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 Post subject: Re: more ruby comparison
PostPosted: Thu Jan 20, 2011 9:44 pm 
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hi Zen

I wasn't aware that synthetic rubies were supposed to transmit more UV, compared to natural... an interesting bit of information. The spectra in my first post here were taken some time ago. The first spectrum in that post is that of a natural ruby, and it doesn't show any transmission in the 300-400 nm (LWUV) range. The second spectrum in that post is that of a flame fusion ruby, and it shows some transmission in the UV range.

As for why you aren't seeing any transmission in the UV, I guess the first question to ask is what are you using as a light source?

At the time of the original post, I was most interested in ruby fluorescence. This semester my students will make some new measurements. This time around we'll consider the absorption more carefully.

As for making a proper measurement, note that the vertical scale of the spectrum (the transmission intensity) is relative. I agree with Bill's last post... I don't see much reason to develop an absolute scale along the vertical axis. These days, I think the primary use of gem transmission spectra is to provide a sort of spectral fingerprint, a little step up from using a spectroscope.

So we just mount a stone in a little bench, where fiber optics direct light from source and to spectrometer. I think I am going to purchase another little bench that should be more useful for transmission spectra.


Last edited by Brian on Fri Jan 21, 2011 1:48 am, edited 1 time in total.

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 Post subject: Re: more ruby comparison
PostPosted: Thu Jan 20, 2011 11:42 pm 
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Bill Hanneman wrote:
Brian wrote:
Ah, so I was correct, measuring absorber concentration.

The latest fad is showing plots of Absorption Coefficients (per cm.) with values on the ordinate ranging
from 0 to over 25.

Can someone explain to me what value that information has to practical gemology and how one determines path length through a rough or faceted stone?

I am referring to an article which compared spectra of heated and unheated stones. The caption reads "The spectra have been offset vertically." and the text says " Abundant cracks were obvious in almost all of the heat-treated stones but in only a few of the unheated samples..." GIGO. :lol:


Hi Bill, I'll take a crack at an explanation....I think the reason that absorption coefficient is used is because it is linearly related to absorber concentration and allows the subtraction of different absorber species within the same sample. This information of available absorbers in a sample is useful is you want to know if or how a sample might respond to a particular treatment. It also normalizes the the spectra for comparison of different samples. Transmission spectra does not provide any information of absorber concentrations and often masks the prescence of absorbers in a sample. Now, I do however realize that this may not be "practical" gemology, but I imagine "practical" gemology will only take the science so far and eventually gemologists will need to rely on less practical and more advanced testing as treatments become more complex. As to the article you mention, can you provide the reference as I would like to see it because I dont understand what they mean by the vertical offset or the bit about fractures. This is just my guess. Certainly this type of measurement isnt for everyone. Just like Brian uses transmission because it works for his application and I use absorbance because it works for mine. If all you want out of a spectrum is a fingerprint for a particulargem species then I think transmision spectra is just fine.

To measure the pathlength in a faceted stone while crude I imagine, the best approach would be to take a measurement through the girdle of a stone or if a stone is badly windowed then from culet to table.


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 Post subject: Re: more ruby comparison
PostPosted: Fri Jan 21, 2011 5:35 am 
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Dear Brian,

Thanks for your reply. Your attached figures are having very good resolution. I wonder if you repost or send me the files? (My email is removed by the admin* ) How many % difference in transmission is observed between natural and synthetics? Also do have a special jig to hold your gemstones during measurements? THanks.


REgards,
Zen

*it's extremely unsafe to publicly post sensible infos like email address, spammers are everywere looking for that. Please, for your safety, use PM :wink:


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 Post subject: Re: more ruby comparison
PostPosted: Fri Jan 21, 2011 4:08 pm 
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hi Zen

As far as mounting the stones, in the previous post there are a couple links (highlighted in blue) to optical benches that can be used to hold the stones. The I use a standard cuvette to hold the stone in the bench... slicing the cuvette in half from top to bottom, putting a flat surface of the stone against the cuvette window, and covering the rest of the window around the stone with blu-tack.

Those spectra are quite old now, and so I'm not sure I can locate the original data files. If I do find them, I'll forward 'em. But as I mentioned, my students will be re-measuring these rubies, and so I could send you the new data when it is available... probably around the end of April, beginning of May.

You probably need to go back and edit out your email addy, or else one of the moderators will do it. Please send me your email via private message.


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 Post subject: Re: more ruby comparison
PostPosted: Tue Jan 25, 2011 2:48 am 
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Hi Brian,

Thanks so much! Please let us know when you have more new information.


Regards,
Zen


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