Wednesday, March 19, 2014

Waterjettting 19b - California gold mining

While mankind has directed the flow of water against earth and rock faces for millennia, as a way of eroding and removing material, it was not until the days of the Gold Rush in California around 1850 that the idea of confining the water into a hose, and through a nozzle crystallized.

Gold was originally found in the gravel in 1848, when James Marshall was helping John Sutter build a sawmill on the banks of the South Fork of the American River, in what is now Coloma, CA.

Figure 1. Sutter’s Mill on the American River (Replica by California Parks ).

The news that gold had been found in the tailrace led into what has been referred to as the greatest mass movement of people in the Western Hemisphere, as people flocked to California over the next few years, during the period of the California Gold Rush.

Figure 2. Location of Coloma, CA (red circle) relative to Sacramento and San Francisco – Lake Tahoe is by the 395 sign in the upper right. (Google Earth)

As the prospectors panned the gold from the stream beds, so they moved north east along the valleys and rivers, seeking the sources of the gold particles that millennia had washed down from the Sierra Nevada. One such source was found at American Hill, just north east of Grass Valley. Here the gold was found in beds of a weak sandstone, lying relatively close to the surface.

Figure 3. The American Hill Diggings, with plaque. The original height of the hill can be seen in the background.

The gold settled to the bottom of the sandstone, and so the miners would tunnel into the side of the hill, seeking to find the richest layer. Unfortunately as you dig out the bottom of a hillside, the overlying rock has a habit of falling down, with mildly fatal results to those caught in its path. This made mining somewhat dangerous, given the soft nature of the rock as Edward E. Matthison found when he was nearly buried when he was working the property. So with partners, he decided that a more remote method of digging out the gold was needed. So, with the help of a local blacksmith named Miller, he fashioned a nozzle on the end of a canvas hose he ran from a water reservoir at the top of the cliff (initially a nail keg) and used the resulting stream to wash the ore (and overlying rock) into a channel that was later turned into a flume, with a series of strips to catch the gold.

Figure 4. Early hydraulic mining

The method had many advantages since, in the process of washing the rock from the solid it was broken down into individual particles. This separated the gold, sand and clay particles, so that while the gold particles would be trapped in the flume, the lighter sand and clay particles would be carried further downstream with the water. By 1853 they were paying a water bill of $153 a week (with water at $0.75 per miners inch this meant they were using 2,000 gal/min) but making the four partners a profit of $50 a day. Larger and larger monitors (the name given to the nozzle and pivoting assembly) were built, throwing water at greater distances, and mining at much faster rates.

Figure 5. Monitors at work at the North Bloomfield mine.

This, in turn, required increasing amounts of water, and this was carried in flumes down through the Sierra Nevada, with agreements being made between companies for distribution, collection and the passing on of water. The nozzle diameters of some of the larger monitors grew to more than 8 inches, and they were capable of mining tens of feet from the operator.

Figure 6. Later design of monitor. The wooden beam usually had a box holding rock on the other end in order to balance the weight of the nozzle section.

The nozzles were made longer, as they were made larger, in order to get the jet to throw to greater distances, but this made steering and control of the jets more difficult. The gooseneck swivel was invented in 1855 to help swivel the nozzle, and a monitor operator noted that when he stuck his shovel into the stream of water it deflected the nozzle. This was Dave Stokes at the Malakoff mine and led to the invention by his Supervisor, Henry Perkins, of the rotating system for sprinklers that is still used to this day.

Figure 7. Modern rotating sprinkler showing the deflection plate. (Aliexpress )

The largest mine in the region was the Malakoff, and in the region around it there were some 425 companies operating and, between 1871 and 1880 they produced $121 worth of gold (at the price of the day).

But there was costs to this operation outside of just the mining ones. For while the gold was captured in the flumes, the sand, and more particularly the clay, was carried in the water until it became less turbulent. And that was when it reached the Yuma, American and Beam rivers flowing out of the Sierra Nevada and down towards Sacramento. As the water slowed, so the clay precipitated out, and the river beds filled with sediment. Thus, when the rains came, the water overflowed its banks, flooding the neighboring fields.

Foregoing the fact that it was the mining that had brought the farmers and many others to the region, the floods were not acceptable, and following the floods of 1880 there was an increasing effort to contain the mining sediments. This led to the court ruling by Judge Sawyer in 1886 restricting the practice of hydraulic mining, and the technology fell into abeyance. It was restarted at the time of both World Wars, but in recent times there was only one small mine that had been “grandfathered” still in production. Its role in developing California is not greatly recognized at present, and the remaining legacy is more seen in the vertical bluffs and large flat areas of mined sand that are left north of Grass Valley, together with the old wooden water flumes that still thread their way around the edges of the valleys.

Figure 8. View of the Malakoff Diggings

I’ll talk more about the spread of the technology next time.

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