Alan-

1200 degrees is not hot enough for some minerals including muscovite. With
the sole object of removing all the OH hydrogen from the mineral (a
treatment that is hot enough and long enough - whatever it takes), I
wouldn't think that a controlled furnace is that important an advantage, if
any advantage at all.  You can follow the decomposition progress with
readings on the vacuum gauge.

Cheers,

Jim

James R. O'Neil
Department of Geological Sciences
University of Michigan
Ann Arbor, MI  48109-1063
Fax: (313) 763-4690