Heat and metal

Heating metal until liquid allowed solid nuggets to be combined into bigger more complex items.

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Melting also enabled minerals to be extracted from rocks that had been crushed.

Pure metals could be mixed to produce alloys.  These had different properties.
  
Pure copper is relatively soft.  Adding tin (another pure metal) to liquid copper creates bronze, a much harder, stronger and tougher metal.  Bronze tools, armour and weapons were much superior - a distinct advantage in armed conflict.
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