Cornwall Iron Furnace - Bloomery

Bloomery

The first furnace built by Peter Grubb at Cornwall Iron Furnace was a bloomery. Grubb built this in 1737 to test the market value of his ore. It was an economical way to test the market without having to invest in building the much more efficient and profitable blast furnace.

A bloomery is basically an enlarged blacksmith's hearth. It consists of a pit or chimney with heat-resistant walls made of earth, clay, or stone. (Sandstone was used at Cornwall.) Near the bottom, one or more clay pipes enter through the side walls. These pipes, called tuyeres, allow air to enter the furnace, either by natural draft or by forced with a 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 bottom.

The first step taken before the bloomery can be used is the preparation of the charcoal and the iron ore. The charcoal is produced by heating wood to produce the nearly pure carbon fuel needed for the refining process. The ore is broken into small pieces and roasted in a fire to remove any moisture in the ore. Any large impurities in the ore can be crushed and removed. Since slag from previous blooms may have a high iron content, slag from previous blooms can be broken up and recycled into the bloomery with the new ore.

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. Inside the furnace, carbon monoxide from the incomplete combustion of the charcoal reduces the iron oxides in the ore to metallic iron, without melting the ore; this allows the bloomery to operate at lower temperatures than the melting temperature of the ore. Since the desired product of a bloomery is easily forgeable, nearly pure iron, with a low carbon content, the temperature and ratio of charcoal to iron ore must be carefully controlled to keep the iron from absorbing the carbon and becoming unforgeable. Limestone could also be added to the bloomery, about 10% of the ore weight, which would act as flux and help carry away impurities.

The small particles of iron produced in this way fall to the bottom of the furnace and become welded together to form a spongy mass of the bloom. The bottom of the furnace also fills with molten slag, often consisting of fayalite, a compound of silicon, oxygen and iron mixed with other impurities from the ore. 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 molten slag out of it. Iron treated this way is said to be wrought, and the resulting nearly pure iron wrought iron.