I recently had a small chunk of volcanic glass or obsidian donated to me. I thought "WOW" what a great idea. Volcanic glass jewellery, now that's a new one on me.
So on with the safety spec and a big hammer. I preceded to smash the living daylights out of it. I did manage to stop before totally breaking the whole piece. As you can see from the next picture. All nice and shiny and black.
I then picked up all the shards of glass and placed them on the kiln shelf and set the kiln going on the normal firing range. I did not worry about the expansion of the glass as I was just trying to slump the bits into jewellery size pieces and not mix it with other types of glass.
As you can see from the following picture the end result was not what I was expecting. This beautiful black glass had turned into this boring dull grey colour stuff that looks like what you would find in the garden.
Well after this disastrous outcome I decided to do a bit of research on this type of glass and it seem I need to get the kiln up about 1500 degrees C that's about 2700F. About 500 degrees C above what my kiln will do.
So the lesson of the day is stick with what you know until full research has been done. But as a man I would probably still have a go just to prove it cant be done. :o)