meta99
has rocks in the head
Ohio Flint Ridge flint
Member since October 2010
Posts: 540
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Post by meta99 on Jun 13, 2012 18:43:06 GMT -5
This is how I check my faceting rough's specific gravity. This technique can be adapted for any stone/rock that you can hold suspended in water. First I glue a thread to my rough with CA (cyanoacrylate) and weigh it: In this case, 25.33 grams. Next I place a cup of water on the scale and zero it out. (If your scale will not tare, then record this measurement and use it to subtract form the next measurement to get the difference): Finally, dangle the stone on a string into the water and record the new weight: In this case 9.71 grams. To calculate specific gravity, divide the first number by the second: 25.33/9.71= 2.61 (the specific gravity of a synthetic faceting stone called Jewel Lite of Laser Gem. Sue
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Post by Jugglerguy on Jun 13, 2012 18:45:29 GMT -5
Aren't you lessening the weight by holding the string?
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meta99
has rocks in the head
Ohio Flint Ridge flint
Member since October 2010
Posts: 540
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Post by meta99 on Jun 13, 2012 20:24:09 GMT -5
I lower the rock until the string is lax so that the water is the only thing holding it up. Here is a link that describes it better than I did: www.prettyrock.com/specific-gravity-calculator.htmI also have an analytical scale that measure to 0.0001 gm and it is surprising how accurate my little $10 scale works... Sue
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Post by helens on Jun 13, 2012 21:08:36 GMT -5
Interesting method! I love all these posts showing different ways of doing something:)!
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Post by Pat on Jun 13, 2012 21:12:13 GMT -5
I could do this!!!
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carloscinco
fully equipped rock polisher
Member since July 2008
Posts: 1,639
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Post by carloscinco on Jun 13, 2012 22:14:15 GMT -5
I just can't wrap my mind around that.
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itsandbits
freely admits to licking rocks
Member since March 2012
Posts: 825
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Post by itsandbits on Jun 13, 2012 22:47:18 GMT -5
Aren't you lessening the weight by holding the string? what you are actually measuring is what that amount of water weighs; the water that is deflected by the volume of the material you are putting into it so the string makes no difference as long as you keep the material off the bottom. This is your 1 SG. then the weight of the dry material is weighed and the division of it or the amount of times the 1 SG of the water goes into it is the real SG of the material. probably more confusing to think of the explanation then how to get the answer. I don't understand the way lots of things work but I still use them. we had this same topic 3-4 days ago
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Deleted
Deleted Member
Member since January 1970
Posts: 0
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Post by Deleted on Jun 13, 2012 22:55:07 GMT -5
I can not grasp it either. If I dangle a stone in the water the string is not going to be lax.
If I am doing ten stones and I have to glue a string to each stone then remove the glue it would add a lot more time than just placing it in the hanging basket. Another problem I see is the limit of a lot of scales. The first scale I used would only go to 100 grams. If I put a glass of water on the scale it would max out the scale. I bought a 500 gram scale and still have to cut a piece off some of the stones to keep from maxing out the scale.
I could see your method working great if you are weighing only small stones and you have a big enough scale for a glass of water but for the nephrite I do not pick up stones that small.
Looks like some draw backs to both methods so your post gives people a second choice.
Thanks for the other point of view. Jim
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Post by 3rdrockfromthefun on Jun 13, 2012 23:42:17 GMT -5
This explanation, and the one provided in the link, are confusing. The water cannot "hold up" the stone - if it could there would be no need for the string. If I'm understanding what's going on here, and as pointed out by Jim's method: forum.rocktumblinghobby.com/index.cgi?board=rocksidlib&action=display&thread=53206then you need only displace the water with the stone (which you're doing - unless you're letting it rest on the bottom - "the string is lax" - in which case you're displacing and adding back part of the original weight). Which is it? Jim's method weighs the stone and then suspends the stone in the water, thereby measure the effect on the water's weight by displacement from the stone, and returns a ratio of the two. I'm not sure either is correct (in the literal, non sarcastic sense) but it would help if you could clarify how you obtain your second weight. As stated - the water cannot hold up the stone. So suppose all your samples come to you as polished spheres (maybe the head of the lab wants to insure there are no trapped air bubbles in holes in the material) - do you release the string once the sphere is in the water or do you suspend the sphere in the water (off the bottom)? Sorry to be a pain but this could be really handy - but only if there's zero gray area in how to do it. Another way to think of it would be - if you were writing robotics code for this to be done by a mechanical arm, how would you tell the arm to get that second reading? Thanks guys, you and Jim - this is neat stuff!
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metalsmith
Cave Dweller
Member since October 2012
Posts: 1,537
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Post by metalsmith on Feb 1, 2013 13:33:54 GMT -5
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