Historically, there have been instances of the use of fire by ancient man to modify the structure of rocks to make them more malleable.
http://www.mainlesso...ibal&story=alps (quite long, the reference is near the bottom of the page).
In response to swede's post #242, I have a correction. Glass is not only present in the samples (http://www.sciencedi...305440308001489), but ferrous crystals embedded within the glass bubbles can be used as a means of dating the time at which the vitrification took place (http://www.sciencedi...305440307001938).
Cool! I learned something new today.
Edited by Kahn, 04 November 2013 - 01:31 PM.