The purpose of the Mohawk Mill – as with any stamp mill – was to separate the copper from the rock that incased it. The process relied on the differing physical properties of the two substances, specifically the weight and hardness. Copper was of a greater density then the igneous rock that surrounded it, resulting in a heavier and tougher substance. This made it much easier to separate the two.
The first step was to take advantage of the relative toughness of copper with the use of the stamp. The stamp is essentially a hammer that smashes the copper rock into smaller pieces. Early stamps, known as Cornish stamps, worked using gravity to smash the rock. A steam engine was used to raise the “hammer” and gravity did the rest. In more modern stamps, as was used at the Mohawk mill, the same steam engine that raised the “hammer” also was used to force it down. This allowed for a much greater force and made the job go a lot faster. A screen on the bottom of the stamp only allowed material of a certain size to pass on, while large piece of copper could be removed directly.
It is also at this first step in the process that the water solution is mixed. The rock leaving the stamps is in solution with water, and both make their way through a sorting machine. These machines were different depending on mill, but the idea was the same. Larger (and heavier) rocks dropped out of the water mixture quicker then smaller pieces. The allowed the rock to be sorted by size and passed onto the next step.
The second step relies not on hardness, but instead on the weights of the particles. This step used a series of machines called Jigs. A jig was basically a water-filled tub, in which a series of screens was laid horizontally. The rock solution was dumped into the tub, in which a plunger was raised and lowered repeatedly. This motion agitated the solution and forced the rocks to float momentarily. A slight pause between strokes, allowed heavier particles to settle, while lighter non-copper bearing particles were allowed to pass.
The last step involved the use of a rather ingenious device that was only added to the process at the end of the 19th century known as Wifley tables. These machines were tables, slightly tilted at an angle. Along the table surface was a series of ridges (or riffles). The water / rock solution was poured over the table. Water and lighter materials (non-copper) simply flowed over the ridges and off the table. Heavy particles (copper bearing) would quickly sink and be held up by the ridges. The table was then shook back and forth, moving the heavy particles down the ridges and off the side of the table.
This, of course, is a simplified version of the process. But it gives you an idea of why the mill is built the way it is. Each level of the building represents a stage in the process. At the top (or upper level) were the stamps themselves. In the middle levels were the jigs of different sizes, and at the bottom were the Wifley tables. After that, the remaining water/stamp sand solution is washed out of the building.