Cornish contribution to mining technology
Copper Country's past and people
As rich as the Cliff Mine’s vein was in 1846, its wealth would make little difference if the company was not willing to invest capital into its surface plant. The first improvements to the surface were most likely creating a carpenter shop, and blacksmith shop, a cooper shop, offices, a storehouse, and similar buildings. The company’s personnel must also focus their thoughts on some sort of concentration plant, where rock could be broken small enough to extract smaller pieces of copper from its adhering rock. The Cliff Mine’s management had an eye toward that from the time the vein showed real promise, which occurred during the winter of 1945-46.
While the majority of the Cliff’s copper production came from mass copper, it would also rely on smaller pieces, which could be shipped pure to the smelter back east in barrels, hence the name “barrel copper.” Additionally, the lode at the Cliff also contained a fair amount of amygdaloid copper; small, sometimes minute, pieces and flecks of copper that had been deposited in gas holes in the rock, or traprock, that could be reclaimed through refining and concentrating.
It was in the early summer of 1846 that the superintendent of both the Cliff and the Copper Harbor mines, William Pettit, visited Charles Gratiot at the Lake Superior Copper Company’s mine on the Eagle River. Pettit was given a grand tour of the mining location, including the stamp mill that had been designed by the geologist, Charles T. Jackson, the first mill to exist in the Lake Superior region — and also the first one to fail.
It is most likely that Pettit discussed Jackson’s mill with the Cliff Mine’s agent, Edward Jennings. It seemed too complicated. Jackson’s mill actually consisted of two batteries of stamps, one for copper and the other for silver. It also contained a grinding wheel to crush silver rock still finer in order to extract more mineral from it.
The Cornish system of rock crushing was less complex, less involving, and less intensive of machinery that could break, fail, or just not work. Jennings, who was born in Gwinear, and grew up in the mines there, understood concentrating and mills, and it is easy to believe that it was Jennings who at least influenced the company to erect a Cornish-style mill.
Whatever convinced the Board of Directors to go with such a mill, it is fortunate to historians that while the U.S. Geological Survey was being conducted at that time, its leaders included descriptions of the mining and milling at the Cliff in what became known simply as Foster & Whitney’s Report of 1850. In his 1955 book, “Lake Superior Milling Practice”, C. Harry Benedict relied heavily on that report to detail the mine’s surface plant. Benedict wrote that the Cliff’s concentrating plant set the standard for milling and concentrating of other fissure vein mines, which included the Minesota, Copper Falls, and Central mines.
“At the Cliff mill,” Benedict wrote, “there were two batteries of five stamps each, each stamp weighing about 500 pounds, operated from a cam shaft, steam driven.”
He went on to state that the wooden-stem stamps had an iron shoe that crushed the ore “in a primitive mortar,” a sort of open trough, called the cover, which had screens in front at the bottom. When fine enough, the crushed ore ran out into a small two-way classifying section which diverted the heavier sands and copper into a small pool, the overflow of which joined the lighter material of the classifier.
“The heavier copper left in the mortar hopper,” he wrote, “and in the pool was ready for barreling and shipment to the smelter. The lighter portions flowed into intermediate storage bins or ‘pits.'”
At this point, what neither Foster & Whitney, nor Benedict reported, was that this crushing system required large amounts of water to keep the crushed ore continually flowing.
“In winter the pits filled up with a material, and no further concentrating of their contents was attempted until water again flowed abundantly in the spring,” Benedict stated. “But in the warmer months, the concentration of values followed the practice of the Cornish tin mines, a combination of hand-operated jig, tie and buddle.”
I lost you by this point, I know. However, three years after the Foster & Whitney Report discussed the milling process, an article was published in the March-April, 1853 Harper’s Monthly, that attempted a less complex description of the washing, or middling, process:
“…a process of washing is performed, technically termed, jiggering. The trench for this purpose is let some 12 or 18 inches into the floor and is provided with a spring-board at the lower end beneath the water, which greatly assists the boy who executes the process. The jigger is a kind of low tum, with a bottom of sheet brass finely perforated. A portion of the deposits from the heads of this trench…having been shoveled into the trench, and by sundry vertical and whirling motions in the water as well as about, gives the proper degree of agitation. Then resting it upon the floor, he with a scraper removes the surface. The copper lies in a deposit of reddish particles beneath. The top is thrown in a heap by itself, to undergo further washing at the buddles. The jigger separates the largest grained and roughest copper which is obtained from the floors.”
The above was the beginning of the concentrating process, so called, because it liberated the copper from the adhering rock, allowing it to be concentrated, which then would be smelted to remove any remaining impurities from the metal.