2010 March

First developed in the goldfields of the state of Georgia, the rocker was an important gold mining tool. At the very dawn of the Gold rush to California, the rocker box also known as a cradle was perhaps the most used piece of gold prospecting equipment. For a time it was perhaps even more important that the gold pan. Mostly this was because the miner could make a rocker for himself in the field from rough sawn lumber cut in the forest. They are also easily portable. Rocker boxes were also popular during the Klondike gold rush for working the hillside placers that were far above the creeks.

The ‘rocker’ is a box with a hopper about 3 to 4 ft. long and 1 to 2 ft. wide, sloped like a cradle, and is mounted on semicircular pieces of wood and worked by a, handle to give it a side motion; and it is also inclined so as to carry the material down to the lower end, which is open. At the upper end is a small hopper that may be removed and which has a sheet-iron bottom perforated with 1/2 -in. holes. Under the hopper is a canvas apron or tray inclined toward the head of the box but touching neither end of the hopper-box. Several wooden riffles are placed across the box. The material is fed into the hopper and screened through by water poured on top; the lighter material is carried over the end, while the riffles in the box catch the gold and magnetic sand. This concentrate is cleaned out and panned at the end of the operation. The rocker is used for the same type of work as the gold pan in that it is mainly a prospecting tool. A man is able to wash 3 to 5 times more yardage than with the gold pan, and the use of the rocker eliminates much of the backbreaking strain of continuous panning. On the other hand, the easy mobility of the pan as a prospecting device is lost.

So why might a modern prospector be interested in building his own rocker box? The principal use of a gold rocker is for mining small deposits where water is scarce. It is not really a desert device and it does use some significant water, but not nearly as much as a sluice. In a rocker, gravel requires about three times its own weight of water to wash it. So perhaps the best use is in streams and waterways with very little water – where some water is present, but not enough to run a sluice box. If enough flowing water to run a sluice is present, a sluice is faster and easier to run than a rocker. The rocker is only a primitive machine, having a capacity but one-fifth as great as that of the sluice box, but because it is cheap, requires but little water, and saves a high percentage of coarse gold, the rocker will continue to be used in many districts.

The operation of a rocker consists of shoveling gravel onto a screen or grizzly, pouring water over it from a dipper, and at the same time giving the device a back-and-forth rocking motion. The grizzly retains all the oversized stones, which are removed by hand when they have been washed clean. The operator briefly examines the oversize rock to be sure no large nuggets or gold specimens are being tossed out. The cradle must be placed on an inclination while being worked, and under the influence of the continued side-to-side rocking the dirt is quickly disintegrated, passes down through the hopper grizzly and the water and the undersize fall down onto the canvas apron which saves most of the gold and places the remainder at the head end of the trough. From the apron it is conveyed to the inner end of the cradle floor (the sluice box like section of the rocker), from which it flows over the riffles, or bars, and out at the mouth. Riffles, canvas, blankets, corduroy, burlap, or cocoa matting with expanded metal have been used to cover the bottom of the trough and all have met with varying degrees of success in saving the gold. The combination of cocoa matting covered with expanded metal lath has proven to be quite effective for most gravels. The frequency of cleaning up depends on the richness and character of the gravel, but clean-ups are usually necessary two or three times a day. The hopper is taken off first, then the apron is slid out, and washed in a bucket or tub containing clean water, and finally the gold is collected with a spoon from behind the riffle bars, and panned out.

The rocking motion used should be sufficient to keep the gravel disturbed, allowing the gold to settle out, but a too vigorous movement will cause a gold loss. The gravel bed should be shifted slightly with each motion and should be evenly distributed across the trough. Generally speaking, the rocker is not known for its ability to save fine gold, but with careful and expert manipulation, decent fine gold recoveries can be achieved. Tailings from both rockers and sluice boxes should be occasionally panned to check for gold losses. When gold is found near the lower end of the rocker or sluice box, the potential for losses should be investigated.

Because there is no one “right” design for a rocker box, I am not actually presenting specific plans, but on my website I am giving you the information you need to plan, design and build your own rocker box if that’s what you decide to do. My recommended design for a rocker is to start buy building a sluice box 40 inches long, 16 inches wide on the bottom, sloped like a cradle, and with rockers at each end. The hopper would be 16 inches square and 6 inches deep, with a sheet metal bottom made of perforated steel with 1/2-inch holes. This hopper box needs to be designed so it can be removed for clean up. A light canvas-covered frame is stretched under the hopper, forming a riffle. Square riffles of wood or steel are placed across the bottom of the sluice portion of the rocker. Curved feet are placed underneath the sluice portion of the box to allow it to be rocked back and forth. Historically, rockers are built of wood, as the early prospectors built them. However, there is no reason that a rocker could not be built from sturdy heavy gauge sheet aluminum. It would be much lighter that the wood version. Remember that wood also absorbs water, and water logged wood is much heavier than dry wood.