuming, as the smith can only work on one object at a time. Metal casting, on the other hand, is much more efficient. Once a skilled craftsman creates a mold, it can be used over and over again by less-skilled workers, with a large degree of certainty that the finished objects will be identical. Indeed, a mold can be created that will produce multiple copies of smaller objects, such as spearheads and daggers.