Transcription discs

The archives that I work with has several large collections of glass
transcription discs from the 40's and 50's and they will be
reformatting a selection of them soon.  I am unfamiliar with this
archival record format and would like some further information
before we proceed with this project.  In particular several discs
have a white crystal formation over their surface.  What is the
black layer of the disc made of?  What might the crystals be?
Finally is there a safe way to remove them?  Thank you for your
input,

Andrea Bowes
Conservator

                                  ***
                  Conservation DistList Instance 12:48
                 Distributed: Monday, November 30, 1998
                       Message Id: cdl-12-48-014
                                  ***