Tom,
 
   Not to worry...
 
   The dark "lining" you are seeing is indeed a cocoon that the larva spin in the cell, just like the
old catterpillars to butterflies trick.  When you melt down old brood comb, you will get less
wax, and you gotta strain it more than comb from supers.  The older the combs, the thicker
they are, (and the less wax.)  A solar wax melter makes it easier.
 
   I saw a post here for a "cheap and dirty" solar melter that used a styrofoam cooler lined with
a black garbage bag, covered with clear plastic or plexiglass, that worked well for small jobs.
 
Take care,
Gerry Visel
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(815) 226-6620
(815) 394-5438 or -2827 (fax)