|MadSci Network: Earth Sciences|
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 perlite. 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, see: http://www.gc.maricopa.edu/earthsci/imagearchive/obsidian.htm For nice background on obsidian, from the perspective of a geologically savy stone tool maker, see http://www.eskimo.com/~knapper/HotStuff.html 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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