(How to) Heating Zircon

(via Wahroongai News, Volume 33, Number 7, July 1999) Mark Liccini writes:

Here is how to heat zircon. You place them flawless……must be real clean or they will crack, and even then some may crack….in a (fire-clay) crucible with activated charcoal. This can be bought from any chemical supply company for about US$30 for a small container. Activated charcoal is the best to use, but there is another way to avoid using of reductive gas. Importantly, activated charcoal will work without creating any smoke. The other way is to put sugar in the crucible, but it starts to put out a lot of smoke when you get up to temperatures of 600°C or more. Although the smoking of sugar stops around 800°C, lots and lots of smoke will be generated. Boy! I mean a lot of smoke!

Then, you then must fill the crucible with zircon and seal the crucible. A good way is window screen and just a layer of plaster (plaster-of-Paris) over the top of the screen. Let it dry; then fill in any cracks after the plaster has dried. You then take the temperature of the crucible up slowly. The slower you go, less breakage will occur….all the way to 1000°C. Hold the temperature there for 2 hours or so. The controlled rise in temperature, depending on your furnace could take all day or longer. Following heating it can take 5-6 hours to cool the crucible down to cold (room temperature). Don’t open the door while the crucible and its contents are hot, or all will crack.

Now here are some tricks of the trade.

You will observe precisely in the bottom of a sealed crucible the best blues will be found. Near the top the heat treated zircons may be white. If you rotate the zircon rough imposition and repeat the heating and cooling cycle again, the white zircons will turn blue. If you overheat the zircons (and bleach them), you can do them again and they will come back blue. Even light blues done again will change to dark blues.

Now there is a trick to produce orange and red colors. You might obtain some oranges and red on the top of a sealed crucible. Indeed, when you first open a sealed crucible you will go crazy. The whole top will be covered with red and oranges. However, beware, for after a few minutes in the air all oranges and reds will revert to white and/or light blue—except a stone or two. These are stable reds and oranges. Now to ensure a high percentage of reds and oranges do the same heat treatment without sugar (or activated charcoal) and in an open crucible.

Note: With both heat treatment methods, you will get better results with a full crucible of zircon rough. Although I have never heat treated Cambodian zircons, I have run tonnage of Nigerian and some from Tanzania and Australia.