Ludwik,
Nice analysis of the conduction of glass at high temperatures.
First- glass as opposed to quartz contains a "flux" impurity that lowers the
melting point, and it may well be the impurity that supplies the conduction
electrons. If you would like to try the experiment, it is written up in the
American Journal of Physics Vol. 58, page 90 (Jan 0f 1990 issue).
Gary
Gary Karshner
St. Mary's University
San Antonio, Texas
KARSHNER@STMARYTX.EDU