Treating Andamooka matrix - with sugar, no acid

Posted by Mark Jones on

If you would like to try this technique, do so at your own risk! 

This blog is provided for interest purposes only in response to multiple requests for information on how we treat Andamooka and Opalton matrix.

Beautiful finished pendant set in sterling silver

This technique does not use acid.  We have another process that uses acid (check out our blog on that process – How to treat Andamooka matrix - with sugar and acid)

Pros =  Makes some beautiful opal, fun to see the colour change.

Cons =  Stones can crack if heated too much, not suited to hard matrix material as they can explode in the oven process, not to mention it looks like you have set up a meth lab in your workshop ;) 

For the heating of the sugar solutions, we use a laboratory hot plate which has a good temperature control. It is very important not to overheat the opal as they can fracture if they get too hot.

 Rough rainbow matrix about to be sliced into slabs

Put 1 cup of raw sugar in a Pyrex container, beaker or other heat resistant bowl with an equivalent amount of water and heat until the sugar is dissolved. Set the temperature to about mid-range for the heating process. The beaker can be placed directly onto the hot plate.

Before treating the matrix opal, it needs to be cut and shaped and sanded but not polished. The opal should be dried thoroughly by lightly heating or air dry to remove any moisture from the fine pores in the stone. Avoid touching the stone with your fingers as oil from your fingers can affect the treatment.

The stones are then placed in the sugar syrup solution and cooked for around 8 hours (give or take). The cooking time is a bit of a guess and depends on the porosity of the stones and what colour tone you are after. Very porous matrix will absorb the sugar solution more quickly than dense matrix. It may be necessary during this stage to add some more water to the sugar solution if it becomes too thick due to evaporation during the cooking process.

 

 rough pieces of matrix cooking in the sugar solution

After the cooking time has elapsed, let the stones cool down in the sugar solution. Avoid touching them with your fingers.

The next step is to wrap the stones individually in some alfoil and cook them in an oven. It’s a good idea to mention here that matrix with a very hard host rock such as ironstone and some hard sandstones will explode if the temperature gets too hot.  You are better off using the acid method if in doubt. We use a cast iron camp oven as we can control the temperature better. Put the wrapped stones into the camp oven, put on its lid and place it on a gas burner ring and cook it for a couple of hours, it will smoke and smell a bit for 20 minutes or so depending on the sugar concentrate. After the desired cooking time, remove the stones and allow to cool fully. The actual cooking process carbonises the sugar in the stone turning it black.

 

sugar cooked stone about to be wrapped in alfoil

 Cooking stones in our camp oven

You can now face, cut, shape and polish the stones as per usual. If the matrix was quite hard with small pores it is better not to use cerium oxide or tin oxide as the polishing compound can get into the pores and be seen as tiny white spots.  This can make the finish look a little pale. Use a leather lap with 50,000 grade diamond paste for the polishing.

 

Cooked stones unwrapped

 

 cooked matrix ready to be shaped and finished

Don’t push too hard when polishing as this will generate a lot of heat and can crack the stone. Take your time. The more porous matrix may not polish at all. If this is the case the surface may have to be coated with a liquid glass to produce the appearance of a highly-polished surface. I have tweaked this process with various temperatures and cooking times and it seems to work well.  You may need to modify a few things to suit your own requirements and the different stones being treated. Either way at the end of this process you should end up with some beautiful stones which are comparable to natural black opal.

 

 Cooked matrix yet to be polished

Check out our rough opal section in our online shop for Andamooka matrix at www.opalquest.com and our range of finished hand crafted jewellery  Stocks vary throughout the year.

Thanks for taking the time to check us out!

Cheers Mark


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