Bloomery Process

a process in which a sponge-like lump of iron (a bloom) is obtained directly from ore by heating the ore on low hearths or bloomeries. The bloomery process is the oldest iron-making process known; it was used as early as the second millennium B.C. and continued in use until the early 20th century.

The original bloomery process was carried out in pits lined with refractory clay or on stone hearths; these furnaces used a natural draft supplied through a tuyere in the lower part of the hearth. A bottom layer of charcoal was ignited and the hearth was then charged from above with alternate layers of iron ore and charcoal. The total quantity of ore loaded was up to 20 kg. The temperature in the working area of the hearth (1100°–1350°C) was not sufficiently high to melt the low-carbon iron. The incandescent bloom was then removed from the hearth and hammered to squeeze the metal and remove some of the slag. The highly ferrous, molten slag was drained from the hearth through a trough.

Such furnaces were gradually improved by using bricks and natural refractory stone for wall linings and by increasing the cross-sectional dimensions and the height of the hearth; the furnace gradually took the shape of a low-shaft bloomery. A forced draft was supplied by bellows and the weight of the bloom was increased to 15–25 kg. The last design improvement, introduced in Finland and Russia near the end of the 19th century, resulted in a continuous process. The Krupp-Renn process is another version of the bloomery process.