Marking microfilm

The acquisition dept. staff in my library have asked my advice about a
"safe" pen to use to write on microfilm.  Part of the receiving
procedure involves writing the microfilm's ID number directly on the
leader footage.  The current pens in use are the generic fat, black
permanent markers that can be bought anywhere.  Does anyone know whether
these markers are safe to use?  (I have my doubts, which is why I'm
asking).  If these markers are not appropriate, where can I find some
that are?  If there are no safe markers available, then I will pursue
changing the procedure. But, I prefer to make life easier for the
receiving staff rather than harder.  Thanks for any help.

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                  Conservation DistList Instance 5:56
                   Distributed: Monday, May 11, 1992
                        Message Id: cdl-5-56-003
                                  ***