MadSci Network: Earth Sciences

Re: How is obsidian affected by heating, e.g., in a kiln?

Date: Tue Feb 14 20:25:07 2006
Posted By: David Smith, Director of Professional Development
Area of science: Earth Sciences
ID: 1138771558.Es

The effect you describe does indeed happen, but only with obsidians that 
have been altered by absorbing water (hydration) and by the decompostion of 
the obsidian glass into very tiny crystals (devitrification)  The result is 
a material called perlite, which is a relatively dense, solid material. 
When you heat it, trapped water expands, fluffing up the rock like popcorn 
and you get the puffy white or grey material that you can buy by the 
bagfull in the garden center for your potting soil, that is also called 

Before heating, the perlite is usually a pearly grey to white in color and 
often weathers into small spheres (pea-sized) made up of concentric layers, 
like a whole bunch of minature glass onions.  The more cloudy it is and the 
more readily it breaks up into spheres, the more water it has absorbed and 
the more lilely it is to "pop."

For a picture of some perlite, 

For nice background on obsidian, from the perspective of a geologically 
savy stone tool maker, see

I wouldn't recommend heating obsidian in your kiln.  Anytime you turn water 
to steam within a rock, the results can be explosive.  There's about a ten-
times increase in volume and so even a small amount of water makes a lot of 
steam and if that steam gets trapped inside - kaboom.  It's one thing to do 
that in a large industrial operation where each burst is small compared to 
the scale of the machine and another thing to do it in a small kiln.

Dave Smith, Da Vinci Discovery Center, Allentown, PA

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