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 Post subject: Heat treatment
PostPosted: Tue Feb 23, 2021 8:18 am 
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A friend has lent me a small electrical enameling kiln, suitable for heat-treating gemstones. It came with a small parcel of faceted greenish-blue aquamarines that the owner would like me to treat. I have a suitable crucible and some investment powder to protect the stones. Does anyone have experienced advice about the temperature to use to treat aquamarine? (Personally I like the greenish-blue of unheated aqua, but this is part of the loan agreement. I intend to use the kiln to heat treat very dark amethyst and tourmaline experimentally.)


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 Post subject: Re: Heat treatment
PostPosted: Tue Feb 23, 2021 10:53 am 
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Hopefully the oven he sent you has a controller that allows you to set the ramp up to temperature, the hold time and shut off.

The theory of heating Aqua is as follows.

Both the green and blue colors are caused by iron.

The green color is caused by iron that is replacing the aluminum site in the crystal. It actually produces a yellow color, which when mixed with the blue in Aqua locks greenish. Heating removes this iron color.

The second iron is situated in a channel site in the crystal. This is what produces the blue color. It is unaffected by heat so after heating the blue color remains.

Enough of the science.

You will want to heat to between 450C and 500 C. Best done in an oxidizing environment. Using dry investment powder in a crucible is great for this.

I am conservative when heating so go slower than many to avoid as much cracking of stones as possible. I ramp up to my selected max temperature over 3 hours, let it soak for 1 -2 hours, and then turn off the heat and let it cool naturally without opening the kiln door. I usually let it run over night so going slow costs me no working time.

I hope this is the information you were looking for. let us know how it comes out.


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 Post subject: Re: Heat treatment
PostPosted: Tue Feb 23, 2021 11:46 am 
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Thank you Steve. This is exactly the information I was looking for. In due course, I will report back here.
The kiln has a temperature controller that allows me to set a temperature and it will hold that once it reaches it. Unfortunately it doesn't have a controlled ramp function, but I could do that manually in stages. I will have to time the soak myself, then switch it off. I'll let it cool overnight.


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 Post subject: Re: Heat treatment
PostPosted: Tue Feb 23, 2021 1:13 pm 
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The most important thing is to ramp up the heat slowly. Otherwise thermal shock, and unequal heating between the stone and inclusions may cause breakage.

Thee thing with electric kilns without controllers is that the operate as switches not rheostats. That is 100% on and 100% off. To dampen this use lots of investment in a larger crucible and burry the stones deep. this will even out the heating and cooling.

Hopefully, it has an accurate temperature gauge for you to read.

My suggestion is that you set the initial temp to about 25C above ambient. Soak for about 15 minutes after it reaches temperature, increase in 25 C steps until you reach your target high temperature. Soak at that temperature for the desired time. Then turn the kiln off and let it cool to ambient without opening the doors.

If you find you are doing a lot of heating, I recommend a small programable kiln. I use this one from TableTop Furnaces. Affordable and it works great.

https://www.tabletopfurnace.com/product ... re-pro-lp/

Not sure if there are dealers for it in SA however.


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 Post subject: Re: Heat treatment
PostPosted: Tue Feb 23, 2021 3:42 pm 
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Steve's methods are exactly as I was taught by a mutual friend. I have a furnace where the temp gauge is not very accurate and always leaves a concern when the gem your heating is valuable, and especially if it may belong to someone else. He also taught me in addition that if you want to eliminate some of the oxidizing environment, was to cap the crucible with wet Plaster of Paris or some of the casting investment, let dry before starting the heating process. Although I found that in higher heats like 675 -800 °C the cap shrinks somewhat and in my opinion I think it kind of eliminates the purpose. Maybe I am just not doing correctly.
I agree a better controllable furnace is needed, the heating I do is only on our gem stock and is experimental mostly.

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 Post subject: Re: Heat treatment
PostPosted: Wed Feb 24, 2021 10:06 pm 
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Snarky sidebar.
Why would anyone choose to remove the green component from aqua?
Do they need an irradiated blue topaz simulant?


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 Post subject: Re: Heat treatment
PostPosted: Wed Feb 24, 2021 11:29 pm 
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Sparky answer is that it brings a higher price.

I think knowledgeable people like the natural slightly greenish blue color in some light to medium tones. But nothing in aqua beats a true Santa Maria dark blue.


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 Post subject: Re: Heat treatment
PostPosted: Thu Feb 25, 2021 2:10 pm 
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Fair enough.


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 Post subject: Re: Heat treatment
PostPosted: Thu Feb 25, 2021 7:05 pm 
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I have not heated a lot of aquamarine, but when I have, I have done it at a little lower temperature. My standard temp is 375 C, ramping up at 100 degrees per hour, holding one hour, then letting the oven cool.

There is always some risk in heating. I had a large flawless aqua develop a nice crack one time heating.

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 Post subject: Re: Heat treatment
PostPosted: Fri Feb 26, 2021 7:56 am 
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Report back. To test the kiln operation I buried a 0.87 ct dark, pinkish-brown tourmaline (with some bubble veils) in platinum casting investment powder in a ceramic crucible, heated it directly to 535 *C (the kiln overshot the set temperature a bit) and let it soak for an hour at 500 *C, then cool overnight. Surprisingly the bright pink stone had not cracked, although the inclusions were more visible, as expected. So the furnace works. But, all the facets on the stone had scattered small rough patches, like little scabs. Under the microscope they looked like shallowly pitted areas and I suspect a reaction with some component in the investment powder. Before heating aquamarine with a stepped ramp to 375 *C, I'll try another tourmaline to see if there is similar pitting. (I don't have any alternative investment powder and the nearest supplier is 100 km away.)


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 Post subject: Re: Heat treatment
PostPosted: Fri Feb 26, 2021 11:20 am 
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The investment powder is just to even the heat around the stone. So alternative materials may work. I have never had a pitting with standard gold casting investment powder.

Maybe just wrap the stones in aluminum foil to sperate them from the investment powder and stop the etching.

Back in the very old days we heated Tanzanite out in the bush by cutting potatoes in half putting the stones inside and wrapping them back together with wire. We would but them in the coals of the camp fire in the evening, and take them out in the morning. It work pretty good as crazy as it sounds.


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 Post subject: Re: Heat treatment
PostPosted: Fri Feb 26, 2021 1:15 pm 
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Right now another test is running, same parameters, but with a smoky quartz, synth. spinel and synth. ruby - only to see if they suffer similar surface damage with the investment powder. It could be a specific tourmaline issue though. Thanks for the Al-foil suggestion, which I will try.


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 Post subject: Re: Heat treatment
PostPosted: Fri Feb 26, 2021 6:47 pm 
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Duncan Miller wrote:
Right now another test is running, same parameters, but with a smoky quartz, synth. spinel and synth. ruby - only to see if they suffer similar surface damage with the investment powder. It could be a specific tourmaline issue though. Thanks for the Al-foil suggestion, which I will try.

I've experimentally heated quite a few tourmalines. My only observation is that they vary a great deal in results depending on color and source. I don't think there's a "once size fits all" solution; experimentation is required. If you can access some of Mark Liccini's old reports on heating you might pick up a few tips.

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 Post subject: Re: Heat treatment
PostPosted: Fri Feb 26, 2021 10:48 pm 
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ROM wrote:
Duncan Miller wrote:
Right now another test is running, same parameters, but with a smoky quartz, synth. spinel and synth. ruby - only to see if they suffer similar surface damage with the investment powder. It could be a specific tourmaline issue though. Thanks for the Al-foil suggestion, which I will try.

I've experimentally heated quite a few tourmalines. My only observation is that they vary a great deal in results depending on color and source. I don't think there's a "once size fits all" solution; experimentation is required. If you can access some of Mark Liccini's old reports on heating you might pick up a few tips.


Very true with the variation. I heat zircons a lot, and even from the same parcel, from the same mine, they can react differently. So I normally start out lower temperature, and work up.

I just heated 2 red zircons from the same parcel, I'll post pictures before and after.

As far as the pitting, I have never seen that, and I too put the stone in some investment powder in a ceramic crucible.

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 Post subject: Re: Heat treatment
PostPosted: Sat Feb 27, 2021 3:05 am 
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ROM wrote:
I've experimentally heated quite a few tourmalines. My only observation is that they vary a great deal in results depending on color and source. I don't think there's a "once size fits all" solution; experimentation is required. If you can access some of Mark Liccini's old reports on heating you might pick up a few tips.

I found an informative conversation here: https://www.pricescope.com/community/threads/description-heat-treatment-process-nigerian-tourmaline.158728/

The result of yesterday's heating experiment with quartz, synth. spinel, and synth. ruby was no pitting or surface reaction on any of the stones. So perhaps it was an issue with that specific tourmaline. I'll try another this weekend.


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