How a Comstock Stamp Mill Crushes Ore

At the edge of Virginia City, Nevada, a stamp mill stands with its back to the dump and its face to the ravine. It crushes ore in a fixed order: the rock crosses a grizzly, waits in a bin, rides a shaking feeder, drops under a stamp, spills over a screen, and slides down a copper table to the tailings flume. Every Comstock mill is built to this order, and the machinery in this one carries it out.

The grizzly

Ore from the mine comes down in ore cars and is dumped onto a grid of iron bars. The bars are spaced a few inches apart, and the whole grid sits at an angle, so rock slides and settles against the gaps. Anything too wide to pass stays on top, and the millman breaks it with a sledge or rolls it back for another drop. In a running mill the grizzly is the first sound, a steady scrape of stone on iron that never quite stops. The bars are worn smooth on the top edge, and the ore side of each bar is polished bright.

The ore bin

Fines drop through the grizzly into the ore bin, a steep-sided timber box built into the mill framing. The bin narrows toward the bottom, where a cast-iron gate closes the mouth. When the mill is at work, the gate is lifted by a handle and the ore slides down onto the feeder. The millman reads the gate by the weight of rock pressing on it: too much feed, and the bin grinds; too little, and the stamps bite on empty steel. The gate is held open by a toothed rack and a pawl, and the pawl drops into the teeth under its own weight.

The feeder

The feeder is a flat pan set between the bin and the mortar box. It shakes on a rod connected to the cam shaft, and each shake moves a layer of ore forward. The motion is deliberately dry at first; water is not added until the rock is inside the mortar. The feeder pushes the ore at the rate the stamps can kill it, which is a slower rate than the men on the floor ever want to believe. The pan rides on a set of leaf springs bolted to the battery frame.

The mortar box

The mortar box is the heart of the battery. It is a cast-iron trough, open at the top, set on timber baulks so the falls of the stamps are absorbed by the building's frame. The stamps rise and drop in sequence, one after another, each head striking the ore bed with a thump that comes up through the floor. Water runs into the box, and the impact turns the rock and water into a pulp. The pulp is driven by the force of the blows toward the front wall of the mortar, where the only way out is the screen. The inner walls of the mortar are smooth and dark from constant friction, and the surface of the pulp in the box has a small standing wave in front of the stamps.

The screen

The front wall of the mortar is a steel screen with punched slots, held rigid by a wooden frame. The pulp presses against the screen while the stamps continue to fall, and only particles small enough to pass through the slots are allowed out. The oversize pieces roll back under the toes of the stamps and are caught again. The screen is the mill's last gate before amalgamation, and the millman keeps a close ear on it: when a screen starts to clog, the splash of the pulp turns from a hiss to a slap. The screen is held by the frame at a slight lean, so the oversize pieces slide back down off its face.

The copper table

The pulp that passes the screen drops onto an inclined table covered with a sheet of copper. The copper has been washed with mercury, and the silver and gold in the pulp wet to the mercury and stay there. The quartz and other waste slide on down the table with the water and off the lower edge. The table is the part of the mill where the order of the flow holds still: the metal stops, the waste goes on. From time to time the millman scrapes the loaded mercury film from the copper and collects it as a heavy silver paste. The lower edge of the copper sheet is bent over a wooden lip, and a thin line of mercury collects along the fold.

The tailings

What leaves the lower edge of the table is tailings, and they run into a launder of planed wood that carries them out through the mill's back door. The launder drops the tailings into an open flume, which follows the hillside down to a settling pond. The pond is a flat bed of sand held by a low earthen dam, and the fine gray pulp spreads across it, losing its water as it goes. The last length of the flume is bolted to the trestle, and a sheet of pulp and water falls from its mouth into the pond.