When it comes to mining gold, there are two methods. One is hard rock mining and the other is placer mining or sometimes called alluvial mining. The easiest method of getting gold is by placer mining and the ways of extracting gold this way really hasn't changed for thousands of years. It was placer mining that started pretty much all the gold rushs you read about. Getting gold in placer gravels is actually quite simple. All you need is a gold pan and a shovel and your in business. If you want to get more gold, you team up with a partner who has a shovel and get yourself a rocker box. If you want even more gold, you get yourself a sluice box or a long tom and maybe another partner to help with the digging. It's all hard bull work and goes well until the easy pickins are gone.
When you are mining alluvial gold the one thing to remember is that gold is heavy and over time this gold ends up on bedrock or some other kind of hard pan material. This depth can be anywhere from a few feet to hundreds of feet down and once you have to start digging deeper the work load just adds up. As a friend of mine once said. Digging deep is an uphill battle. But in order to get to the gold that laid deeper in the ground took more than manpower. It took money and horse power. This is where the dredges of goldfields came into play.
The old pictures you see of these dredges were known as bucket line dredges. These floating machines were monsters. Some of the larger dredges were over 100 feet long and 30 or more feet wide. The depth of the pontoon base could reach 8 feet in height. As an example, Dredge No. 4 that rests on Bonanza Creek in the Klondike, was eight storeys high and two-thirds the size of a football field. it weighed of over 3000 tons. It could dig gravels 47 feet below water level and reach up to 17 feet above water level using iron buckets that each had a 16 cubic foot capacity. This dredge could process 18000 cubic yards of pay gravels every day and these dredges worked 24 hours a day for 365 days a year. It just wasn't in the Klondike that these dredges mined the gold. These machines were all around the world. You could find them working in California after the big rush was over, all the way up to the tundras of Alaska. Some of the very first dredges to be built were in New Zealand and the designs were modified over time and specific to location. Many of these old dredges at first were powered by steam but eventually a lot of them were switched over to electric or internal combustion engines. Each dredge depending on the size could have anywhere from a 6-12 workers and because these machines never quit there would be shifts that worked 24 hours a day.
One of the biggest challenges at the time was getting these huge dredges to the mining location. Take for instance the number 4 dredge up in Dawson City Yukon. This huge dredge was built in Ohio and had to be shipped across the country to west coast where it was loaded onto a ship that took it to Skagway Alaska. From there everything was shipped on the White Pass Yukon rail road to Whitehorse Yukon where it was once again reloaded onto paddle wheel boat and sent up the Yukon River to Dawson. From there it would be unloaded and then moved peice by peice with horse teams to where it was goin to mine and then had to be assembled. This was an amazing feat considering there were no roads and very little infastructure at the time.
The operating principle of these dredges was pretty simple. In the front was the bucket ladder that reached down into the ground. Each bucket was connected to a large chain much like a bicycle chain and as it turned upward, these buckets will scoop up gravels and slowly bring them up towards the dredge. Once these buckets reached the top the contents would drop out into a large steel cylinder inside the dredge. The steel cylinder called a trommel, was perforated with holes and had water pumped inside to help wash the gravels. This trommel was at a slight angle also turned, causing the gravels to be washed and worked towards the end the trommel. All the small rock and bits of gold would fall through these many holes and the larger rocks would continue down this cylinder and fall out onto a conyeour belt or better known as a stacker where these rocks would end up in what is called tailings.
The small rocks and gold particles that fell through the many holes along with the water would flow down a long wide tray call called a slice box. Inside the sluice box would be carpet materials that the gold and concentrates would get trapped. After a certain amount of hours of dredging, the dredge would stop and this carpet material was removed and washed out so the gold could eventually be separated and cleaned. The carpet material would then be replaced in the trays and the dredge would start back up.
Today most of these old dredges have fallen into disrepair. Some of these machines have been taken over by local governments and are being rebuilt as heritage pieces and others have been known to start rebuilding and modernizing them for actual mining purposes again. If you eve wanted to go and actually see one in person, you could go to Dawson City and tour the number 4 dredge that is now owned by the parks. There are other dredges also like the Dredge 8 in Fairbanks and also in Sumpter Oregon. If your an old mining fan and haven't seen one of these old dredges, make sure you take a visit one day.