The techniques of gold mining

In California, the Argonauts ran the sand and gravel through a long tom, or rocker, on the bottom of which were slats nailed across to act as riffles for catching the gold, while the waste washed away.

Another ancient people, the Phoenicians, sailed the Mediterranean and discovered vast quantities of gold, mercury, copper, and such in what we now call Spain. The Romans finally came into possession of this area after defeating the Carthaginians, who followed the Phoenicians.

Some of the gold in Spain was scattered in sands and gravels impossible to recover. So, the Romans developed a method, which we today call hydraulicing. This was developed to tear away the gold-bearing banks of sand and gravel and recover the metal. They did this by damming water at the head of a gravel bank, then releasing it in a deluge over the gravels. If the gravel bank was recalcitrant, they would tunnel under the bank hollowing out areas to reduce support. When the gravel bank collapsed the gravels ran through sluices lined with brush. But this was no ordinary brush. It was wild Rosemary which oozes a sticky gum resin which captured much of the gold. By burning off the brush, the gold could be recovered.


Hydraulicing in California was the same basic technique, with the advantage of a powerful water jet that precluded the need to undermine the gravel. You can imagine the devastating effects this mining had on the land and waterways, however. This method also requires a considerable input of labor and cash, but it was very effective in the pediment highlands, as opposed to dredging, which was most effective in the lower reaches of the rivers.

In the United States, Hydraulicing got its start in 1853 at American Hill, near Nevada City. One E. E. Mattson worked with Anthony Chabot and Eli Miller, to devise a system of damming water at a high elevation, lead it through flumes to the base of the target gravel bank and, by running the water through a narrow-mouthed monitor nozzle under gravitational pressure, the gravel would give way and be carried by the huge volume of water to be treated like river gravels in separating devices. The waste ran off into the watershed, creating havoc downstream. This destructive practice was stopped by the Sawyer Decision in 1884, as it was ruining a vast area of farmland and silting the rivers.

Early Argonauts used shovels to dig the gold-bearing gravels from the bottom of a stream. In California, the gold dredge was employed at times. This used all the established techniques for running gravels through a recovery process just as the old-time prospector did. The only difference is that the dredge was mechanized and could process tons of gravel a day, instead of a few shovelfuls. Once the gold was extracted, the gravel was cast aside like a trail of excrement, spoiling the land. Dredging did not, however, silt up the rivers, as hydraulicing, because as the dredge worked, the silty spoils were dropped back into the river first, then covered with the cobbles and gravel from the operation. This limited the silting of the rivers, leaving only a snaking pile of cobbles in it wake.

More recently, when a gold dredge works, it reclaims the land as it goes. In fact, I spoke to one California dredge operator on the Yuba River not too many years ago. I wondered what his biggest problems might be. I assumed it would be environmental. It was, but not as I expected. He said the biggest problem was beavers! The mining company reclaimed the land by planting shrubs and trees along the new riverbanks. The beavers realized these fresh new trees were great for lunch and ate them, forcing the mining company to plant and re-seed repeatedly.

The Romans also developed a means of hoisting ore. Instead of hauling it out on the backs of slaves, they used a rope and winch system. This later developed into the horse whim, a huge barrel-like wheel being turned by an animal to haul ore up a shaft and to lower men and supplies down a shaft. Today, we use electrical power to do the same thing.

The Romans also used mercury to recover gold. Since mercury will form an amalgam with gold, separating it from any other rock debris with which it is found, the amalgamation process was very popular. When the California Argonauts recovered gold, they would put a small puddle of mercury at the end of a long tom or sluice to catch the gold fines. Using mercury to recover gold is still a valid, through infrequently used, technique.

To crush ore centuries ago, hammers were used. Then crushing devices were developed, primarily the arrastra and the Chilean wheel. Both worked on the principal of dragging or rolling a huge rock over the ore to crush it down. Power was supplied by animal or humans, whichever were cheaper.

In the early days of California, these methods were still used and you can see the remains of such devices at museums today. The arrastra and Chilean wheel were hardly models of efficiency and California's mines soon began to employ new mechanical devices if they could afford them. So, the stamp mill was born.

The stamp mill consists of a series of very heavy steel hammers which could be lifted and dropped by mechanical means usually a water wheel, later a steam engine. Below each stamp was a heavy steel plate tipped at a slight angle. The partially crushed ore was carried by water under these huge stamps, which crushed the ore down fine. Below the stamps was a copper sheet coated with mercury. Mercury was them retorted, which means it was boiled away in a vessel so the mercury vapor could be caught and condensed for use again while the gold was left behind as a nearly pure-gold sponge.

My favorite story about using mercury to amalgamate and recover gold has to do with a potato. Old-timers used to hollow out a potato, put the gold/mercury amalgam in it, wire the potato closed and throw the potato on the fire. The mercury would vaporize into the potato, and the gold sponge would be left behind. The story goes that a prospector decided to get rid of his partner and keep the diggings for himself. He fed his unsuspecting partner the mercury-laden potato!

The early use of fire and water continued into relatively recent times. By the time the Spaniards conquered Mexico, it was still being used in a very limited way, and only in the absence of black powder, or the money to obtain this explosive.

The use of black powder was very dangerous. With the least spark, it ignites. Just packing it into a crack or drill hole can set it off. More dangerous is the act of setting off the blast, since very unpredictable fuses of black powder were the tried and true method of ignition in the early days. The fuse might burn just right...or too rapidly, whereby the miners would be caught in the explosion. Or it might not go off at all, until the miner returned to investigate the cause.

Things got better when the Englishman, Bickford, developed the first dependable fuse that could be timed. His work saved a lot of fingers, arms, even heads. And this was followed by Noble's discovery of dynamite, shortly after the California rush, thus making the handling of explosives almost routine. Yet, either with fire and water, or these more shocking techniques, the goal is the same, Break up the rock for extraction.

Reviewing the development of mining and milling of gold, you come to the realization that the earlier techniques discovered through trial and error, thousands of years ago, in some cases were still valid in mining until 1887. In that year, an event occurred that should be considered the most dramatic change in mining since the development of explosives that replaced fire and water. I refer here to the chemical extraction of gold using cyanide.

Cyanide is a genie in a bottle. It can be more dangerous than black powder, yet it has saved many a mining project, including the great reef mines of South Africa.

Cyanide is a stable chemical compound. It is not explosive or corrosive. Unfortunately, it is a deadly poison, a property that causes environmentalists to go ballistic, and rightly so. Even in dilute condition, as used in gold recovery it can despoil the environment in a flash. Yet, it can also squeeze every last gram of gold from a crushed rock.

It was in 1887 that three men, a metallurgical chemist, J.W. MacArthur, and two doctor brothers, J. and J.W. Forrest, through long experimentation, found that sodium cyanide had as great an affinity for gold as did mercury, itself a quite poisonous chemical. Their discovery had a profound effect on mining. With cyanide, all the gold that had been lost during the wasteful milling processes of the past could be recovered. Old dumps could be reworked. New mines could become more profitable. Low-grade deposits suddenly became viable.

The process is relatively simple: crush the gold ore to a flour consistency, add it to a 0.005 percent solution of sodium cyanide, stir it in an open container, and the cyanide will effect a marriage with the gold. Once consummated, this metallurgical marriage is easily broken by draining off the gold-laden liquid and running it through a vat filled with powdered or flaked zinc. The zinc has an affinity for the cyanide and the gold comes free. With zinc there might be clinging to the gold is easily vaporized off just like mercury from a gold amalgam.

For gold mines, cyanide has been a boon. For the environment it is a disaster if the cyanide genie gets out of the bottle. That is the biggest problem facing gold mining today, keeping the cyanide under control.

From time immemorial, right into the days of the great California Gold Rush and beyond, the mining and milling methods of gold ore have changed but little. They are still valid and useful today. That's why a visit to California's gold country is such a treat. You can relive the thrill of the Rush while enjoying a window into mining history that spans thousands of years.