[Histonet] paraffin carry-over problem

[Monfils, Paul]

I have never used Clear Rite or any other xylene substitute in my lab.
Part of the reason may involve something about teaching an old dog new
tricks. But there is another reason too.  A few years ago I had to
paraffin embed and section tissues containing fibers of a new polymer
that was to be used in an artificial kidney device.  Xylene severely
damaged the polymer.  So, at that time I did a bit of research on
clearing agents in an attempt to find one that could process these
particular specimens without destroying the polymer fibers.  I did
comparative tests with xylene, toluene, chloroform, tetrahydrofuran,
dioxane, and seven different commercial "xylene substitutes". Since I
would have to use this solvent not only for processing the tissue, but
also for deparaffinizing the slides, I compared the various solvents for
their efficiency in dissolving paraffin.  I put 20 ml of each solvent in
a glass vial, added 10 pellets of the embedding paraffin we use here,
placed them on a rotator to swirl them gently, then timed how long it
took for the 10 pellets to disappear.  What I discovered surprised me.
All the "xylene substitutes" took a lot longer than xylene to dissolve
the paraffin, in some cases 5 or 6 times as long.  I did find a product
that worked beautifully for this particular project - "Safeclear" - but
I have been reluctant to replace xylene with any of these "xylene
substitutes" based on the results of those tests, even though I know a
lot of people use them and are apparently satisfied with their results.