David:
If you are looking for an alternative to the standard SG fluids,
Hanneman developed sodium polytungstate (I hope I’ve got that
right) as an alternative. The pure liquid has an SG of about 3.2, I
think, and then it is diluted with water until the sample is
suspended. Then the RI of the sodium polytungstate - water mixture
is measured and the RI is related on a graph to the SG of the
mixture. The water can be evaporated off by leaving the mixture
out, and pure sodium polytungstate results.
Unfortunately, I do not know what the toxicity of this material
is, but you could ask Hanneman. As you say, weighing might be
better.
Roy (Jess)