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Author: Subject: Growing Synthetic Corundums (Ruby)
Pasrules
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[*] posted on 29-6-2023 at 00:30
Growing Synthetic Corundums (Ruby)


Preface:
Its been many years since I first found this community. I'm glad it's still operating. I was originally completing a Ph.D in synthetic drug chemistry at the time and then moved into the XRD/XRF industry. These days projects just sit on the shelf.

Intro:
The plan is to make sapphires for an engagement ring which are a type of corundum with TiO2/Fe2O3 inclusions. However it is regarded as the most difficult of the corundums so the following is an attempt to discover a method for producing them, that isn't the flame Verneuli (standard method).

Method:
170 Inverter Arc welder with 2x carbon gouging rods (BOC), arcing between them.
Mix of 3% chromium(III) oxide and 97% calcined alumina, shipped from walker ceramics, Victoria.
Graphite crucible (1Kg) from Australian Jewelry Supplies, Sydney.

Results:
First run was powder on the bench and arcing over it:
https://cdn.discordapp.com/attachments/291251122914721793/11...
https://cdn.discordapp.com/attachments/291251122914721793/11...
https://cdn.discordapp.com/attachments/291251122914721793/11...
https://cdn.discordapp.com/attachments/291251122914721793/11...

Most of the smaller beads are very brittle with voids, the larger piece has a surface layer of ruby about 0.5-1mm thick which a black corundum substrate, again very voided and brittle.


Second run was in a crucible with 30g of mix:
https://cdn.discordapp.com/attachments/291251122914721793/11...
https://cdn.discordapp.com/attachments/291251122914721793/11...
https://cdn.discordapp.com/attachments/291251122914721793/11...

Attempting to reheat the solidified mass (bottom)
https://cdn.discordapp.com/attachments/291251122914721793/11...

Destructive analysis
https://cdn.discordapp.com/attachments/291251122914721793/11...
https://cdn.discordapp.com/attachments/291251122914721793/11...
https://cdn.discordapp.com/attachments/291251122914721793/11...
https://cdn.discordapp.com/attachments/291251122914721793/11...

Discussion:
The first test on the bench had minimal sprays to a radius of 200mm from the arc until a puddle of molten material forms which slowly accumulates. The products created are undesirable, they are very brittle and have a lot of voids, they shatter easily with pliers. This would indicate rapid cooling causing embrittlement and crystallization not occurring during the fusion.

The crucible melt of powder took about 2 minutes under arc, it formed a molten mass sitting in the center of the powder shot without touching the crucible walls. The heating process creates a brilliant light paying homage to the Stefan-Boltzmann law. No violent stress fracturing occurred and a boule of product can be seen in the images. The bottom side of this boule was crusty with pink powder. I should have cracked it open to see the depth of ruby formation, but from the bottom view we can see corundum (black) substrate present. The boule was resistant to blows from the slag chisel.

Upon remelting the solid boule, great difficulty was encountered to melt the product owing to its poor thermal conductivity. Most of the energy input created a puddle of product ontop of the boule which would re-solidify multiple times as attempts were made to heat the surrounding material. The molten material may have been conductive but no difference in performance was observed with the electrodes above or in the molten puddle. Also to note the consistency of the puddle against the rod was like pressing into wax. The welder is under powered to push enough energy into the sample, although the steel bench popped during heating which ended the run without consequence.
Inspection of the remelted boule was black corundum with a bottom of powdered pink. This new boule broke easily and could be cracked apart by hand. Inspection under magnification shows a thin 1mm layer of ruby on a corundum substrate formed. The ruby layer easily destroyed corundum emery paper abrasive and was reasonably resistant to a stone bench grinder.

The formation of corundum is under question, the following theories may account for it:
Alumina melts at ~2000°C, chromia ~2400°C, per flame Verneuli the best temperature to create synthetic corundum is as low as possible to melt the alumina then allowing the chromia to dissolve in solution. This means the welder at ~3000°C was too hot. Possibly allowing separation to occur.
Insufficient power could have kept the alumina liquid which let it settle down to dissolve cooler powder leading to pink bottoms, or the chromia being denser could have had the same effect. Where the less dense alumina forms at the top. It is difficult to tell as the remelted boule had inverted formation to its parent.
The black material may have carbon inclusions, it is unsure what effect this has, possibly owing to the blackness or mis-identification.

Going forward, the arc method is not suitable but potential improvements would include a higher power unit, inert blanketing, longer heating cycle.
A oxyhydrogen torch method similar to flame Verneuli is to be the target of new experiments.




Atropine, Bicarb, Calcium.
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Metacelsus
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[*] posted on 29-6-2023 at 04:00


Quote: Originally posted by Pasrules  

Intro:
The plan is to make sapphires for an engagement ring


Lol, don't give my girlfriend any ideas, I'm not sure I want to operate an arc welder in my apartment!

But it looks like you are having some success already, just not getting all the melt to form ruby.




As below, so above.

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Fleaker
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[*] posted on 9-7-2023 at 04:38


Have you considered hydrothermal growth?

https://www.gia.edu/gems-gemology/spring-2022-lab-notes-hydr...

[Edited on 9-7-2023 by Fleaker]




Neither flask nor beaker.


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Admagistr
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[*] posted on 9-7-2023 at 11:19


Quote: Originally posted by Fleaker  
Have you considered hydrothermal growth?

https://www.gia.edu/gems-gemology/spring-2022-lab-notes-hydr...

[Edited on 9-7-2023 by Fleaker]


This is a nice method,but where to get an autoclave for temperature 350 C to 500 C?In specialized shops is offered German autoclave for temperature only up to 300 C with silver seal and it is very expensive.If you grow corundum at temperature lower than 350 C you will get diaspore AlOOH,in case of trying to synthesize ruby it will be chromdiaspore...
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highpower48
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[*] posted on 9-7-2023 at 11:36


I'm a silversmith and gemologist as a day job. Why bother you can buy a cut and polished flux grown
(Verneuli method ) corundum stone for about $5.00 a caret. The only reason I could see doing it yourself would just be for the satisfaction, i doubt you will produce anything even close to a usable stone.

Sorry if I sound negative.
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Admagistr
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[*] posted on 9-7-2023 at 11:48


Quote: Originally posted by highpower48  
I'm a silversmith and gemologist as a day job. Why bother you can buy a cut and polished flux grown
(Verneuli method ) corundum stone for about $5.00 a caret. The only reason I could see doing it yourself would just be for the satisfaction, i doubt you will produce anything even close to a usable stone.

Sorry if I sound negative.

You are right,but rubies from the Verneuil method are the furthest from natural of all synthetic stones,you can tell right away under a microscope that the stone is synthetic.If you use MoO3 and alkaline carbonate as a flux,then the jeweler will have a much harder job to tell that the stone is synthetic,especially with a small stone.Also to get crystals that look like natural,not Verneuil "pears". Stones made by the Verneuil method also have a lot of internal tension,as they cool quite quickly...They have to break off the shank to crack them in half and then they can only be cut...There is no substitute for the feeling of making a own gem!
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