heat-treating slabs

[Lavenderfish]

Have any of you done any experimentation on heat-treating slabs to intensify color of the material? I know of the procedure whereby you bury a slab in sand, put it into the oven, then ramp up the temp slowly.

But how well does it really work? Does it make the material too brittle to cab very well afterward?

I'm curious about materials like Indo fossil coral & palmwood in particular. Am sure there are many others besides tigereye, carnelian and flint where this is a common practice.

[Taogem]

Another little something I have yet to do Carol..

Yes, the Indo Fossil Coral really turns out exceptionally nice. Lots of great coloring.

Hopefully the members who have experience will answer your question about possible brittleness for cabbing.

[seth]

I have done it with several different materials with mixed results. Some make a very nice change some do nothing great. I never had a problem cutting any of it.

[Lavenderfish]

While re-reading thru some of the Fischerstone procedures on the Ganoksin website that I sent Tao a link to in another thread, ran onto this EXCELLENT, concise tutorial by Dr. Fischer on how to heat treat slabs. THought it was well worth sharing for anyone else interested.....


Heat Treatment
Some kinds of gemstone (e.g. Brazil carnelian) can be improved in color by heat treatment, without benefit of any chemical soaks. This is possible when the gemstone has some native "impurity" in it, particularly iron compounds that impart improved color changes when heated sufficiently.

Two of the chemical coloration processes described in this book involve the introduction of chemical compounds that result in marked desirable color changes when the soaked slabs are properly heated. This heat treatment must be accomplished so that the slabs (or other pieces) are heated very gradually, and then allowed to cool very gradually. A "sand bath" is ideal for this. This simply means that the slabs are embedded in sand during the heat treatment. This promotes gradual heating and gradual cooling.

Some kind of metal container is necessary such as a bread pan or refrigerator pan. It should be five to seven inches deep. The sand I use is common plaster sand, available where building materials are sold. Pour a layer of sand into the bottom of the pan to a depth of about an inch. Place a layer of the chemically treated slabs to be heat-treated on this layer of sand. Cover with another layer of sand about one-half inch deep. Follow this with alternate layers of sand and slabs until the pan is full. The top layer must be sand, of course, and approximately one inch deep. If the pan has a lid, so much the better.

When the sand bath has been packed, it is ready to be heated. The oven in an electric kitchen range serves nicely for this and is the only kind I have ever used. Probably a gas oven would be just as satisfactory or a kiln. The important thing is temperature control. The following heating schedule is recommended:

175°F Several hours (overnight is fine)
275°F Four hours
375°F Four hours
475°F - 500°F Four hours 

Allow the pan and contents to cool in the oven overnight or longer, or at least until the pan feels only warm, not hot. If you can stand the suspense of waiting to see the results, allow the pan to cool until it does not even feel warm. If the sand bath is opened prematurely, fracturing of the suddenly cooled slabs is likely to result. Once you have experienced the dismay of having a beautifully colored slab thus fractured, you will become quite patient in waiting until the sand bath has entirely cooled. After the sand bath has properly cooled, remove the slabs, wash and dry them. The sand may be re-used indefinitely. You may notice that even the sand changes color, usually taking on a reddish tinge. 

[seth]

I would be careful when you get over 400deg. Some agates and jaspers with inclusions will fall apart. Always do a test on an end cut piece if it is good material. Nothing worse than waiting that long to see a bunch of cracked over heated material. Been there done that. The most important part is to dry the stone (1st step). Yes I said dry the stone. You would not think agates would have water in them but they do. Some carnelian like the Indonesian material I have will only need 300 at a slow heat to turn cherry red.

[Taogem]

I do not have any of the chemicals that are necessary when bathing a slab prior to heat treating.

I did read in Ganoksin' s site the following:

"Some kinds of gemstone (e.g. Brazil carnelian) can be improved in color by heat treatment, without benefit of any chemical soaks. This is possible when the gemstone has some native "impurity" in it, particularly iron compounds that impart improved color changes when heated sufficiently."

This is a picture of the Carnelian I just got in the mail today. I have no idea how to tell if it has any native impurities in it. Like iron?

When I purchased it had hoped there might be some areas in it that could just cut some solid jelly like preforms from. No such luck. It is banded. With the exception of the end cuts. The outer part of it. I have it in the saw right now slabbing that part that appears to not have any banding. That is the part I am going to place in the oven tonight.

Any way, I am going to try just straight up heat treating in the oven tonight.

Will post any progress, or lack of...  ;) in the morning.

With a little luck will gain a nice solid jelly kind of slabs!

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[Lavenderfish]

Did you get to heat any of the slabs up yet? How'd it go?

[Taogem]

Thanks for asking Carol... I totally forgot to come back here and post the results.

Which there is absolutely nothing to show!

Cleaned all the slabs real well.

I included with the Carnelian, some of Seths Fossil Coral, and also some Brazilian Rainbow.

Pre-heated for a couple of hours at 300. Then bumped it up to 400 for about 7 hours. Oh.... yes in the sand.

Turned the oven off and let cool for a couple hours.

Nothing.... Not even a slight difference.

[Lavenderfish]

Hmmm, would think you'd have seen at least SOMEthing happen. Maybe the temp wasn't quite high enuf.......looking for something.....

Check this link out below, about 3/4 down the page is a more specific protocol for heat-treating as described by Dr. Fischer (as in Fischerstone). The final temp is a bit higher too and that might help make a difference if you wanna try it again. I'd give it a whirl too but my husband would keeel me if I cooked rocks in "his" oven, he's the cook!

http://www.ganoksin.com/borisat/nenam/gemstone-dye-technique.htm

[theimage1]

One other place you might get info, but I have no specific recommended site is the Arrow Head Knappers. They heat treat a bunch of their rock to make it chip better and it changes colors sometime. They have a lot of "potential" recipes, and I have seen (in the past) info on some of their forums about which can be easily heated and which get too brittle etc.

[Taogem]

Gosh, right off the bat it becomes obvious from that site Carol that my temps and time were way off.

# 175°F Several hours (overnight is fine)
# 275°F Four hours
# 375°F Four hours
# 475°F - 500°F Four hours

Yes, I am going to give it another whirl....

[Taogem]

have water in them but they do. Some carnelian like the Indonesian material I have will only need 300 at a slow heat to turn cherry red.

I am at it again.

Would you say Seth that the first suggested temp from Ganoksin website of 175 degrees overnight would be considered drying out?

I have several slabs in the oven at about 150 or so. Not in sand. Just drying them out for a while at low temp.

I thought would dry them out like that and then pack them and start again at the:

# 175°F Several hours (overnight is fine)
# 275°F Four hours
# 375°F Four hours
# 475°F - 500°F Four hours

Not entirely sure if the 175 as suggested from his site is a drying period or not. Don't imagine it will hurt to end up sorta double drying them if it is.

Bought a bag of sand and dried it out as well in the oven for the past 5 hours. It was wet in the bag and thought should be dry too.

[Taogem]

Finally, the heating process is completed.  :)

Now the slabs are in cool down. Will probably take several hours to completely cool.

This is the part talked about in regards to having patience. Letting them cool completely is the best.

Excited to see how these turn out !

[Lavenderfish]

Got my fingers crossed for you for some good results!

What did you try heating?

[Taogem]

Because the success of heat treating without chemicals seemed to be for Brazilian and Carnellian Agates, most of the slabs are these two.

What I am curious to see is if the best success is going to turn out to be the Brazilian type. It is already full of reds. I think, not sure that the red is related to iron. Apparently it is specifically the iron content in these two that makes heat treating alone possible. So curious if these Brazilian slabs will be most successful.

The Carnellian slabs had no real color at all prior to heating. So not sure where the iron content comes from.

Guess will see.....

Then for fun, I included a few Fossilized Coral slabs. I have seen Fossil Coral have results from just heat treating. So thought would give a few a whirl.

I don't think I have anything else that would have qualified for color and or intensity changes.

Crossing my fingers.......  :)