By Joseph A. McGeough
In casting, a liquid metal is poured into a cavity or a mould, where it takes the shape of the mould when it congeals; casting shapes the metal to essentially final form once a proper cavity has been prepared. Some touch-up work may be needed; for an edged copper tool, such as an axe or knife for example, hammering the cutting side gives a keen edge.
A great step forward was made with the discovery that gold, silver, and copper could be melted and cast with many advantages. Casting meant that the size of the tool was no longer dependent on the size of a chunk of available copper. Old tools could be added to a melt instead of being thrown out. This reuse of old metal accounts in part for the scarcity of virgin-copper implements.
To make the procedures of melting and casting possible, several innovations were required. Pottery making, already well established, provided the knowledge of heat-based processes. Clay vessels were essential to working with fluid metal, for, in all but the most primitive operations, it was necessary to convey the melt from furnace to mould. Aside from providing crucibles, pottery making taught how to restructure a fire with a deep bed of prepared charcoal to provide a heat superior to that of a simple campfire. Tongs of some sort had to be devised to carry the hot crucible; it is surmised that green branches were bent around the pot and replaced as needed.
A number of forms of moulds were developed. The most primitive was simply an impression of a rock tool in clay or sand to give a cavity of the desired form. A more durable mould resulted when the cavity was worked into stone. Cavities of uniform depth allowed flat but profiled pieces to be cast. For example, some axe blade castings were roughly T-shaped, the arms of the T being afterward bent around to clasp a handle of some sort, with the bottom of the T becoming the cutting edge. A one-piece mould, prepared for a dagger, could have a groove for most of the length of the cavity to provide a stiffening rib on one side. With experience, closed but longitudinally split and, hence, two-piece moulds were devised, each side having a groove down the middle to furnish a strengthening rib on both sides of the blade.
Split moulds for copper were not desirable because pure copper is a poor metal for casting. It contracts a good deal on cooling and has a tendency to absorb gases and thereby become porous, blistered, and weak. Also, molten copper exposed to atmospheric oxygen contains embrittling cuprous oxide.