non-toxic heavy liquids

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Dear Paleontologists,
  Does anyone have experience using Na polytungstate that they are willing
to share with me? Alternative does anyone know of other low-toxic heavy liquids?
 I am deciding whether or not to buy some Na polytungstate to use in
flotation of foraminifers. I asked a chemical company that sells it about
adjustment of the specific gravity, and they were clueless. I assume that
one just adds water to it until the density is reduced to the level desired.
When using other heavy liquids, I adjust density with the appropriate
solvent until a gypsum crystal (2.3) is neutrally bouyant. 
  The chemical can be purchased in liquid or powder form. Has anyone used
the powdered form? Was there any problem diluting it with water? Is it
readily soluble? The powder form is less expensive than the liquid form, but
if the powder is difficult to use, then I would like to avoid it. 
  I have some correspondence from Chris Barnes from years ago. He said that
he evaporated the water to recover the Na polytungstate. He said that the
last stage of evaporation goes fast and to be careful or I would end up with
a cooked brick too hard to redissolve very well. Have you had this problem
or any others?
  The stuff is, apparently, more viscous than some the more toxic and
traditional heavy liquids like TBE. How long does it take to do a
separation, 5 minutes, 1/2 hour, more?
  Thanks in advance for your help.

Cheers, CB

Charlotte A. Brunner
Center for Marine Sciences
The University of Southern Mississippi
Bld. 1103, Rm. 102
John C. Stennis Space Center
MS 39529

Office phone: 601-688-3402
Dept. phone: 601-688-3177
FAX: 601-688-1121
Internet: cbrunner@whale.st.usm.edu