Duncan Miller

Inspired by the dramatic change in colour of the large tourmaline illustrated in last month’s Mineralogical Chatter, that went from autumn brown to a purplish-pink on heating by the client for whom I had cut it, I decided to experiment myself. A friend lent me a small ‘enamelling’ kiln; I bought a suitable crucible from jewellers’ supplier Lipman & Son in Cape Town (https://lipmanson.co.za/); and Ian Lipman generously gave me jewellery casting investment powder to protect the stones. My first experiment was to find out how the kiln’s temperature controller worked and how long it took to reach the required temperatures. I also had to research what those temperatures were for different gemstones. An enquiry to GemologyOnline led to a longish online conversation, with detailed advice from some very experienced lapidaries and gemstone dealers (https://www.gemologyonline.com/Forum/phpBB2/viewtopic.php?f=8&t=26065).

My first experiment was on a small, dark red tourmaline that looked like a garnet. It was buried in two tablespoons of investment powder in the crucible and heated in one step to 500 °C. It took only 15 minutes to reach this temperature, overshot to 535 °C, and then settled down for a one-hour soak at 500 °C. Obviously the kiln control would have to be nursed when approaching a set temperature. The kiln was left to cool overnight before removing the stone. The tourmaline had changed colour to a bright pink, but the internal flaws became more visible and the facets all developed scattered pitted areas. I thought that may be due to some reaction with the investment powder.

Then I tried more controlled heating of three khaki-brown tourmalines, taking the kiln up in steps of 200 °C per hour, with a one-hour soak at 500 °C. Two of the stones were wrapped in aluminium foil to isolate them from the investment powder. The third was ‘naked’ to see if it developed surface damage. It didn’t, and all three stones changed to a not very appealing yellowish-green with serious internal crazing. It was clear from these very limited tests with tourmaline that heating has a low potential of success unless the stones are free from inclusions and flaws to start with.

The owner of the kiln had given me two pairs of light greenish-blue aquamarines to heat to try to remove the green cast. I only heated one pair, to keep the other for comparison. The pair of stones when heated slowly to 450 °C and cooled overnight lost the green and became a more pure blue, but they were pale to start with so the change was not dramatic. They didn’t suffer any damage, which was a pleasant relief after the lack of success with the tourmalines.

My experiment with heat-treating dark amethyst was a bit more successful. Many years ago I had lightened dark Namaqualand amethyst in a kiln at UCT, but had forgotten the temperature used. A quick search on the internet produced a link to a recent, detailed, open-source article on the effects of heating amethyst (https://www.nature.com/articles/s41598-020-71786-1). So I buried four stones in the dry investment powder and set the kiln controller to 300 °C, knowing that it would overshoot that mark. It reached 360 °C, which was the actual temperature I was aiming for, and then I switched it off to cool overnight. The result was that all four stones lightened slightly. Three stones suffered no damage, while the larger oval developed visible internal cracks that were not present originally. But they were still rather dark, so they were heated again to 385 °C. The additional 25 °C made a bigger difference. Interestingly, all the stones did not lighten to the same degree. Even different portions within some individual stones responded differently.

My conclusion from reading about heat treatment of gemstones online and this brief experiment is that it is a somewhat hit-and-miss business with a substantial failure rate. The advice of experienced heat treaters is that even gem stones from the same source may respond differently to heating, with rather unpredictable outcomes. I have satisfied my curiosity, and don’t plan on doing any more gemstone heat treatment for now.

Amethyst before heating

Amethyst after heating to 360 °C

Amethyst after heating to 385 °C