Many carbon dust drawings will adhere well from the constant brushing
and burnishing and do not need to be sprayed. You could see if dust
picks up on a tissue lightly dabbed on a surface.
I have used Krylon Workable Fixative, but sometimes it and others
will spot or, worse yet, spray out white or yellow particles. It is
important to test whatever you use on a scrap of the same drawing
surface.with sample areas of dust and line before using it on your
drawing. Shake the can well before spraying.
Alcohol does set the dust. You can use any kind of clear alcohol
(not whiskey, in other words, and not even gin/vodka) such as rubbing
alcohol or 70% alcohol plus 30% water, which is a common percentage
used in museums. It can be sprayed with a perfume or other atomizer
that gives a fine spray. Actually, you can gently pour the alcohol
evenly on the drawing (with fear and trepidation) - test this first,
also - and let it dry. The alcohol will turn clay-coated surfaces
yellow, but when it dries the surface will return to white. The
alcohol re-texturizes the dusted areas, allowing additional layers of
dust to adhere and make the areas blacker, if desired. So it is used
to help get more intense blacks, not just as a fixative.
Clara is correct; acrylic glazing could lift off a drawing, or
parts of the dust, if it builds up static electricity, as will
acetate (or polyester) coverings. I have seen drawings lifted up and
ruined when they were covered with acetate directly on the drawing -
then stored in stacks in a humid climate. The "plasticizer" in the
acetate, in this case, leached out and made the acetate sticky, which
added to the problem. Double mats should help prevent lift-off, but
it is safest to use glass as glazing over friable media over a long
period, but also with mats.
Good luck - but do test first.