When Cornish mining native Edward Jennings, the first superintendent of the Cliff Mine, on Keweenaw Point, constructed the first successful copper concentrating plant in the copper district in 1849, the Cornish system he built had scarcely evolved from concentrating technology of the 1500s, when the mining processes of that period were described by Georgius Agricola’s Book VIII of “De Re Metallica,” which was published in 1556. The only significant change in three centuries was that Jennings applied steam power to his mill rather than water power.

Agricola wrote that “Ore is crushed with iron-shod stamps, in order that the metal may be separated from the stone and the hangingwall [sic] rock.” He went on to write that the stamp-stems were made of small square timbers, nine feet long and half-a-foot wide each way.

“For each stamp there are on a cam-shaft, two cams, rounded on the outer end, which alternately raise the stamp, in order that, by its dropping into the mortar, it may with its iron head pound and crush the rock which has been thrown under it.”

As with many early 19th century stamp mills in the Lake Superior district, Agricola also provided intricate details in explaining that the stamp mills of his time were powered by water wheels. At other locations, by the 1700s, stamp mills were often powered by steam engines rather than water-power. The same launders and copper-washing techniques installed at the Cliff in 1849, likewise, would have easily been recognized by Agricola.

Even the method of reducing mine rock sufficiently to be stamped had changed very little in over four centuries. As Agricola wrote:

“I now come to the methods of roasting, and first of all to that one which is common to all ores. The earth is dug out to the required extent, and thus is made a quadrangular area of fair size, open at the front, and above this, firewood is laid close together, and on it other wood is laid transversely, likewise close together, for which reason our countrymen call this pile of wood a crate; this is repeated until the pile attains a height of one or two cubits.” He continued in the next paragraph: “By digging out the earth they make the areas much larger, and square; walls should be built along the sides and back to hold the heat of the fire more effectively, and the front should be left open.” In the Lake Superior region, in a period of less than 30 years, those practices would change.

The mining companies that organized in the Lake Superior copper region after the mid-19th century benefitted immensely from the earlier companies that had introduced modern equipment, conducted the experiments, ran trials and invested hundreds of thousands of dollars into new technology that may or may not work. The Franklin and Pewabic companies, both under the same management, were the first to introduce Edwin Ball’s steam-powered ore stamp in the Lake Superior copper district, in 1858.

A couple of years later, the Bohemian mine experimented with Eli Whitney Blake’s mechanized rock crusher as a means of replacing the old kiln houses used for roasting mine rock before sending it on to the stamp mill for further crushing.

A decade later, when Alexander Agassiz, president of the Calumet and the Hecla mining companies, began construction on the mills for those mines, he did not begin with the now obsolete Cornish-style gravity drop stamps, but installed the new, modern steam-powered stamps. The rock received at both mills had never undergone calcining in kiln houses, but was reduced by Blake crushers. Agassiz’s mills were state-of-the-art facilities, thanks to the modern milling practices developed by older mines, like the Huron and the Atlantic.

Both the Huron and the Atlantic operated on extremely low-grade copper lodes; the Atlantic, between 1873 and 1910, averaged just 13 pounds of copper per ton of rock mined, stated the U.S. Geological Survey’s Professional Paper 144, published in 1929. In order for lower-grade mines, like the Atlantic, to survive, they had to find ways to maximize production while at the same time cutting production costs. Ball’s steam-powered ore stamp became necessary to the Atlantic’s survival. Blake’s crusher was another invention that greatly decreased the time between copper rock came from the mine to the time it could be reduced small enough to be received by the mortar boxes of the stamps. The crushers also slashed labor costs. The old kiln houses could require as many as 25 men to work them to best advantage.

John Collom, at the Huron mine, invented a jig that captured more of the finer particles of copper from the stamped rock, or gangue, that previously was washed out with the tailings. Collom’s jig was, in simple terms, a tank that received the stamped rock and copper in a flow of water on a fine, wire screen. By use of a plunger in the tank, the particles and water were kept in continuous motion, causing them to separate according to density. The denser copper settled beneath the less dense rock, allowing more of the rock to flow off to be sent to the tailing pile.

As explained by Swapan Kumar Haldar, in his book Mineral Exploration (Second Edition):

“The elementary jig (Fig. 13.29) is an open tank filled with water. A thick bed of coarse heavy particles (ragging) is placed on a perforated horizontal jig screen. The feed material is poured from the top. Water is pulsated up and down (the jigging action) by pneumatic or mechanical plunger. The feed moves across the jig bed. The heavier particles penetrate through the ragging and screen to settle down quickly as concentrate. The concentrate is removed from the bottom of the device. The jigging action causes the lighter particles to be carried away by the crossflow supplemented by a large amount of water continuously supplied to the concentrate chamber.”

In addition to Collom’s jig, William Evan’s slime table added one more component to flotation mineral concentration. Later referred to as round concentrating tables, William J. Evans’ ore separator was patented on May 4, 1875. According to documents in the United States Patent Office, Evans, a resident of Houghton County, described his invention, what became known as the Evans Slime Table, as having “a horizontally-rotating table whereon the ore (sand) is distributed by water and washed by jets of clear water.”

The clear water was supplied by a pipe to the distributor, and ran over one half of the table, while the slime water ran over the other half. The sand and water being on one side of the distributor, ran through its perforated bottom, and were distributed equally over one-half of the stationary head, and ran on the rotating table into a circular launder, then through the waste pipes. As 911metallurgist.com explains, through the action of clear water the proper grades of ore were washed about half-way down the rotating table. They then came in contact with the diagonal perforated pipe, and were rewashed by a succession of small jets from perforations of small pipes.

It’s a complicated way of comparing a slime table to panning for gold, whereby rocks, mixed with gold, are swirled together in a pie tin with water. Swirling the water slowly around keeps the contents in motion, allowing the heavier gold to settle to the bottom of the tin while the rock continues to be swirled around above it.

In a short period of time, Collom’s initial jig underwent modifications. Soon there were roughing jigs, middling jigs and finishing jigs; Evan’s slime table was replaced by Wilfley, or shaker, vibrating tables. Then came “rotating duplex trommels, triplex rolls,” then came regrinding mills that re-ground the stamp sands to wring still more copper from it. The companies of the Copper Country were compelled to adapt, invent, and innovate modern technology at their mills if they were going to survive into the 20th century.