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The Alchemy of Gemstone Heat Treatment

January 8, 2020

By Justin Prim

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Blue Ratanakiri Zircon Photo Courtesy of Alexey Spirit Yakhlakov

Before the advent of modern science, Renaissance-era proto-scientists known as alchemists thought that they could change the fundamental chemistry of lead through an elaborate and semi-mystical process and purify it into gold. These ancient seekers were doing all sorts of experiments with chemicals and rigorously keeping track of their results in order to be able to share their findings with other alchemists. They believed that if they, through their experiments and research, conquered the four classical elements of Earth, Air, Fire, and Water and became masters of the physical world, they would be able to purify lead into gold and then through the same process, proceed to purify the human soul into a state of divine perfection.

It’s doubtful that any alchemist ever succeeded in turning lead into gold but their experiments, theories, and early deaths due to chemical poisoning might have lead to the discovery of the phenomenon that I witnessed in a homemade shed behind a house in Ratanakiri, Cambodia.

Banlung, Ratanakiri Province, Cambodia Photo by Justin K Prim

Witnessing the Incredible

The process of transforming a poorly colored, undesirable stone into something that is bright, beautifully colored, and full of value is usually a highly guarded secret that many people in the West know nothing about. There aren’t many books and there aren’t many people willing to discuss the techniques of heating a stone in order to improve its color.

In late March of 2017, I was traveling as part of a gemology expedition lead by Bangkok-based field gemologist Vincent Pardieu. With the help of our Cambodian gemstone guide and translator Sovanny, we were able to meet with a Cambodian stone burner and get permission to come and witness him do his alchemical magic.

There surely was magic in the air as we walked into his house. The first thing we encountered was a talisman against evil spirits; a hanging sculpture created from the mixed remains of a cat and a chicken in a sort of homemade taxidermic fashion. We seemed to be going deep into a secret and distinctively different part of that gem trade than we were used to.

The stone burner, Mr. M, took us out into his backyard where he had a shed built out of galvanized steel. The setting seemed normal enough. There were chickens in the neighbors yard with a rooster occasionally crowing. The other neighbor was sitting outside grinding herbs with a mortar and pestle.

Grinding Station Photo by Justin K Prim

Mr. M prepared his makeshift laboratory: Outside in the yard he had a small hand-cranked 350 grit lapidary wheel that he used to grind off any included areas of the stone that he was about to burn. If there are inclusions or fractures in the stone, there is the possibility that the stone will explode during the heating process so it’s very important to look closely at your stone and make sure it’s totally clean and inclusion-free before beginning.

The Burning Lab Photo by Justin K Prim

The Process

We watched Mr. M prepare a brown, root-beer colored Zircon crystal from a local mine. The Zircon from Ratanakiri is one of the only Zircon sources in the world that has just the right chemical combination inside it to be able to transform from it’s dark reddish brown origin color into a light and clear sky blue. It’s also the only place that’s producing large enough clean rough to be able to create a large, nicely saturated blue Zircon stone.

After he was satisfied with the state of his stone, Mr. M took it, along with about 8 smaller Zircons that Vincent brought for this experiment, and placed them inside a crucible. The crucible is a small ceramic cup with a lid. The small crucible he put the stones in seemed to be white on the inside, almost like a child’s porcelain tea cup. He placed the lid on and covered the seam of the lid with a cement that we watched him make. He said it was a special cement for this sort of firing application that comes from Thailand. He mixed it with water and a little sand and experimented with the consistency a bit before he felt it was ready. He applied the mixture all around the seam, totally covering it and locking the stones and the air around them inside.

Sealing the Crucible with Cement Photo by Justin K Prim

Once this was finished he put the crucible into a larger crucible and proceeded to put a lid on and cover the seam with cement in the same way. This creates an air buffer so that the heat of the flame doesn’t hit the stones too hard. Like a double boiler with chocolate, you don’t want your stones getting scorched so you insulate them from the flame with this chamber of air. Vincent told me that the crucible is made of Alumina, a special ceramic corundum compound that is able to withstand very high heat without melting.

The Double Crucible is Ready for the Kiln Photo by Justin K Prim

Once the double crucible was airtight with cement he placed it inside his homemade kiln. The kiln is a simple device that I have seen used in other places for melting metal for casting. Essentially, it’s a small cylindrical drum, lined with fire bricks with an open top, fed with air from the base by a tube sticking out at one side. It looks like a large cannon and we would soon find out that it really does shoot fire. The bricks that the kiln is made out of helps to hold the heat from the fire inside the kiln and enables it to reach temperatures of almost 1000 degrees Celsius. The exact temperature in this kind of homemade device is unknown and variable but based on the kind of wood charcoal that Mr. M was using, Vincent estimated that he couldn’t have gotten the inside of the kiln hotter than 800 degrees Celsius. The tube near the base of the apparatus is used to pump in air to feed the fire and get it really hot. Mr. M had a small electric blower and he attached it to the tube with a hose.

Spitting Fire Photo by Vincent Pardieu

The bottom of the kiln was full of wood charcoal. Within the charcoal sat the double crucible. Mr. M then started a fire in the charcoal and turned on the blower. This is when things started getting exciting. His little shed quickly began to fill up with disgusting black smoke and a shower of sparks began raining out of the top of the forge. The alchemical experiment had begun. We couldn’t be sure what would happen but we were excited to find out and document our findings just as the Medieval alchemists had done generations before us. The shed quickly became toxic with smoke and sparks, so we waited outside. Mr. M let the blower run for 20 minutes before turning it off. He let the flames and the smoke die down enough to take the crucible out. He used special tongs to pick up the sealed receptacle and when he pulled it out, it was glowing red hot.

Glowing Crucible Photo by Justin K Prim

The Result

He let the crucible sit on the dirt floor for 10 minutes with the blower helping to cool it down faster. Once it was cool enough to touch, he started breaking the hardened cement off lid on with a knife. He methodically worked around it until all the fired cement came off and he was able to take the lid off. He pulled out the smaller crucible from inside the bigger one and and proceeded to do the same methodical breaking of the cement.

He finished breaking the seal and looked at us. We looked back at him with our breath held tightly in our chests. This was the moment of the final reveal. Did our experiment work? Did we destroy or melt our stones? Did we transform them in the the way we meant to or did something else happen completely? He looked back down at the small metallic container and pulled the lid off. Inside the crucible, on its shining white porcelain-esque walls was a perfect pile of baby blue gemstones where once sat a group of ugly, muddy looking rocks. The experiment worked and the results were incredible! Not only did the color change but the clarity improved as well.

The process of physically changing Zircon from red to blue is still somewhat unclear. The uranium that the Zircon contains is obviously interacting uniquely with each stone’s chemical composition when it’s engulfed in the fire but for now, the exact mechanics of it remain a mystery. I took a look at Mr. M’s stone which was the largest and easiest to inspect the changes on. The stone was perfectly translucent and the color was remarkable. Beautiful. Desirable. I was in love with what had just happened.

The Before and After Zircon Photo by Justin K Prim

My Attempt

I was so inspired by what I witnessed that I had to try and reproduce the results myself. I bought a small bag of the Cambodian Zircons while I was there and luckily enough, a friend of mine had almost the exact same kind of wood burning kiln, used for melting aluminum for casting. I tried to follow the exact steps that I had seen in Ratanakiri. I separated out the stones that I wanted to heat and made sure they were free of inclusions and then I put them in the small crucible that I bought in Bangkok. I used the Thai cement powder mixed with water and sand and painted it on to the seems with my fingers. Next, I placed the small crucible inside the large crucible and cemented that the lid onto that one. I filled the kiln with firewood and started the fire. There were a couple of differences between my setup and the one in Cambodia. First, the burning material was different. Second, instead of an electric blower to fuel the flames with oxygen, I had a pedal powered one made from an old bike.

Pedal Powered Kiln Photo by Justin K Prim

I tucked the kiln into the wood charcoal and started to pedal. Due to the way the pedal powered blower was made, it was very hard to pedal fast for 20 straight minutes so I did the best I could and then took small breaks and then pedaled for more. After 20 minutes I took the lid of the kiln and discovered that something had gone wrong. The outer crucible had cracked and broke. Nothing was glowing red hot. I took the small crucible out with tongs and let it cool. I the broke off the cement and poured out my stones. I was disappointed to see that instead of turning blue the rusty red stones had lost much of their color and were mostly clear with some red spots.

Before and After Zircons that I Heated Photo by Justin K Prim

Final Thoughts

After doing some research and asking around, I discovered a few things I did wrong. First, I think I should have started pedaling slowly to let the heat build up slowly. I think the sudden flames might have shocked the outer crucible and broken it. Secondly I discovered one thing I hadn’t seen in Ratanakiri. Mr M buried his crucible inside the charcoal so that no oxygen could get to it. Zircon needs to be heated in an oxygen deprived environment for the blue transformation to occur. Since my crucible was placed on top of the the coals instead of underneath, there was no chance they could turn blue.

Unfortunately, I didn’t have more time to use my friends kiln before I had to return to Thailand. I still have more unheated Zircons so I am hoping to correct my mistakes the next time. I still have a ways to go before I can call myself a Master Alchemist. Thanks to Mr. M. in Ratanakiri, our group got to become modern day alchemists for an hour and witness a process of true alchemy that changed the chemical properties of our stones and thereby dramatically changed the color and clarity of the stones. Now if only we could figure out how to use this process to refine our souls…

If you’d like to see a video made about this trip check out this one made by Vincent Pardieu

About the Author

Justin K Prim is an American lapidary and gemologist living and working in Bangkok, Thailand. He has studied gemcutting traditions all over the world as well as attending gemology programs at GIA and AIGS. He is currently working on a book about the worldwide history of gemstone faceting. He works as a Lapidary Instructor for the Institute of Gem Trading as well as writing articles, producing videos, and giving talks about gem cutting history.