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 Post subject: A dead-simple "new" specific gravity technique
PostPosted: Fri Jan 27, 2017 10:50 am 
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Hello everyone,

And first of all to Barbra and other admins, I'm not sure whether the present post may constitute an infraction of GO rules, good taste or other general principles inasmuch as it refers to my magazine-ish workshop and techniques page on facebook. If it is questionable, by all means delete, move or do whatever your admin discretion suggests.

That being said, I've been experimenting with a "new" technique of specific gravity determination. It's dead simple, and dead obvious when you think of it. Although I wouldn't want to use it (yet) for peer reviewed analytical work, first experiments suggest that it's amply accurate enough for at least a kind of rough, general everyday use and in addition, it seems convenient enough that you could probably use it in the field. I say "new" in quotation marks because I cannot imagine that this technique has not been done before by somebody else. And yet I've not seen it described in any gemological literature or discussed in any internet gem group.

The details are here - on my facebook magazine page, "Luddite Laboratories." Why not check it out?

https://www.facebook.com/notes/luddite- ... 0689296154

Cheers all,
Hans Durstling
Moncton, Canada

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 Post subject: Re: A dead-simple "new" specific gravity technique
PostPosted: Fri Jan 27, 2017 1:40 pm 
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Absolutely brilliant.


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 Post subject: Re: A dead-simple "new" specific gravity technique
PostPosted: Fri Jan 27, 2017 8:02 pm 
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This principle has been used for a long time but has gone out of fashion this past couple of decades. Look up "Pyncnometer" in any (oldish) physics textbook.


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 Post subject: Re: A dead-simple "new" specific gravity technique
PostPosted: Sat Jan 28, 2017 12:00 am 
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There is an ASTM method that applies to soil samples, which would apply to gemstones as well.

This is a link to a nice u tube put out by a university that describes the process, as well as the formulas.

I like your write-up.

https://www.youtube.com/watch?v=_PqKWiUlbYg


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 Post subject: Re: A dead-simple "new" specific gravity technique
PostPosted: Sat Jan 28, 2017 12:13 pm 
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Let's face it, for the average cutter, enthusiast, buyer, what-have-you, who does not have a separate gemmological lab area in his home, determining specific gravity by the usual suspend-the-stone-in-water method is likely to be somewhat of a nuisance procedure, as often postponed as performed.

But the method I describe is almost as simple as owning a UV light - "let's see if this glows, let's see if that glows...", so simple that it's actually fun.

Moreover since the volume of the container is always the same you only need a one-time weighing to establish its filled weight. That done, all that's needed is to weigh the unknown stones, put them in the (full, of course) container, weigh again, do the calculation, and voila. You could do that at the market in Mombasa or Mozambique; you could make two or three containers of different sizes for different size stones or parcels...and so on.

It's the simplicity and convenience of it that make me wonder that I've not seen this method described or discussed, that it isn't more widely used.

Cheers all
Hans Durstling

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 Post subject: Re: A dead-simple "new" specific gravity technique
PostPosted: Sat Jan 28, 2017 12:40 pm 
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Sorry, another terse "Dr Hannemann" style post, my excuse is that it was late at night. So, to expand it into an intelligible form - the pynknometer is an apparatus for measuring specific gravity to a high degree of accuracy. It consists of a finely made glass vessel, perhaps 25 or 50 millilitres in capacity, complete with a precision fitted, ground glass stopper which has a very fine hole drilled through the centre. In use, the specimen is weighed in air, the pynknometer is filled with water, dried and the hole checked to see that the water has completely filled it. It is then weighed. The specimen is then placed in the pynknometer, which is dried and the hole checked to see that the water has completely filled it again. The pynknometer and specimen are then weighed again.
Subtracting the weight of the specimen from the weight of the pynknometer and specimen and then subtracting the adjusted weight of the pynknometer from the original weight of the pynknometer gives the volume (corrected if need be for temperature). Divide the volume into the weight to give the specific gravity.
I think the method fell into disuse with the ready availability of small, sensitive, strain gauge, balances, which make the "bridge" method much easier.


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 Post subject: Re: A dead-simple "new" specific gravity technique
PostPosted: Sat Jan 28, 2017 1:39 pm 
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I know what a pycnometer is and indeed mentioned it in the post. Perhaps you missed that. The hole in the cover should have signaled this also.

"Small sensitive strain gauge balance" - well right, I'll just go get mine. I use a cheap Chinese gem scale. Hands up, who else? Let's be honest here. Also I make jewellery. That often involves seriously picky wire work. And still I find it a pain in the arse to make that wire bridge. And stone basket and whatnot.

At the market in Mozambique am I going to take my small sensitive strain gauge balance out of its foam protected carrying case, accurately level a table to put it on, take my pliers and the spool of wire out of my pocket to make the bridge and stone cage (most likely while swatting the flies off)? Slim chance. A small glass pot and a pocket scale, now that's different.

The point of the presentation is simply that the method outlined is real-world simple, real-word convenient.

Cheers again
Hans Durstling

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 Post subject: Re: A dead-simple "new" specific gravity technique
PostPosted: Sat Jan 28, 2017 6:38 pm 
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I prefer the bridge method above all others for ease, simplicity and accuracy. I have made myself a folding kit and can be ready to measure in 30/40 seconds, including leveling the balance.


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 Post subject: Re: A dead-simple "new" specific gravity technique
PostPosted: Sat Jan 28, 2017 10:57 pm 
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Okay, so maybe this is a joke? EVERY reference book I have, including Schumann says the S.G. for quartz is 2.65. Where did you get 2.73 from?

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 Post subject: Re: A dead-simple "new" specific gravity technique
PostPosted: Sun Jan 29, 2017 12:00 am 
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Hahaha, it is! That makes things both a little worse and much better.

That said making a permanent wire basket isn't hard. I like the niftiness factor of this method though.


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 Post subject: Re: A dead-simple "new" specific gravity technique
PostPosted: Sun Jan 29, 2017 1:15 am 
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Let me look in Schumann again. Hold on. Actually no, the 2.72 was a google search. Let me see if I can duplicate that. Damn. 2.65 on google. Mea culpa.

Still, it doesn't invalidate the process. If you read the story (and obviously you did), you'll have seen that my initial runs with more "bulk-ish" quartz were in the 2.65 range, and the one where I used the small cut stone - which I expected to be less accurate - now, and thanks to your checking, turns out to have indeed been the less accurate.

Peer review is a good thing not a bad thing! (Except better done before publication not after; saves eating some crow that way; I shall go back and do some after-the-fact editing, and not to conceal the error, but to point it out.)

Cheers again
Hans Durstling

PS - Also "facetor" may I in the revision give you credit for having spotted the error, and if so, may I have your real name by PM (either here or on facebook). Thanks. :)

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 Post subject: Re: A dead-simple "new" specific gravity technique
PostPosted: Sun Jan 29, 2017 10:25 am 
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Hans,
After that Lapis chalice you can do no wrong!

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 Post subject: Re: A dead-simple "new" specific gravity technique
PostPosted: Sun Jan 29, 2017 11:03 am 
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You are kind, Bob, you are kind. Haven't made the revisions yet but will do. As a side note I think the present "incident" actually might be a good one for the science textbooks, as a living example of how peer review works, and particularly in these days of "alternate facts."

-H

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 Post subject: Re: A dead-simple "new" specific gravity technique
PostPosted: Sun Jan 29, 2017 4:54 pm 
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Alan F. wrote:
I prefer the bridge method above all others for ease, simplicity and accuracy. I have made myself a folding kit and can be ready to measure in 30/40 seconds, including leveling the balance.



I see an advantage to this approach if you are mobile. It would fit in your pocket with a scale which wold make it very portable. With just 2 quick weights you can get the sg. (assuming that the container empty and full are known

I can see a easy smart phone app, where you input the container weight dry, container weight with water only (these can be stored) then the stone dry weight and stone in the water and the dry stone.

I think that you should be able to make a cheat sheet with dry weight and wet weight and a line would be the SG. No calculations then needed. You could have bands of likely materials (for those of us who cannot remember them all)

The diagram would not be as accurate as a full calculation, but it could probably get you in the ball park, which is all you need if you want to do a sanity check.


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 Post subject: Re: A dead-simple "new" specific gravity technique
PostPosted: Sun Jan 29, 2017 11:26 pm 
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I finally figured out where the SG of 2.72 came from: right here, first thing that came up on a google search.

Attachment:
spec.g-quartz.jpg
spec.g-quartz.jpg [ 110.21 KiB | Viewed 757 times ]



Which makes the results of my first two trials, 2.64 and 2.65 (where the specimen was large in relation to the container) more, not less gratifying, and the result for the 3 carat faceted amethyst (2.73) exactly in line with my expectation that the determination would be less accurate when the stone was small in relation to the container.

Would a smaller container in fact yield better results? Might as well make a container and see.

Attachment:
sm-pot-GO-7488 copy 2.jpg
sm-pot-GO-7488 copy 2.jpg [ 244.21 KiB | Viewed 757 times ]


Attachment:
sm-pot-cut-CU-GO-7490 copy 2.jpg
sm-pot-cut-CU-GO-7490 copy 2.jpg [ 234.82 KiB | Viewed 757 times ]



After grinding the rim of the glass pot and its gem-pot plastic cover to a good snug fit I did three tests with the same amethyst (0.60 grams) used in the facebook story, and a little 0.77 gram chip of synthetic ruby.

Oops - have just hit the 3-attachment maximum. OK, continued next post.

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