4.6.
DESIRABLE CHARACTERISTICS OF EMBEDMENT
93
proposed as desirable represent a judgment on the part of the investigator. As will be seen below some formulations have been designed to
allow a controlled variation in hardness over a moderate range so
that the investigator can suit his own taste or fit his own problems.
Talking again of the cross-linked plastics only, there is a curious
phenomenon often seen in good blocks which might be characterized
as a conflict between plasticity and brittleness. One can have a block
that can be slowly deformed, and indeed actually may seem to be rather
soft. Yet a sudden strain, as when trimming a spot with a razor blade,
will create a chip or flake. It is sometimes surprising how big a flake can
be chipped off a block as soft and pliable as it may be. This phenomenon, though, demonstrates crudely that such a block can be expected
to behave as a relatively rigid structure in relation to strains of short
duration, such as one would expect to exist at the knife edge during
actual sectioning.
Ordinary methacrylate blocks behave quite differently in their cutting
characteristics from the usual epoxy and polyester ones. Good sections
can be cut from much softer blocks, which easily can be indented with a
fingernail. This in turn means that much larger sections can be obtained.
This apparently is not simply a distinction between plastics which are
cross-linked and those which are not, for it is possible to cross-link
methacrylate (Chapter 4.15), and this does not importantly alter its
cutting properties. It is my personal opinion that the wetting properties
of methacrylate are substantially different from the epoxy or polyester
plastics. In sectioning it, the block face is not so apt to be wet from the
trough fluid as when the other plastics are used, and thus fluids with
very little surface tension can be employed that would not be practical
with the other plastics. If a glass knife is wet to its edge, methacrylate
seemingly does not have any strong tendency to wipe away the fluid, and
thus establish contact with the glass as do the epoxy and polyester
plastics. I suspect that such contact is dangerous, for then adhesive forces
tend to bind the plastic to the glass, and a drag is put upon the section
as it floats away from the knife, and a compressive force where the section is being cut. In cutting any plastic, one need only lower the meniscus
of the trough fluid below a critical point to observe a rapid deterioration
in sectioning, presumably related to this sort of sticking at the knife
edge just where the section is being cut. Thus it is that any plastic which
is to be used easily as an embedding medium must be reasonably hydrophobic, presumably the more so the better.
DESIRABLE CHARACTERISTICS OF EMBEDMENT
93
proposed as desirable represent a judgment on the part of the investigator. As will be seen below some formulations have been designed to
allow a controlled variation in hardness over a moderate range so
that the investigator can suit his own taste or fit his own problems.
Talking again of the cross-linked plastics only, there is a curious
phenomenon often seen in good blocks which might be characterized
as a conflict between plasticity and brittleness. One can have a block
that can be slowly deformed, and indeed actually may seem to be rather
soft. Yet a sudden strain, as when trimming a spot with a razor blade,
will create a chip or flake. It is sometimes surprising how big a flake can
be chipped off a block as soft and pliable as it may be. This phenomenon, though, demonstrates crudely that such a block can be expected
to behave as a relatively rigid structure in relation to strains of short
duration, such as one would expect to exist at the knife edge during
actual sectioning.
Ordinary methacrylate blocks behave quite differently in their cutting
characteristics from the usual epoxy and polyester ones. Good sections
can be cut from much softer blocks, which easily can be indented with a
fingernail. This in turn means that much larger sections can be obtained.
This apparently is not simply a distinction between plastics which are
cross-linked and those which are not, for it is possible to cross-link
methacrylate (Chapter 4.15), and this does not importantly alter its
cutting properties. It is my personal opinion that the wetting properties
of methacrylate are substantially different from the epoxy or polyester
plastics. In sectioning it, the block face is not so apt to be wet from the
trough fluid as when the other plastics are used, and thus fluids with
very little surface tension can be employed that would not be practical
with the other plastics. If a glass knife is wet to its edge, methacrylate
seemingly does not have any strong tendency to wipe away the fluid, and
thus establish contact with the glass as do the epoxy and polyester
plastics. I suspect that such contact is dangerous, for then adhesive forces
tend to bind the plastic to the glass, and a drag is put upon the section
as it floats away from the knife, and a compressive force where the section is being cut. In cutting any plastic, one need only lower the meniscus
of the trough fluid below a critical point to observe a rapid deterioration
in sectioning, presumably related to this sort of sticking at the knife
edge just where the section is being cut. Thus it is that any plastic which
is to be used easily as an embedding medium must be reasonably hydrophobic, presumably the more so the better.