In reply to Flinticus:
Many types of Sandstone have microscopic gaps between the grains - thats why some absorb water as you may know.
You may be interested to know that those rocks which do not absorb water - rocks which are generally heavy, dense, compact materials (with no spaces between the particles) are thus very heavy are often used in places subject to extremes of heat such as furnace linings. Up here in North Yorkshire there were small areas containing rocks such as Basalt and a sandstone called Gannister, Both extremely heavy, and dense - it too was commercially quarried, and used to line furnaces. Because it was dense they could not absorb water, nor air. Therefore they did not break on being exposed to large amounts of heat.
Where both these stones occur on the surface I sometimes come across them walling. They are so dense they are impossible for me to break them with a sledge hammer. I've never tried putting these on a fire but I doubt they'd crack or explode.
Having exploded stones on a fire when I was much younger, like you, its a wonder we've still got 4 eyes between us. '
Post edited at 15:36
