The copper that arrived to the smelter was anywhere from 70 to 90 percent pure. Due to the unique nature of Lake Superior copper, most of those impurities manifested themselves as pieces of foreign rock imbedded within the copper itself. As a result Copper Country smelters – including the Quincy Smelting Works – utilized a rather uncomplicated procedure to purify their copper wares. A process that essentially boiled down to melting the copper and skimming off its impurities. The central piece of equipment used during this process was the reverberatory furnace.
A reverberatory furnace consisted of a large brick-lined pot – known as a hearth – in which the copper was melted down to a liquid. This was done not with direct heat, but with reflected – or reverberated – heat emanating from an adjacent firebox. The natural draft created by the furnace’s chimney would pull the hot gases from the firebox up across the specially curved top of the hearth before being pulled up the flue. Along the way those hot gasses would reverberate off the roof and bounce down into the copper filled hearth – slowly melting it.
While rather simple in its aggregate, the actually reverberatory process actually involved several procedures and nearly a full day before copper could be poured into waiting ingots. The first of these steps was to load the copper into the furnace in a process known as “charging”.
During the charging process the furnace’s firebox is doused and its doors and top are opened. While copper could have been loaded into the furnace from any of these side doors, most of the mineral was loaded from the top – thanks to the furnace’s removable top. The furnace is first filled with the smaller pieces of copper to create a carpet of mineral along the furnace’s bottom. Then the larger pieces – mass copper – are placed on top of that copper carpet. After the furnace reaches is maximum load – about 36,000 pounds – its top is dropped back in place and all the doors are closed.
With the furnace charged it was now time to ignite the firebox and beginning the heating process. A layer of coal was placed in the firebox atop a metal grate spanning across the ash pit and lit on fire. The fire is fed by air drafting up through the open ash pit, air that is forced through the grate and directly across the hot coals before its drafted over to the hearth and up the flue.
The natural draft created between the flue and ash pit opening creates a constant flow of hot gasses that drift out of the firebox, up over the hearth’s roof, and finally up the flue. As mentioned before, the curved top of the hearth forces those gases to reverberate down into the copper itself facilitating its melting. During this stage the furnace is kept tightly shut to maximize the heat within the hearth.
After a few hours in the furnace, the copper and rock will begin to melt and the next step of the process can commence. Due to the differences in density between copper and the rock that surrounds it, the molten contents of the hearth began to separate naturally into two distinct layers. At the bottom is the copper itself – known as the matte – while floating on top forms a layer of molten rock known as slag. Once formed, this slag would be skimmed off the top of the copper and removed through the furnace door. Skimming would continue for another 12-15 hours until all the remaining copper is thoroughly melted and all the slag that forms is removed.
With the melting stage now complete (16 hours later) the entire smelting process is three-quarters of the way complete. Now the final refining stage begins, were the last drops of impurities are removed from the molten copper in a process known as “rabbling”. Rabbling involves agitating the copper to expose it to air and oxidize and remaining impurities within it. This is done by blowing compressed air into the molten mixture, or more traditionally by splashing the copper around using a paddle-like tool known as a rabble. This oxidizing method forces further impurities out of the copper matte and to the surface. This is done for another 1-2 hours to insure the entire matte has been oxidized, at which time the resultant slag is again skimmed off and removed.
As a result of this rabbling a great deal of the copper matte has also been oxidized, creating a copper oxide mixture. In order to remove this excess oxygen from the mix and return the copper to a more native state, the matte undergoes a final stage in the smelting process known as “poling”. Here workers insert into the molten copper several poles of wood – usually poplar – measuring between 15 and 20 feet in length. As the wood is consumed in the hot copper, it releases carbon into the mixture. This carbon attaches itself to the oxygen in the copper to form carbon oxides which are then drawn up out of the furnace through the chimney.
Here’s a shot of this poling process being performed at the C&H smelter. The “pole” here looks more like a log as its dragged into the furnace by means of an overhead crane. Several more poles can be seen stacked up in the foreground. This process would continue for sevearl hours until the majority of the copper oxide had been reduced. At this point the copper is as pure as the reverberatory furnace is going to make it and the final step can finally commence.
That final step would be the casting process itself, by which the molten copper is removed from the furnace in large ladles and then poured into rows of ingot molds. Those molds would then be dropped into a water bath to cool them before being removed and sent out to the warehouse – some 22 hours after the copper was first loaded into the furnace.
Information for this post was obtained from “The Copper Mines of Lake Superior” by Thomas Arthur Rickard (1905).