A tip from the late, great Nick Brolon:
If you have a sensitive DC volt meter which will
read millivolts, put it across these "transmit micas."
On many, you will read a small voltage.
This is from moisture which has entered over
80 years. 

Cover a cookie sheet with parchment paper.
Put your micas in a 180-200F oven for an hour or so.
The heat will drive out the moisture from most
of them.  Let them cool and many will check "good" again.
Worked for me.  Had a few "stubborn" which
required a second treatment.  Only two
failed to recover from about a dozen.

GL OM DE Dave AB5S