Production in Early Times

The next development was the application of hand, or foot, operated bellows to provide an air blast to the hearth, which enabled the smelting site to be more conveniently located near ore supplies. The furnace was constructed as a short, square shaft with a bottom opening for the bellows and to allow metal and slag to run out. The shaft was packed with charcoal and ore. An example of this type of furnace is the Catalan Eorge, introduced in Spain about 700 AD. 

The furnace generally operated only for a few days and was then cleared of accretions and the walls were replastered with lute , a paste of clay and fine charcoal. The skill of the furnace operator was most important in regulating the air blast from the bellows and in the placement of the ore charge towards the front of the shaft so as to avoid the formation of a sintered mass or sow . Natural fluxing materials such as fluorspar were often also added to the charge depending on the nature of the ore. 

For the standard reverberatory furnace, a batch of galena was added to the furnace hearth and was roasted with hand-rabbling for about two hours, in which time part of the lead sulfide was directly oxidised to lead sulfate. The resulting mixture of lead sulfate and unreacted lead sulfide was thoroughly mixed and the temperature of the furnace was increased. This allowed the roast reaction , as given in Equation 2.1, to take place, with copious emission of sulfur dioxide  

Any silica in the concentrate tended to react with lead oxide (PbO) to form lead silicate, and in the final stage of the process lime was added to the furnace charge and mixed in with the slag and umeacted ore, for the purpose of decomposing the lead silicate in accordance with Equation 2.2  

Following this step, molten lead bullion was tapped from the base of the furnace hearth as a crude impure or hard lead. 

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