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

I once heard that an obsidian sample will expand (by a large amount)and turn 
into a light gray mineral if heated in a kiln. My students want to try this, 
and our art teacher is (cautiously) interested, but we want to know more... 
First, will anything interesting actually happen?  Secondly, is it safe?  will 
clay pieces in the kiln be at risk? (It is expensive to fire the kiln, so we 
would like to include student art projects.) I have been unable to find any 
more than vague references to "physical changes" at high temperatures.

Thank you for any insight you can provide on this.  Sincerely, R. Johnson

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