Bloomery

Bloomery

A single-stage method of directly smelting iron

Iron

The product of the single-stage method of directly smelting or reducing iron from its ores using charcoal as the fuel and reducing agent. Reduction from ores to give a mixture of iron particles and waste products, mainly non-metallic slag and cinders, is possible as low as 800̊C, but the production of iron in this way is not practicable until a large proportion of the slag can be separated from the iron in the furnace, only possible when the free-running temperature of the slag (in most cases, approximately 1100-1200̊C) is reached. During reduction solid particles of iron are formed and after the slag has run to the bottom of the furnace or been tapped away these gradually coalesce to form a semi-congealed, porous or spongy mass of iron, the ‘bloom’. (The term derives from the Old English word bloma, meaning a flower: Anglo-Saxon iron-makers presumably thus described the appearance of the mass of metal in the furnace). The bloom is still mixed with much slag which must be mostly removed by bloom consolidation. Most of the impurities present in the ore, such as the oxides of manganese, magnesium, calcium and aluminium, will be lost in the slag since they are far less readily reduced than the iron oxides, although some impurities such as phosphorus are readily reduced.

Steel

The direct production of steel from iron ore by making the conditions in a bloomery smelting furnace more reducing than is necessary to produce plain or low-carbon iron. A higher charcoal to ore ratio is required and control over the air blast is more critical, factors which make the production of bloomery steel a more difficult and expensive operation than that of plain iron. In practice, the difficulty of maintaining even reducing conditions in early bloomery furnaces probably resulted in the production of blooms with varying carbon content.