Saw this in a description of Arthur C Clarks prog on the subject:
It was evident, therefore, that a rude foundation of boulder stones was first formed upon the original rock, and then a thick layer of loose, mostly flat stones of feldspatic sand, and of a different kind from those found in the immediate neighborhood, were placed on this foundation, and then vitrified by heat applied externally. This foundation of loose stones is found also in the vitrified fort of Dun Mac Snuichan, on Loch Etive.

That sandstone sounds like the stuff at Wincobank.

on your ceramics line a wee thought when making raku pottery you paint on oxides on to the clay then fire it then throw it in vats of sawdust which causes reduction of the oxide to its base metal giving the metalalic appearance seen on raku pots. like you've said sand could give a glassy appearance, especially if used to put out a fire and thrown on hot rocks. I wondered if there was any metalicallyness also on these forts which may occur if oxides, such as iron oxide, where present and burning occured in an oxygen starved enviroment thus reducing the oxide.
mmm I think i'm probably going off the track a little here !
love
carolinexx