After looking through some of the later 20th c. literature and finding
nothing, I checked <Taxidermy and Zoological Collecting> by William T.
Hornaday; Charles Scribner's Sons, New York, 1892.
 
After some general comments about skinning turtles and preserving them
with an arsenic dressing inside and out, Mr. Hornaday goes on to say (pp.
206,7 "...and the huge, lumbering, greasy 800-pound leather-back is a
first class calamity.  Shun him, unless there is plenty of money behind
him.  I once had the misfortune to be chief-mourner over a leather-back
which pulled down 940 pounds dead weight - mostly oil. ...vowing that
neither gold nor glory (neither of which is yielded by <Sphargis
coriacea> should ever again tempt us to 'strike oil' in that manner.  The
soft and gelatinous shell of that monster dripped clear oil for several
months, and actually yielded several gallons."
 
Based on the foregoing, I would caution against handling the piece until
someone knowledgable tested for the presence of arsenic, a common enough
substance in natural history collections.  What appears to be a varnish
or coating may be oxidized oil which oozed out of the skin over time and
has darkened and hardened.  Hard to tell from here.
 
Good luck.
 
Jack C. Thompson
Thompson Conservation Lab.
Portland, OR
 
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