Anatomy of a Mill (p1)

In the beginning of the Copper Empire, very little importance was put on milling. It was the mines themselves that produced copper, usually in the form of large pieces known as mass (or barrel) copper. These large pieces required no milling, and fetched high prices out east. Milling at that time was considered only a secondary producer of copper for a mine, relegated to simply processing a mine’s left-overs. The small amount of copper that would be recovered would often barely cover the operating costs of the mill itself.

In only a short time, however, this mindset would be drastically altered. It turns out that Mass Copper deposits such as those enjoyed by the Cliff and Central Mines were a rare breed in the Keweenaw. Companies quickly discovered that most copper here existed in small pieces, mixed in with a great deal of basalt and other rocks. These Amygdaloid deposits - as they are known - could not be simply removed from the earth through blasting and chiseling. Extensive milling would have to take place before any of that copper could be recovered. After a generation as a second-class producer, stamp mills would finally have their day.

Luckily for mine companies, by the time the mills were ready for prime time stamp technology had improved dramatically from its early cam-shaft and gravity powered days. It was during the 1850’s that the industry turned from Cornish style stamps to steam powered Ball Stamps. These stamps used steam power to not only lift the stamps, but to also drive them downward. It was the Copper Falls Mine that first used such a stamp and the improvements were immediately apparent. By the time the Mohawk Mill was constructed at the turn of the century (pictured above), the technology had been perfected. It is here that we’ll take our tour.

Using the Sanborn Map from 1928 we get a nice overview of the Mohawk Mill, typical for most Copper Country mills of the time. Accompanying the mill is a collection of support structures, which include a boiler house, engine house, pump house, mineral house, and sand house. Also existing at the site but not pictured here are a few warehouses, a water reservoir (for fire-fighting I assume), blacksmith shop, carpenters shop, and oil house.

As for the mill itself, it can be divided up into five distinct sections: the rock bins, the stamps, the roughing jigs, the refining jigs, and finally the wash floor. These sections are built along a step-stair foundation, with each subsequent level a few feet lower the one previous to it. This allows gravity to be used to transport the copper/rock/water solution (slimes) from one machine to the next.

The first step in the milling process required a great deal of water, and Copper Country mills were the thirstiest of all. A single stamp required as much as 3.5 million gallons of water each and every day to operate; 30 tons of water for every ton of stamped rock. At the Mohawk Mill, where 4 stamps were in constant operation, 14 million gallons of water had to be supplied to the mill every day. To feed this thirst, mills were equipped with enormous steam powered pumps. At Mohawk the pump-house was equipped with pumps capable of supplying nearly 30 million gallons of water a day, enough to supply not only the Mohawk Mill but the Wolverine Mill next door as well.

The Mohawk / Wolverine pump-house sat to the north of the mill, far outside the ability of the stamp sands to clog up the intakes. (The ruins of this pump-house can still be found at the Tobacco River Park, along with the long wood cribs used to protect the intake pipe from ice in the winter) Water from the pump-house is sent to the mill along a large pipe - which you can see in the photo above.

In addition to water, mills required two more important ingredients. The first was copper rock, which was delivered from the mine along the Mohawk and Traverse Bay Railroad (later controlled by the Copper Range). At the Mohawk, car loads of copper rock were pulled up a tramway (which you can see on the left in the photo) by means of a chain drive. Once at the top, the cars would dump their loads down into large storage bins which were placed above and behind the stamps themselves.

The last ingredient for a mill was what all mine sites required - coal. In the photo above you can see the coal piles to the right. The coal was also brought by train, but from a much shorter distance - a nearby coal dock south of the mills. The coal would be used to fire two sets of boilers - one at the mill and another at the pump house. The boilers at the mill were used to create the steam necessary to run both the stamps themselves as well as any other pieces of machinery at the mill.

After the milling process was completed, a mill would produce two outputs. The first was the copper itself, which was loaded onto waiting rock cars to be sent south to a Portage smelter. The second was the mill’s waste product - stamp sand. These rock sands were flushed out of the mill along several wooden launders. These launders would then divert all the sands to a central building known as a Sand House. Here the sands wold be lifted a few dozen feet by means of a water wheel type machine (known as a sand wheel in the biz) and loaded onto an elevated conveyor. The sand house and inclosed conveyor is shown in the photo above. The conveyor would then bring the sands far out into the lake where they would be dumped into the water.

To Be Continued…

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