[Histonet] xylene substitutes

[Monfils, Paul]

I have used xylene for close to forty years, and consider it the safest of
all commonly available clearing agents.  For certain special projects over
the years I have also used toluene, carbon tetrachloride, dioxane,
tetrahydrofuran, and a few other compounds that require much more stringent
safety precautions than xylene does. Certainly xylene does pose some health
concerns. What chemical doesn't? You shouldn't wash your hands in it to
remove paraffin.  But my attitude toward the various recently-offered
"xylene substitutes" is this:  I would rather use xylene, the potential
hazards of which are well documented after many decades of general use, than
an unknown mixture of organic solvents which have not been in use long
enough to allow accurate assessment of possible longterm health hazards.
Most of the "xylene substitutes" on the market are not pure compounds, but
mixtures of several (in some cases many) different organic solvents. The
manufacturers won't reveal the solvents contained in the mix, and in some
cases they have not even identified all the organic compounds present in the
product!  I don't feel comfortable exposing myself on a daily basis to large
volumes of a product of unknown chemical composition and largely untested
health effects.  

That having been said, I recently did test several "xylene substitutes"
because of a special project where I needed to paraffin-embed synthetic
hollow fibers which were broken down by xylene. First I tested a number of
pure compounds, many of which attacked the fibers much more vigorously than
xylene did. Some of them completely dissolved the fibers within seconds. I
did discover that carbon tetrachloride worked exceptionally well, but was
reluctant to use it due to the combination of high toxicity and high price.
Then I turned my attention to "xylene substitutes". I tested eight different
ones, from various manufacturers, and while most of them caused no apparent
damage to the fibers, they differed greatly in their ability to dissolve
paraffin, as well as in their odor. I know from past experience that they
also differ greatly in their ability to dissolve fat, though that was not an
issue in this study. I finally settled on "Safe-Clear" from Fisher
Scientific, which gave me results comparable to carbon tetrachloride, and
has a mild odor. But is it really safe? I don't know!  I handle it with care
since I don't really know what I am handling!  Of course, the fact that this
product produced good results for this specific purpose doesn't necessarily
mean that it would be the best choice for general tissue processing.

	Paul M.