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