Bloom smelting

Bloom smelters made wrought iron directly from iron ore... placed the ore in a small hearth with a charcoal fire blown with bellows or a mechanical air pump. CO from the fire reduced the ore to solid iron, and impurities in the ore simultaneously formed liquid slag. The bloom smelter took a ball of white-hot iron and slag (the bloom) from the hearth and hammered it to consolidate the iron and expel the slag. (pg 14)

A bloomer had to accomplish simultaneously two separate objectives in his work: reduce the iron oxides in the ore to metal and separate the metal from the gangue in the ore. He placed the ore in a small hearth containing a charcoal fire blown with air from one tuyere. In the heat of the fire, the gangue reacted with iron oxide in the ore to make liquid slag, and CO gas formed by the burning charcoal reduced the rest of the iron oxide to particles of solid, metallic iron. A coating of liquid slag protected the newly formed iron from reoxidizing as it passed in front of the tuyere on its way to the bottom of the hearth. As smelting progressed, the metal particles began to adhere, forming a lump of solid iron and liquid slag called a bloom or loup. The bloomer pulled the loup from the hearth when it was big enough and hammered it while still hot to weld the iron particles firmly together and force out as much of the liquid slag as possible.

To smelt successfully, a bloomer had to have a fire that would reach 1,200 C (the lowest temperature at which he could keep the slag liquid) and simultaneously release gas rich enough in CO to reduce the ore... To make iron, the bloomer had to get the ore into the 'iron' field and on the CO side of the curve for the reaction C + CO2 = 2CO (otherwise CO decomposed to C + CO2). To obtain the high temperature necessary to melt the slag, the bloomer had to have a strong flow of air through the fuel. This increased the amount of CO2 in the furnace and could lower the ratio CO/CO2 below what was needed for smelting. A bloomer had to achieve just the right balance between air flow and heat loss to get simultaneously the necessary temperature and CO/CO2 ratio. Additionally, he had to have rich ore because he got metal only from the iron oxide that remained after the slag formed. Bloomery slag had to contain about 55% iron to be liquid in the hearth: if the ore had less than about 55% iron (and if chemical equilibrium were reached), no metal would form...

(from Robert B. Gordon American Iron 1607-1900 [Johns Hopkins Press 1996, pp 90-92]
TN704 .N5 G67 1996)