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  • Writer's pictureLori Dee

To Classify Or Not...


To classify or not to classify, that is the question. Some expert miners will tell you that you must classify your materials. And other experts will tell you that it is unnecessary. I plan to show you both sides. I think the deciding factor is the amount of risk you are willing to accept. That is, how much gold are you willing to lose for the sake of speed and reduced work?


First, let's clarify what we are talking about.


"Classifying" is just a way of sorting your materials before further processing. Many dredges and high bankers use "grizzly bars" which are just metal bars mounted at a steep angle with a gap between them. As the material is shoveled or dumped onto the bars, the smaller material will fall through the bars, while larger rocks just roll off. Often a spray bar is mounted so the rocks are rinsed off before being discarded.


The most popular method of classifying is using screens, sometimes called sieves, of various sizes. In larger operations, a drum made out of a heavy mesh called a trommel, is used. Material is fed into a hopper which feeds into the trommel. As the trommel rotates, smaller material falls through for further processing, while the larger pieces roll out the end to a tailings pile.


For small operations, portable screens are used. Some screen kits are designed to stack on top of a 5-gallon bucket. This allows the classifying and separation of the different sizes. Some screen kits are smaller for processing small amounts at a time.


Screens are sized (their mesh size) using a numbering system that is easy to learn. A number 2 Mesh means there are two holes per inch in the screen, or the hole size is 1/2 inch.


2-Mesh = 2 gaps per inch = 1/2" gaps

4-Mesh = 4 gaps per inch = 1/4th" gaps

8-Mesh = 8 gaps per inch = 1/8th" gaps

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100-Mesh = 100 gaps per inch = 1/100th" gaps


If something is said to be 4-mesh size, that means it fell through the 2-mesh (smaller than 1/2") but got caught in the 4-mesh (larger than 1/4"). Sometimes you will hear someone using "plus" or "minus" and a number. A +100 mesh is something that was caught in the 100-mesh. A -100 or 100 minus mesh is something that fell through (smaller than) the 100-mesh screen. Usually, the minus indicates a size you did not screen for. So -100 means it fell through the 100-mesh and I did not have a smaller mesh to catch it. So we know it is smaller than 100-mesh, but we don't know how much smaller. It could be 200- or 250- or 300-mesh, or smaller.


Standard sieve and wire mesh charts will disagree with these size figures because some uses of wire mesh or sieves require more precise measurements. So those charts will account for the size of the mesh wire as well as the size of the gaps in between the wires.


Classifying material can be done wet or dry. I prefer wet so that water rinses off the material into the lower screens and there is no dust. If you are dry panning or have some other need to keep material dry, the screens will work just fine using them dry. You might want to wear a dust mask. Silicosis may sound funny but it is no laughing matter.


When you are classifying if you find three or more "colors" (gold) in your screen, use a larger screen.


Back to our original question: Should you classify or not?


There are two different sides to this. I have heard arguments both for and against by experts that I trust. These guys know what they are talking about, and both are correct from their perspective.


If you are just using a pan, you do not NEED to classify material first. It does make panning easier if you do because you are getting the bigger stuff out of your way. But then you are investing extra time and work to do the classifying.


Remember the Golden Rule: If all things are equal, gold rules. In other words, if everything in your pan is the same size, gold will be the heaviest of all and will sink to the bottom easily. If you have large rocks in your pan, they could be the same weight as smaller pieces of gold. But then again, if you have a good panning technique, no gold is going to escape your gold pan whether you classify it or not. If you are classifying, make sure you check your screens very carefully before throwing out those rocks. I found a rock about the size of a dime that didn't look like gold, but it had a lot of gold inside it. I'm glad I caught it before I tossed it. There is a picture of it in my Gold Gallery and another of it magnified 40x so you can see the gold. If I am just sampling in the field, I don't classify. I save time and can pan many more samples and be on my way. But if I am bringing material home to sample, I will classify it so I have a very thorough understanding of all of the contents of the sample. That requires classifying the material and examining each size. I classify using mesh sizes 2, 4, 8, 12, 20, 50, and 100. Gold is not visible to the naked eye when it is smaller than 250-mesh so I don't bother screening smaller than 100. At 100 minus, the material is like the dust in your vacuum cleaner. It is really tiny stuff.

If you are using a dredge, you won't be classifying materials except through whatever system your dredge uses, (grizzly bars, expanded metal, punched metal, etc.). That is because of the way that dredges function.


How you run your sluice will determine if you should classify material or not. Doc over at Gold Hog came out with a Stream Sluice that he claims requires no classifying. I own one and have used it many times. It works better if you classify first. I will explain why.


Gold Hog produces sluice/dredge mats for big commercial operations, so it makes sense that those customers will spend very little time and effort on classifying, (they mostly use dredges, which I discussed above). Other large mining operations have different ways of processing the ore. Doc claims that large rocks just roll right out of the sluice and do not affect the exchange happening down in the riffles. Yes... but... To roll a large rock out of the sluice, your water pressure must be pretty high. That puts you in danger of losing fine gold. Here in the Black Hills of South Dakota, the rocks are usually flat, so they don't roll. They sit on top of the riffles and that affects the water flow and disrupts the exchange happening below. So my recommendation is to classify anyway. Get those rocks out of your way. Now, you don't need to get everything down to a 100-mesh size. The size of your material is determined by the riffles in your sluice.



 

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