The Old Method of Smelting
The ore, which came from the mines in Røros contained mainly, copper, iron, sulphur and other minerals. The ore was considered rich if the content of copper reached, 4% – 6%.
The purpose of the process in the smelting houses was to separate the metallic copper from the other substances in the ore. To achieve this, an extraction process was employed whereby the ore was alternately roasted and smelted. This process remained unchanged from the time production started in 1646 until 1887, and can be divided into the following five steps: (1) first roasting or reducing by calcination, (2) copper or matte smelting, (3) turn-roasting, (secondary roasting), (4) black-copper smelting and (5) refining.
In the early days of production the process of first roasting was done close to the mine. However, from 1730 all roasting work was done at the smelting houses.
|Photo E. Olsen Trondheim|
Copper or matte-smelting
The first-roasted ore was transported into the smelting houses where it was smelted. The ore was placed in the furnaces in layers together with charcoal and quartz, which was used as a slag-producing substance. In order to achieve high enough temperatures it was necessary to force air into the furnaces in enormous quantities this was achieved by the use of bellows powered by a water wheel driven by the flow of the river.
During the smelting process much of the iron oxide from the roasting process was converted into slag, which was tapped out of the furnaces and transported to the slag heaps. The in-between product from the smelting was a residue still containing impurities, which was taken outside the smelting house for another round of roasting.
‘Turn-roasting’ was carried out in stone pens or bins approx. 2 X 2.5 m. The residue from the smelters was piled up on top of firewood stacked inside the bins where it was fired for 1 – 4 days. Subsequently it was ‘turned’ and built up again on top of wood in another stone pen where it was again roasted. This process was repeated up to 10 times. During ‘turn-roasting’ most of the remaining sulphur was released and the residue now comprised mostly copper and iron oxide.
The product of turn-roasting was then transported back into the smelting house and heated up together with charcoal, in this process the majority of the remaining iron oxide was burnt off. The end result was called, black copper, which had a copper content of approx. 90%.
Black copper was still impure, mainly with residual iron and sulphur and had to be further refined. The refining of black copper did not take place at all the smelting houses, in some of them black copper was the final product. The end refining process was done in open iron vats. The black copper was melted and at a certain point a newly cut pole of either fir or birch was thrust and stirred into the vat of melted copper, a process called ‘poling’. The next operation was to throw water on the melted copper, which caused the topmost layer to harden so that it could be scooped off. At the end of this 5 step process the end product was copper that was 98 – 99% pure, This ‘gar’ copper, or refined copper, was excellent for alloys but not suitable for forging. In order to make the copper suitable for other products yet another process, called hammer refining had to be done.