Heat Treating Lapidary Cabochon Minerals For Silver Solder Jewelry Design

by georgeingraham on February 9, 2010

Heat Treating Minerals

There are a great many minerals
that can be heat treated as shown in
the forum post. Heat treating these
different minerals can result in some
outstanding color enhancements !

Here is a picture of some showing
a Brazilian Agate before, intermediate,
and a fully heat treated cabs.

Slow increments of raising the heat
and lowering the heat makes a
difference between highly fractured
results and fracture-free results.

There is a great reference on Gaonksin by George W. Fischer. He has conducted hundreds of experiments using chemical coloration of gemstone minerals. Using dozens of chemical compounds to induce their color.

This is a process I tried following using what appeared to be a fairly heavily iron concentrated Brazilian Agate. I did not have any luck with it, but believe it was because the slabs just did not have the iron I thought they did.

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.

Rather than continue on and on within this blog entry. Would much rather viewers here check out one of the main “Heat Treating” discussion thread on the forum. That way each members posting will be properly credited.
There are a great many more images, and references available within that thread. Plus links to other archived threads on the subject.

Thanks for stopping in !

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