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 Post subject: Tutorial on Specific Gravity Using the Hydrostatic Method
PostPosted: Sun Feb 26, 2012 5:50 pm 
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A brief tutorial I made several years ago:



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 Post subject: Re: Tutorial on Specific Gravity Using the Hydrostatic Metho
PostPosted: Tue Feb 28, 2012 11:39 am 
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As good as the first time I saw it! =D>

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 Post subject: Re: Tutorial on Specific Gravity Using the Hydrostatic Metho
PostPosted: Fri Mar 02, 2012 7:08 am 
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Very good tutorial! =D>

Additional point: If you have a digital scale with large weight capacity (some 500ct min), you can also use easier setting for SG measurements.
Put a cell with water directly on the scale and press “T” button to put it on zero.
Now just introduce the stone placed on wire spiral into the water. It can be hooked to some external support or simply supported by hand! (less precision).
In these conditions the scale will give you directly the value of Archimedes force, equivalent to the weight of water displaced by the stone, and iqual to the loss of weight of stone in water.
To calculate SG just divide the weight of stone in air by this value.

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 Post subject: Re: Tutorial on Specific Gravity Using the Hydrostatic Metho
PostPosted: Sat Mar 03, 2012 2:52 pm 
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in the method you described Egor, tare the scale while the spiral wire basket is already immersed in water just like in your picture (that will eliminate the archimede force linked to it).
Also, the thinner the wire (1mm diameter is a maximum), the more accurate the SG.


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 Post subject: Re: Tutorial on Specific Gravity Using the Hydrostatic Metho
PostPosted: Sat Mar 03, 2012 3:12 pm 
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Yes, exactly, good points!

Also, when you have the scale tared, you can measure the weight of the stone in air simply placing the stone on the plate near to the water cell, no need to take the water cell out!

Curiously, in a scale with good precision (0.001 ct) you will see how quickly the water evaporates, some 0.001ct/2sec (in Madrid :) )

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 Post subject: Re: Tutorial on Specific Gravity Using the Hydrostatic Metho
PostPosted: Mon Sep 21, 2015 5:38 pm 
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cascaillou wrote:
in the method you described Egor, tare the scale while the spiral wire basket is already immersed in water just like in your picture (that will eliminate the archimede force linked to it).
Also, the thinner the wire (1mm diameter is a maximum), the more accurate the SG.

Why don't you just eliminate the wire basket by using a nylon thread, and use a minute piece of Scotch Tape or Superglue to hold the specimen?

But, one still has to buy an expensive balance and make the calculations. What a waste of time and money. It is no wonder gemologists don't use SG more.

If they were to use Relative Density, as recommended by B.W. Anderson some 50 years ago, they would have no problems and get their answers directly.


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 Post subject: Re: Tutorial on Specific Gravity Using the Hydrostatic Metho
PostPosted: Tue Sep 22, 2015 11:21 pm 
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Bill Hanneman wrote:
cascaillou wrote:
in the method you described Egor, tare the scale while the spiral wire basket is already immersed in water just like in your picture (that will eliminate the archimede force linked to it).
Also, the thinner the wire (1mm diameter is a maximum), the more accurate the SG.

Why don't you just eliminate the wire basket by using a nylon thread, and use a minute piece of Scotch Tape or Superglue to hold the specimen?

But, one still has to buy an expensive balance and make the calculations. What a waste of time and money. It is no wonder gemologists don't use SG more.

If they were to use Relative Density, as recommended by B.W. Anderson some 50 years ago, they would have no problems and get their answers directly.


Bill, I still don't understand why you think gemologists don't use SG anymore. I can confirm that myself, and all the gemologists I work with use SG every day. Also, it is a lot easier to remove a stone from a wire basket than to scrape superglue off of it. The wire basket is practical, which is why it is used, especially when measurements on several stones in a production environment is required.

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 Post subject: Re: Tutorial on Specific Gravity Using the Hydrostatic Metho
PostPosted: Tue Sep 29, 2015 4:26 pm 
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Nathan Renfro wrote:
Bill, I still don't understand why you think gemologists don't use SG anymore.

How about these reasons? :D :D
1. GIA does not consider SG a necessary instrument for basic gem identification. :(

2. I have never encountered a GG who could easily identify mineral specimens, i.e. faceting rough. :( :(

3. quote=Brabra Voltaire "… we all make mistakes. Specific Gravity is still a major pain in the butt for me. (Thank gawd for the gemmoraman.) …” =D> =D> :( :(
Quote:
I can confirm that myself, and all the gemologists I work with use SG every day. The wire basket is practical, which is why it is used, especially when measurements on several stones in a production environment is required.

It is interesting to note that in the latest Kassoy advertisement for Gemological Instruments, there is no mention of SG or how to measure it.
:smt003 On the other hand, they have shown nine different Scales, but not a single one claims to be useful for SG work, so I guess the secrete lies in the wire basket. I say eliminate it, as very few are interested in production. Use Relative Density. It is cheaper, faster, more easily understood and should be taught to all budding gemologists. :smt003

With the advent of Hodginson's new book, I believe GIA will be forced to revise their teaching methods of gem characterization involving SG, filters, dispersion, Luster, and B:D ratio. If I can be of any help, perhaps we should talk. Be aware, I have in little interest in artificial gems or appraising. I am only interested in what a matrial should be called. :D


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 Post subject: Re: Tutorial on Specific Gravity Using the Hydrostatic Metho
PostPosted: Tue Sep 29, 2015 4:55 pm 
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Since I wrote my response (cough cough) I've set up a permanent SG station in my lab.
Now, it is no longer a series of rearrangements of scales and water and fumbling and grumbling.

Although my permanent SG set up was inspired by shear laziness, it's actually rather efficient.


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 Post subject: Re: Tutorial on Specific Gravity Using the Hydrostatic Metho
PostPosted: Wed Sep 30, 2015 1:47 pm 
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It is nice to know you have a SG station Barbra, yes i agree it is difficult to take SG of whatever a mineral or gemstone everyday, at least i have to rearrange all setup again and again...

By the way how would you take a SG of an aggregate crystal having same weights without breaking it?
E.G Phenakite or Heliodor or Goshenite in Quartz cluster

can anyone answer?


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 Post subject: Re: Tutorial on Specific Gravity Using the Hydrostatic Metho
PostPosted: Wed Sep 30, 2015 2:13 pm 
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Yes. Meaningful SG values require the sample be homogenious.


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 Post subject: Re: Tutorial on Specific Gravity Using the Hydrostatic Metho
PostPosted: Wed Sep 30, 2015 3:33 pm 
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Ok, how people take SG of Johachidolite in colorless/white quartz without breaking it?, Johachidolite is found colorless as well, otherwise it could be citrine or heliodor association having the same red top,


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 Post subject: Re: Tutorial on Specific Gravity Using the Hydrostatic Metho
PostPosted: Wed Sep 30, 2015 3:39 pm 
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You don't. SG isn't a magic bullet, it's just another metric. RI, luster, B/D ratio etc will do nothing for you either in that case. Mineral specimens are a different beast anyway so naturally they'll require different techniques. Of course this does marginally overlap with gemological concerns (things like cutting Mexican topaz with a splash of rutile at the culet for color) but in that case microscopy would be your buddy.

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 Post subject: Re: Tutorial on Specific Gravity Using the Hydrostatic Metho
PostPosted: Wed Sep 30, 2015 4:02 pm 
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I don't contradict as you' re more experienced but SG is a root to any mineral origin like what i have in my hands so as crossed polarized filters or polariscope, rest gemological instruments are for later cut stones, but i have seen experienced man (only geologist over 70 years old) don't even need those tools. he just use 10x loupe and will tell if you have red zircon, spinel, garnet or topaz (treated or untreated) in your hands.
I have tested such experience all by myself and was stunned, anyway, that's sort of experience now i'm trying to learn, using basic gemological tools along.

If luck favored

P.S. And believe me Bill Hanneman is not wrong, its his experience that speaks people should respect and try to take advantage of such experience...


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 Post subject: Re: Tutorial on Specific Gravity Using the Hydrostatic Metho
PostPosted: Wed Sep 30, 2015 4:39 pm 
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Gabroo wrote:
I don't contradict as you' re more experienced but SG is a root to any mineral origin

SG is of some, but limited, value in identifying minerals, like any simple measurement. But what you're talking about is identifying mineral inclusions hosted in another mineral. That's a totally different thing and requires a totally different approach. Part of the problem is that, in the absence of some serious testing firepower, you aren't really doing the identification yourself--you have to rely on someone else having correctly identified that kind of inclusion and recognizing the similar characteristics between the inclusions they identified and yours. Even then you could get in trouble--golden goethite in amethyst has been wrongly identified as cacoxenite for ages. Fuchsite-included quartz from Pakistan is often sold as ajoite-included, and they do look somewhat similar. If you read the gem literature, you'll see that most identifications of inclusions are tentative if they give one at all, because only the most notable inclusions are likely to have had testing firepower applied to them.
Gabroo wrote:
but i have seen experienced man (only geologist over 70 years old) don't even need those tools. he just use 10x loupe and will tell if you have red zircon, spinel, garnet or topaz (treated or untreated) in your hands.

Most of the time it's not too hard to tell rough gems apart by sight thanks to things like crystal form and cleavage. It's also not too hard to identify cut stones by sight with good accuracy if you have a lot of experience--but the reason we don't just look at gems to identify them is that term 'good.' Some gems are really distinct, but others are not. Try telling apart red garnet and red spinel by sight alone--sometimes it's trivial, sometimes it's not. On the other hand, red zircon will probably be trivial to distinguish from red spinel and red garnet thanks to its dispersion and double refraction. Some treatments are also obvious--if the coating is peeling off the back of your stone or you can see blue flashes from lead glass filling you have a good idea it's treated. Others, like irradiation, might leave no trace at all. That's why we measure material properties in the first place--the numbers are less likely to lie than our eyes.

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