Bloomery (Edited from Wikipedia)

SUMMARY

A bloomery is a type of furnace once widely used for smelting iron from its oxides. The bloomery was the earliest form of smelter capable of smelting iron. The result of using a bloomery is a mass of iron and slag called a bloom. This is called sponge iron, because it is porous like a sponge. Sponge iron is usually refined into wrought iron. Bloomeries are no longer used in smelting, because blast furnaces are better.

HISTORY

China

China has long been considered the exception to the general use of bloomeries. It is thought that the Chinese skipped the bloomery process completely, starting with the blast furnace and the finery forge to get wrought iron: by the 5th century BC, metalworkers had invented the blast furnace.

Sub-Saharan Africa

Smelting in bloomery type furnaces in West Africa and forging of tools appeared by 500 BC.

Medieval Europe

Early European bloomeries were relatively small, smelting less than 1 kg of iron with each firing. Progressively larger bloomeries were constructed in the late fourteenth century, with a capacity of about 15 kg on average, though exceptions did exist. The use of waterwheels to power the bellows allowed the bloomery to become larger and hotter. European average bloom sizes quickly rose to 300 kg, where they leveled off until the demise of the bloomery.

As a bloomery's size is increased, the iron ore is exposed to burning charcoal for a longer time. When combined with the strong air blast required to penetrate the large ore and charcoal stack, this may cause part of the iron to melt and become saturated with carbon in the process, producing unforgeable pig iron which requires oxidation to be reduced into cast iron, steel, and iron. This pig iron was considered a waste product

1 detracting from the largest bloomeries' yield, and it is not until the 14th century that early blast furnaces, identical in construction but dedicated to the production of molten iron, were built.

Bloomery type furnaces typically produced a range of iron products from very low carbon iron to steel containing approximately 0.2% to 1.5% carbon. The master smith had to select bits of low carbon iron, carburize them, and pattern-weld them together to make steel sheets. Even when applied to a non-carburized bloom, this pound, fold and weld process resulted in a more homogeneous product and removed much of the slag. The process had to be repeated up to 15 times when high quality steel was needed, as for a sword. The alternative was to carburize the surface of a finished product. Each welding's heat oxidises some carbon, so the master smith had to make sure there was enough carbon in the starting mixture.

In England and Wales, despite the arrival of the blast furnace in the Weald in about 1491, bloomery forges, probably using water-power for the hammer as well as the bellows, were operating in the West Midlands region beyond 1580. In Furness and Cumberland, they operated into the early 17th century and the last one in England did not close until about 1770.

PROCESS

A bloomery consists of a pit or chimney with heat-resistant walls made of earth, clay, or stone. Near the bottom, one or more pipes (made of clay or metal) enter through the side walls. These pipes allow air to enter the furnace, either by natural draught, or forced with bellows. An opening at the bottom of the bloomery may be used to remove the bloom, or the bloomery can be tipped over and the bloom removed from the top.

The first step taken before the bloomery can be used is the preparation of the charcoal and the iron ore. The ore is broken into small pieces and usually roasted in a fire to remove any moisture in the ore. Any large impurities in the ore can be crushed and removed.

In operation, the bloomery is preheated by burning charcoal, and once hot, iron ore and additional charcoal are introduced through the top, in a roughly one to one ratio.

The small particles of iron produced in this way fall to the bottom of the furnace, where they combine with molten slag, a compound of silicon, oxygen and iron mixed with other impurities from the ore. The mixed iron and slag cool to form a spongy mass referred to as the bloom. Because the bloom is highly porous, and its open spaces are full of slag, the bloom must later be reheated and beaten with a hammer to drive the

2 molten slag out of it. Iron treated this way is said to be wrought (worked), and the resulting iron, with reduced amounts of slag, is called wrought iron or bar iron.

PIG IRON

Pig Iron is Iron with a high carbon content. Pig iron is made by smelting iron ore into ingot form, in a blast furnace. It’s called pig iron because when being cast, each ingot being cast looks like a Piglet suckling milk from a sow.

Ancient Egyptians, Greeks, and Chinese knew about pig iron, and were producing it as early as 1100BC.

Pig Iron is refined into Wrought iron by melting it and blowing hot air through it. Pig Iron can also be turned into gray iron.

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