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Lapidary / Gemstone Community Forum
December 20, 2014, 07:32:52 pm
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Heat treating stones

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Bluesssman
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« on: January 19, 2009, 11:08:27 am »

The subject of heat treating stones was brought up in another thread.

Could someone explain what is involved in this, why do this, and what stones benefit from this process?


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1rockhound
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« Reply #1 on: January 19, 2009, 11:42:32 am »

Heat treating will usually brighten the color of a stone.  A lot of Sapphires are heat treated.  For example Tiger Eye which is normally a yellow color if heat treated it will become red.

Jason
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Bluesssman
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« Reply #2 on: January 19, 2009, 12:03:49 pm »

If I am understanding, heat treating will bring out brighter more vibrant color?

Are there only certain stones that this process works?

Is the heat treating process one of bringing up the temperature of the rock and then either slowly or quickly cool it?

What temperatures are we talking about?

Does the time at the high temperature come into effect?

I understand the aspect of heat treating steel, but that process is not done to achieve a change in color.

Thanks....

Gary
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Lavenderfish
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« Reply #3 on: January 19, 2009, 12:48:44 pm »

Bluesssman,

If you google the forum for "heat-treated slabs" you'll pull up a good discussion we had about this awhile back, which also follows Tao's experiment with it.

It might help answer your questions. Or bring up more! LOL
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Taogem
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« Reply #4 on: January 19, 2009, 01:14:14 pm »

I did not have any luck trying it myself. Not sure why......

Here is the thread
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Seth
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« Reply #5 on: January 19, 2009, 02:07:43 pm »

Heat treating does work on many rocks. Brazil agate and carnelian have been treated this way for years. Gemstones like ruby are often done this way.Some material will not work if iron oxide and or other minerals are not present. Flint knappers do it all the time to make material knap better and in many cases changes the color of flint or jaspers. I have even heated graveyard point slabs and they turn more red. Again it depends on minerals dry time and final heat.
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Bluesssman
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« Reply #6 on: January 19, 2009, 06:50:18 pm »

I guess I am not very good at doing the search thing, so thank you for directing me to the thread!!

How many here do heat treating of their stones?
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bobby1
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« Reply #7 on: January 19, 2009, 07:23:50 pm »

I have dabbled in heat treating some Agates. Here is a picture of some cabs thatI I cut showing a before, an intermediate treating and a fully treated cab. It is Brazilian Agate. I don't recall the temperatures that I used, but I've documented them some where. I have a casting burnout oven that I use. Slow increments of raising the heat and lowering the heat makes a difference between highly fractured results and fracture-free results.
Bob
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Taogem
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« Reply #8 on: January 19, 2009, 08:45:50 pm »

I don't recall the temperatures that I used, but I've documented them some where.

If you think of it sometime and are able to post your heating temp stages, I would be interested in comparing to my failed attempt.

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bobby1
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« Reply #9 on: January 20, 2009, 08:08:43 am »

I have a document on heat treating that I will post. It is a hard copy of notes someone made of a presentation at a flint knapping event. I will take pictures of the pages and try to post them. It will take a day or two to do it.
Bob
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sodagar
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« Reply #10 on: January 21, 2009, 11:55:36 am »

Certain Gemstones improves or completely change its color by means to controlled " Heat treatment ". Some times its  permanent like in the case of burnt amethyst to produce yellow , green and brownish red variety of Quartz  another example can be brown and yellow Topaz from Brazil converted into  Pink topaz or greenish Aquamarine to lovely tone of Blue .
 In Brazil , sand bath of 500 C turns reddish brown topaz into lovely Pink topaz and it happens due to the presence of Chromium in the process. For Tourmaline , red and pink variety become colorless if heated up to 260 C where as numerous are stable up till 400 C .Some dark green tourmaline turns into lovely shade of emerald green if heated up to 700 C . Aquamarine changes from greenish tone into lovely blue when heated somewhere between 300 C to 500 C . for Ruby and sapphires , five main  methods are used for heat treatment . sometimes 1600 C to 1900 C in reducing atmosphere and sometimes 800 C to 1900 C in Oxidizing Atmosphere . To give star effects in Ruby and sapphire , heat treatment is given at 1300 C to 1900 C for 1 to 14 days .
In Chalcedony varieties , heat treatment is seldom used, mostly Brownish Carnelian from India is heat treated to produce  its trade mark color for centuries .Agates are mostly dyed, I  never heard or read about heat treatment for agates .I will look forward to hear about Agates heat treatments .
 
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bobby1
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« Reply #11 on: January 21, 2009, 08:43:24 pm »

Here are pictures of a document that I got somewhere, I think it was a knapping site. I haven't a clue which site nor who the author is. I believe it is notes from a presentation made at a  meeting of flint knappers rather than a lapidary group. I'm sorry about the color of the examples but my copy isn't exactly the best.
I hope it helps. Generally I treat mostly Brazilian Agate. I embed slabs (not rough) in an aluminum bread pan filled about 4" deep with sand. I slowly ramp the temperature as noted in the presentation to about 500 degrees F. I leave it at that temperature for 6 hours and then ramp back down for 3 hours then turn the oven off and leave the door shut for another 4 hours. DO NOT at any time in the process do you EVER open the oven door to check the process. The thermal shock will always fracture the slabs. I rarely treat finished cabs.
Bob










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wywyrd
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« Reply #12 on: January 21, 2009, 09:20:02 pm »

Thanks bobby1 !!! A wealth of information there in those few pages I want to start experimenting!
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Taogem
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« Reply #13 on: January 21, 2009, 10:14:08 pm »

Yes, thanks Bob,

The heat ramping and holding times are quite a bit different from the process I tried my last attempt with some Carnelian and Brazilian.

It was mentioned before about how the iron contents are a big factor in successful color change.

I am not sure how to tell if the Carnelian and or Brazilian that I tried had enough iron content nor do I know how to tell for sure if there is enough in them to make them a good candidate for heat treating.

That would be something I would be interested in learning how to tell. The only thing that I thought of that might indicate the iron was areas of red coloring. Like in the banding. Not sure and just sorta thought that the red might represent iron?

Thanks for posting this.

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bobby1
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« Reply #14 on: January 21, 2009, 11:08:58 pm »

It is difficult to determine exactly if or how much iron there is in any particular material. Generally a brown or red "skin" is a hint. This is particularly evident in some carnelians from the Northwest. They often have what my uncle called "sunburn" that is caused by many years of exposure on the surface to the heating effects of the sun. A similar effect on Brazilian Agate sometimes is a hint. In the example of the three cabs that I posted, the  original color had hints of a possible content of Iron. That is what inspired me to try heat treating the material. My first attempt didn't show much coloring so I heated longer and at about 100 degrees hotter. I think I went to perhaps 550 to 600 degrees. If you start out a little lower and see some color change reheating the same piece is OK. This material was heat treated as slabs and cabbed later. I would rather find out if it heats to a good color and remains fracture-free before I invest the time to cab the material. I haven't noticed a significant difference in brittleness in the material after the heat treating process. the blue colored Brazilian definitely won't heat treat  with any noticable color shift. Again, blue  means low Iron content. Sometimes a translucent material will become less so after treatment. Most Agate material that you want to heat treat isn't very expensive so you won't suffer much if it doesn't work.
Bob
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