Q&A: Heating Zircon

Posted on Dec 18, 2015 in Q&A, Transformations, Uncategorized | No Comments
Q&A: Heating Zircon

Question:

I am curious about the process of changing the color to colorless. I had read online that the color can be changed by heating at home, is this so (or just silly internet advice)? I am considering changing its color if so…

Answer:

Yes, you can heat zircon yourself and without special equipment. Many cutters and suppliers do it routinely in their own offices. I started with just an alcohol lamp & a pair of tweezers, but now I use a computer controlled kiln. Really any heat source will work provided it gets hot enough. Though a source that does not produce soot is probably best.

A quick Google search will give you some ideas of the different ways people heat zircon. But what many of those pages lack is good information about the hazards of heating zircon yourself:

Keep in mind, zircon has to reach a temperature someplace between 400C-550C (750F-1020F)* to change color. Using tweezers to hold something this hot can be serious risk. If the stone slips, it’ll likely burn whatever it falls on from wood to carpet to your hand if you try to catch it. (NB: burning carpet smells really bad. Really. Trust me.)

There is also a possibility the stone can shatter due to inclusions or internal structural strain. I haven’t had this happen with zircon, but it’s possible with any natural material. (Incidentally, zircons are usually cut before heating to eliminate any inclusions that may prove problematic.)

Heat shock is also a risk to the stone. That is, internal cracks may develop if the stone undergoes a rapid temperature change. e.g., if a 500C stone gets put in room temperature water or if a room temperature stone is suddenly put into a full flame.

The best way to mitigate all of these safety issues is to use a computer controlled kiln to heat your gemstones. Computer controlled kilns are able to ramp up to a target temperature slowly, hold at that temperature for a specific amount of time and then stop the heat to begin slowly cooling. Since the stone is generally held in a crucible or similar container with some kind of investment medium, there is no risk of fumbling a very hot rock and minimal risk of heat shock. Any risk of a stone explosively shattering is also mitigated, since the gem is well contained.

Of course, the down side of using a kiln is that you cannot watch the zircon change color during the heating process. Hitting the target color can be challenging; the heater must rely on experience from previous attempts.

It’s not a safety issue, but it’s worth knowing that not all zircons will achieve a permanent color change. Some zircons only change color while they are hot, but will revert to their original color as they cool down. Other cutters have described this phenomena in Tanzanian zircon, but I’ve only seen it in Nigerian zircon. Your mileage may vary.

Disclaimer: I am only answering your question, not recommending you heat your own stones. If you want to heat zircon, I recommend you contact someone with a kiln & experience heating gemstones.

If you’d like to see an illustrated example of how heating changes the color of Tanzanian zircon, please see my previous Journal entry, “Heating Red Zircon.

*Other sources, such as the Gemology Project, state that Tanzanian zircon will change from dark brown/red to colorless around 900C. In my own experiments, I’ve produced colorless zircon at 625C. It’s well known that zircons are variable in how they heat, so if you’re trying to achieve a very specific color it’s best to heat incrementally.

White Zircon after heating

Purple zircon before heating