Subject: why glass conducts electricity when it is hot , not when it is cold.

 Hi, My name is Walter Lieberman and I am teaching a program called Science of
Art (for elementary school kids) at the Museum of Glass in Tacoma Washington.

http://www.museumofglass.org/education/science-of-art/

We are doing a lesson involving the conductivity of glass. We are going to do an
experiment involving using molten glass to conduct electricity.  I am looking
for a simple coherent explanation why glass conducts electricity when it is hot
, and not when it is cold. 
    Thanks for your help

Re: why glass conducts electricity when it is hot , not when it is cold.