My question going into this experiment was how can rainbow matrix opals from Andamooka, Australia be treated safely and efficiently in a small lapidary studio?
The Andamooka variety of rainbow matrix is know in the bush as cement. It is an opalescent play of fire within a matrix of dull gray and tan limestone, and it needs a carbon imbedded into the stone to bring out the brilliant plays in color. The carbon gives a dark back ground to the limestone that contrasts with the colors of the fire within the stone.
Typically the stones are soaked in a sugar mix, and then burned with either sulfuric acid or a charcoal bonfire. But both of these traditional treatments are not beneficial for the small studio. The sulfuric treatment means the storage and handling of dangerous carcinogenic acid that can seriously burn the skin from just the fumes, while leaving the surface dehydrated worse than a typical heat burn. And, the bonfire method is taking a risk of exploding stone due to thermal shock. And, for the small studio to lose stones that have money and time invested in them is a big risk.
Also potential buyers may be turned off by the idea of wearing a stone on their body that may or may not have traces of sulfuric acids. With today’s green conscious society, people are becoming more aware of chemicals used in their merchandise. Therefore, we have a need for a “Greener” more economical way to carbonize the sugars that we soak into the stones.
After much discussion with chemist friends of mine and research on the effects of heat and sulfuric acids on sugars, I hypothesized that the sugars could be most efficiently carbonized in a small jewelers kiln using charcoal to create a reduction atmosphere and sand to moderate the heat absorption of the stones. This along with a slow ramp of temperature within the kiln reduces the risk of thermal shock to null, possibly pushing the carbon deeper into the stone than the acid.
The key to this whole experiment is the 50% by volume to 50% sand and charcoal mixture, in which I tightly pack the stones. A slow ramp of the kiln may push the carbon in by itself, but the sand, charcoal mix creates an even more shock resistant environment that is also used in many other facets of gemstone heat treating (mookaite for example).
Ok here is my experiment.
First, I cleaned the cabbed, pre-polished stones with a mixture of Dawn dishwashing detergent and a bit of ammonia. Then I wiped them down with alcohol to remove all traces of oils from handling them while I was forming the cabs. Do not use an oil and diamond paste mix while cabbing these, the limestone is very porous and will absorb the oils. After you wipe them with alcohol only touch them with clean tweezers.
I pre-warmed the cabs on a coffee mug warmer. In Alabama, the humidity is 100% all year long, or close. So, I wanted to drive off excess moisture that may have built up in the stones while I was working with them.
I used Black strap molasses and corn whiskey. Corn whiskey cuts the sugars well, and has less water in it than water :o) (In the South we have sorghum syrup which is sold as molasses, but it’s not the same thing. Avoid it.) Make sure it is pure cane, but I’m sure that brown sugar would work just as well. I poured the molasses into a crock pot and diluted it with whiskey till it rolled off of the wooden spoon about as viscous as cooking oil. Then I put the stones in just as the mixture was warming up. Wait till the temperature is about as hot as a baby’s bottle. Cook on high for 6 hours. Why six hours? I don’t know. It just seemed to be a magic number for me, lol. Cook till the whiskey is driven off and it is like a caramel consistency. I had to dig the cabs out. But, be careful. If you burn the sugar it will smell very bad.
Wipe the sugars off with a rag without touching the stones with your fingers. Then lay them onto a mug warmer. You don’t want water to get back into the stones.
While the cabs are in the sugar pot, crush up some charcoal. Get the kind without an accelerant added. Avoid Matchlight, compressed and flavored gourmet charcoals. Although a good chipotle flavor might make the studio smell good, ha! I used a non-compressed natural charcoal that is sold in hardware stores. I got a 15lb bag for $5. I think it is used in swimming pool filters. But, I’m sure any charcoal would work. Crush it up and mix equal volumes of washed construction sand to the crushed charcoal (volume, not weight). You can wash the sand in a wire mesh coffee filter. I filled a tin container with the mix, one of those containers people put cookies in to give at Christmas time.
I then put the tin in the kiln and took the temperature up to 600F to drive off the water. I held it there for an hour, but I’m sure that the times for the drying of the sand could be modified a little more scientifically.
After the sand has cooled down to just above room temperature, I buried the cabs in the sand and charcoal mix. Make sure the stones don’t touch and have at least an inch of sand all around.
I put this in the kiln and ramped the temperature up one hour per 100F to 550F, and I held it there for a couple of hours. I want to work on getting the time here correct, so I will play with holding times. But 2 hours worked. 500-600F simulates a charcoal fire. The charcoal provides a reducing atmosphere, and the sand evens out the temperature spread throughout the stones, preventing thermal shock. I am certain that there is a threshold where the carbon burns out of the stone and the water that gives the opal its flash of color is destroyed. So, I will play with times to find the point of destruction later. I will tweak this to get the greatest dept of carbon, while preventing it from burning back out.
Here is the cab coming out of the tin with the sand and charcoal mixture.
After 2 hours I ramped the temperature back down, allowing the stones to soak in the sand mix for 12 hours. When the sand is cool enough to touch by hand, take the stones out and soak in a mixture of Dawn detergent and warm water to remove the outside carbon. I used a bit of steel wool to help this along, but don’t use something too abrasive that would destroy your pre-polish finish.
The polished cabochon. The hole was created for a gemstone to be mounted in the center, and I wanted to create a shape that would test the boundaries of thermal shock.
I then finished polishing the stones with cerium oxide on a leather flatlap. I am still playing with finishes. Some recommend lacquers, Epoxy 330, or various other thick layers of polymers to protect the stone. But, I cringe at that. Opals have to breathe and have access to the moistures in the air to keep its fire. So, I have experimented with Renaissance wax, which will impart a microcrystalline layer, which will allow the stone to breathe. I am also going to experiment with natural damar crystals to make a varnish, which will also allow the stone to breathe, and I have friends who have recommended using Opticon lapidary epoxy. I will try some of that as soon as I can get some ordered, but I am unfamiliar with how much it seals. Finishing is still in the experimental phases for me :o)
I want to experiment further, and encourage others to do so, to find the perfect time of soaking the stones in sugars, temperature and times for soaking the stones in heat, and the perfect finish for these cabs. There is still much room for improvement, and I am anxious to hear results from various other lapidaries.
The idea is that the sugars turn to carbon as deep into the limestone matrix as it can be absorbed, the deeper the better. This creates a backdrop for the play in colors that is otherwise invisible in the tan to grey colored Andamooka rainbow matrix opals. The sand is used to evenly heat and insulate the stones, preventing an explosion, so as the slow ramping up of the temperature. The charcoal is used to create a reducing atmosphere pulling oxygen off of the stones as it is released from the sugars, leaving only pure carbon. The charcoal also keeps down the smell of burning sugars, which is very noxious. Let the whole thing cool slowly.
Important things to keep in mind;
Drive off as much water from the stones as possible. Water expansion can make the stone explode in the heat, no matter how much sand is used.
Dry the sand very well. Make sure that the stone is packed into the sand charcoal mix well. Air gaps can cause an uneven increase in temperature, cause an explosion of the stone.
Do not use an accelerated charcoal. This will definitely ignite in the kiln.
Ventilate your kiln to the outside. You should do this anyway.
And, don’t burn yourself!
While some may still want to hold onto traditional methods. This experiment might provide some insights into further production experimentation of Andamooka matrix opals in the small studio.
Also, I will post pictures of these opals in their settings very soon :o) Stayed tuned…
