Armour, construction tools, agricultural tools, and other building materials were mostly made of iron; thus, making iron one of the most in demand metals at all times. There was always a supply for iron in many parts of the Roman Empire to allow for self sufficiency.
There were many iron mines in Roman Britain. The index to the Ordnance Survey Map of Roman Britain lists 33 iron mines: 67% of these are in the Weald and 15% in the Forest of Dean. Because iron ores were widespread and iron was relatively cheap, the location of iron mines was often determined by the availability of wood, which Britain had in abundance, to make charcoal smelting fuel. Great amounts of iron were needed for the Roman war machine, and Britain was the perfect place to fill that need.
Many underground mines were constructed by the Romans. Once the raw ore was removed from the mine, it would be crushed, then washed. The less dense rock would wash away, leaving behind the iron oxide, which would then be smelted using the bloomery method. The iron was heated up to 1500 °C using charcoal. The remaining slag was removed and generally dumped.
After being smelted, the iron was sent to forges, where it was reheated, and formed into weapons or other useful items.
Roman iron was thought to hold more value than other metals due to the tedious production through direct or bloomery smelting. A recovered Vindolanda tablet documents the purchase of 90 Roman pounds of iron for 32 denarii by a man named Ascanius. This amounted to 1.1 denarii per kilogram of iron.