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Post by Deleted on Jun 10, 2012 10:15:35 GMT -5
I thought some of you would like to know how to do a specific gravity test but thought it was a difficult process. It is quite simple if you have an accurate scale. I test all of the rocks that look like nephrite and some others to see if I can get one step closer to getting an ID. The SG weights overlap and you can not get a positive ID from just this test but it will eliminate some things and maybe with this test and some other tests like hardness you can get a lot closer to an ID. Here is how simple it is. I made a hanging basket with craft sticks and real fine copper wire. I clamped a board to a table so I had room underneath for the basket. Put the basket on the scale and turn the scale on. This will give you a zero reading so you are not weighing the basket along with the rock. Put the rock in the basket and write down the weight. Hold a container of water (very still) up high enough so the stone is covered in water. Write down the weight of the stone while it is in the water. If the stone has a lot of cavities try to turn them up so they fill with water instead of having air trapped underneath. Subtract the floating wet weight from the dry weight. Divide that answer into the dry weight. That answer is the SG of that stone. Here are some that fell into the range of nephrite which is about 2.92 to 3.3. That is a pretty wide range and a lot of other things fall into it but the second photo is some that I eliminated. If I did not explain this very good tell me and I will try to answer any questions. I have done this test on quartz which I can ID by eye and the accuracy is right on the money of 2.65. I got this method out of a cheap little geology book and I have not seen it anywhere else which surprises me. In the last year I have used it a lot and eliminated a lot of rocks from my nephrite pile. Jim
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Post by mohs on Jun 10, 2012 11:03:32 GMT -5
Eureka Jim! Cool setup
so once you get the weight of the rock dry submerging the rock in water changes it weight?
The rock is always heavier submerged than dry?
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Post by Deleted on Jun 10, 2012 13:29:39 GMT -5
hahahaha Ed. To a certain degree the rock tries to float and is lighter when it is in water so it weighs less. I am not sure why but that is how it is. So you subtract the wet weight which is lighter from the dry weight. Then the answer from that is divided into the dry weight.
I should add here that this will work with any unit of weight. Pounds, ounces, grams, carrots.
Here is an example of one rock that I did.
193.oo grams dry weight 125.50 grams rock submerged in water 67.50 grams dry weight minus submerged weight
193.00 divided by 67.50 = 2.68 So the specific gravity is 2.68
Sometimes I am not so good at explaining things. Jim
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Post by mohs on Jun 10, 2012 14:30:00 GMT -5
Well I think your doing a fine job with explanation the pictures are real helpful too I'm fascinated with the process and the science of it something I like to try 3 thing I lack are space & time the third is just about everything else that is necessary Ha Ha mostly
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Post by mohs on Jun 10, 2012 14:36:47 GMT -5
O i might add if you run naked into the public square yelling Eureka ya might get arrested !! you could only get away with that in Archimedes days
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Post by Pat on Jun 10, 2012 14:53:39 GMT -5
Jim, thanks for the easy-to-understand explanation.
Ed, I should tell you that about 10+ years ago, a student at UC Berkeley (CA) exercised his right to go naked everywhere. He was a tall handsome young man, but his presence made it hard for others to concentrate in his classes. Berkeley is very liberal. The press kept up with him. He committed suicide several years ago. He was known as Naked Guy. Here's a link:http://en.wikipedia.org/wiki/Andrew_Martinez
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Post by mohs on Jun 10, 2012 15:07:43 GMT -5
That's a sad & interesting story, Pat never heard of him thanks for the link Ed
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Post by Deleted on Jun 10, 2012 16:26:09 GMT -5
hahaha Ed. If you ran through the street yelling ANYTHING you would NOT get arrested. There would not be anyone to arrest you because they would be rolling around on the ground laughing to hard to do mostly anything. Jim
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itsandbits
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Post by itsandbits on Jun 10, 2012 16:38:29 GMT -5
different version of the same thing Weigh the stone dry, put a container of water on your scale that is big enough to emmerse your stone in completely without it spilling over, zero the scale with the container of water on it. hang the stone in the water emmersed completely without touching bottom and read the weight. devide the dry weight by the wet weight for the answer.
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Post by mohs on Jun 10, 2012 16:42:17 GMT -5
well... maybe in my case they would need a loupe to prove my nakedness wait a minute !! wait a minute ! I'm a ROCK! with very a specific gravitas mOostly !!
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Post by Deleted on Jun 10, 2012 16:54:12 GMT -5
hahahahahaha Now we are getting into an area where I plead the fifth. Jim
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Post by 3rdrockfromthefun on Jun 10, 2012 16:55:05 GMT -5
Very neat!
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Post by mohs on Jun 10, 2012 18:51:52 GMT -5
hey Jim have you an I.D. on 3.14 ?
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Post by Deleted on Jun 10, 2012 19:37:11 GMT -5
I prefer cherry pi. But nobody has ever topped my mothers butterscotch pi. Jim
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Post by mohs on Jun 10, 2012 19:48:43 GMT -5
so what your telling me is that you have some of that rare PInite? I heard it was going around mostly
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Post by Deleted on Jun 11, 2012 8:44:15 GMT -5
My mother died a long time ago so I do not get PI nights any more. I do have PI days once in a while when I travel with my circle of friends and we have to divide up and go our separate ways. I always take the circuitous route home. Jim
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Post by mohs on Jun 11, 2012 11:58:19 GMT -5
Hi Jim This subject is taking the long way around Ha Ha
I read an account about an old fence line built in perhaps in the 1700’s When the distance between the posts were measured the uncanny number that came up was like nine feet --four inches. Why this odd number? Why not a rounded number like ten feet?
Well the builders used a cart and used the revolution of the wheel on the cart to mark out the distance for each post | | | | | The would come out to PI X 3 mostly
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Post by Hard Rock Cafe on Jun 11, 2012 12:43:32 GMT -5
Just to clarify the matter of specific gravity and "floating" a little: You're actually comparing the density of the stone with the density of the water. If you were to put the rock in the water without the wire suspending it, it would obviously sink, not float. However, because the water has some density (much more than air), it provides a little resistance to the rock sinking. The rock weighs the same whether it is submerged or out of the water, but the water is exerting force (buoyancy) against the rock. If you were to weigh the rock on top of the scale, weigh the water on top of the scale, then weigh both together on the scale (rock in the water or out, it doesn't matter), you would see that the combined weight is equal to the sum of the individual weights.
I would actually try to put the container of water on a table or stand underneath the rock so that it was absolutely still, rather than moving and changing the force characteristics.
Sorry to be a nerdlinger scientist type, just thought it might help to clarify what is actually happening.
Chuck
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Post by Deleted on Jun 11, 2012 14:10:23 GMT -5
Trying to float and buoyancy seems like the same thing to me. A person would have to be pretty dumb to think that the rock sitting on the bottom of the cup would weigh less. It only weighs less when it is suspended. If you want to get really technical the stone does not weigh the same when it is sitting on the bottom of the cup because the water is still TRYING to float the rock but the force of the water pushing the rock up is also pushing the water down making it equal out. This could be proven with a waterproof scale that you could put in the bottom of the cup but I do not have one. lol
I could build a fancy thing on top of the table so the cup was sitting still but for a quick and easy way to check SG holding the cup gets a pretty accurate reading if I put my hand against the clamp while I am holding the cup. The older I get the less accurate it will be especially if I have had too much coffee. lol Rock hound accuracy is a lot different than scientific accuracy, for me anyway. Jim
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Post by Deleted on Jun 11, 2012 16:59:52 GMT -5
That is very interesting. I never would have thought of marking out distance that way but then I have a tape measure. Thanks for bringing that up. Pretty cool fact. Jim
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