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 Post subject: Dark sapphires. Should I heat 'em?
PostPosted: Sun Feb 28, 2016 2:28 pm 
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Location: Moncton, Canada
Hi all,

Some weeks ago, from a very old jeweler who worked at a local jewelry store that closed way back in the late 1980's, I inherited a batch of tools and stones. While most of the stones were little calibrated commercial thingies of no interest to me, there was one little plastic snap box containing 16 approximately pea size chunks of what I presume is rough sapphire. "Presume" for the sake of truth-in-advertising, because to be honest I haven't even put them on the refractometer yet.

Most of the stones are a mix of blue and green in the same stone. Some show exceedingly fine parallel growth lines intersecting at 120 degrees, the fineness and precision of which makes me suspect that they may have not been heated.

As you can see in the picture/s the stones are dark, and since they cost me nothing, I have a mind to heat them, in a kitchen table, primitive sort of way, in the hope of lightening the color.

To do this I have an ancient kiln with a broken thermostat but which, going by the almost white incandescence inside at maximum temperature, I suspect may reach 1,200 to 1,300 celsius.

I'd be grateful if anyone who has done heat treating would offer some speculation on what the outcome might be, or helpful suggestions; pack them in charcoal for a reducing atmosphere perhaps, or any other stuff like that.

Cheers all, and thanks in advance
Hans Durstling
Moncton, Canada


In their little plastic box, under the desk light.

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At the edge of the light. Now you can see the blue/green color mix and indications at least of the zoning lines

Attachment:
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A single piece at the edge of the light

Attachment:
Sapph-rough-1-3894 copy 2.jpg
Sapph-rough-1-3894 copy 2.jpg [ 213.08 KiB | Viewed 1716 times ]

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 Post subject: Re: Dark sapphires. Should I heat 'em?
PostPosted: Sun Feb 28, 2016 3:10 pm 
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I don't think you'll get close to hot enough to lighten sapphire. On the plus side you've got some silk in some of those--they might cab up as stars or flat polish to show a nice pattern. Or you could of course send the to someone with the kind of kiln that can get them up to 2000~c in the right atmosphere. I mean, by all means give it a try--I'd love to see the results, I just don't think you'll accomplish much. That temperature range reportedly can sharpen ruby's color but I don't think much else for corundum.

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 Post subject: Re: Dark sapphires. Should I heat 'em?
PostPosted: Sun Feb 28, 2016 5:23 pm 
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Hans Durstling wrote:
... there was one little plastic snap box containing 16 approximately pea size chunks of what I presume is rough sapphire. "Presume" for the sake of truth-in-advertising, because to be honest I haven't even put them on the refractometer yet.

Use Relative Density and you won't have to polish a flat on each one before trying to use your refractometer.

Or, determine Hardness.


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 Post subject: Re: Dark sapphires. Should I heat 'em?
PostPosted: Sun Feb 28, 2016 10:55 pm 
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Not to go off the gemological deep end, but there's no testing neccessary here. These stones are extremely distinctive. At least one is a trapiche for goodness sakes.

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 Post subject: Re: Dark sapphires. Should I heat 'em?
PostPosted: Mon Feb 29, 2016 11:17 am 
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I have some of the same and thought about heating them. My little muffle furnace only went to 1200-1300 before it fell apart. My husband is considering hitting them with a MAP torch to see what he can do. If they get trashed, we haven't lost anything of value.

Will take before and after pics and will post if successful.


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 Post subject: Re: Dark sapphires. Should I heat 'em?
PostPosted: Mon Feb 29, 2016 4:29 pm 
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mess with the torch, and you will most likely kill them completely.
They all appear to be sapphires, and as Stephen has already said, some appear to be candidates for cabbing, you may be surprised at the asterism .
The lighter ones shown are fine to facet. once again, it is surprising what will come out from apparently poor rough.
Have fun with them.


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