174
5.
SECTIONING
succeeding section, and sections will not spread smoothly over the fluid
surface of the trough. They may be severely folded at one end and tend
to form a circle rather than a straight ribbon. This can be avoided by
trimming the block so that the leading edge is slightly longer than the
trailing edge and the upper corners stick out laterally farther than the
lower corners.
5.20. Section Flotation
If plastic sections are given a chance, they will stick with great tenacity
to dry clean glass. Thus, one cannot collect them on a dry knife surface.
The introduction of a trough and flotation fluid by Gettner and Hillier
(1950) was a most useful development.
A desirable fluid to use for section flotation would seem to be one
which wets glass vigorously, yet does not wet the plastic block face more
easily than can be helped. Dilute acetone solutions ordinarily answer
these requirements best, and are generally used. In this respect different
plastics have different characteristics (and diamond knives wet differently
than glass ones, Chapter 5.16), so one has to adjust the solution accordingly. Speaking first of glass knives, 20-40% acetone is advantageously
used with methacrylate, 10-20% with epoxy and polyester resins, and
pure water with glycol methacrylate and other highly polar plastics. Pure
water or 10% acetone is apt to be best with diamond knives except perhaps when methacrylate is being sectioned.
In general, one uses a nearly flat meniscus at the knife edge for sectioning, although this depends somewhat on the angle of the knife. Thus,
quite a highly convex mensicus may be necessary when the knife has an
obtuse angle, while a somewhat concave meniscus will work best with an
acute angle.
Considerable latitude is permissible in the shape of the meniscus. If it
gets too high, however, the block face will be wet and fluid will be carried
over the edge of the knife and down its front face. This, of course, is apt
to contaminate the edge. In any event one has to wait for the face to dry,
and it is at the very least a considerable nuisance. On the other hand, if
the meniscus is too low the sections will not glide away from the knife
edge in an unimpeded fashion. There will be some back pressure and a
tendency for sections to rumple and not flatten out properly. In extreme
cases, of course, they will pile up on each other.
5.
SECTIONING
succeeding section, and sections will not spread smoothly over the fluid
surface of the trough. They may be severely folded at one end and tend
to form a circle rather than a straight ribbon. This can be avoided by
trimming the block so that the leading edge is slightly longer than the
trailing edge and the upper corners stick out laterally farther than the
lower corners.
5.20. Section Flotation
If plastic sections are given a chance, they will stick with great tenacity
to dry clean glass. Thus, one cannot collect them on a dry knife surface.
The introduction of a trough and flotation fluid by Gettner and Hillier
(1950) was a most useful development.
A desirable fluid to use for section flotation would seem to be one
which wets glass vigorously, yet does not wet the plastic block face more
easily than can be helped. Dilute acetone solutions ordinarily answer
these requirements best, and are generally used. In this respect different
plastics have different characteristics (and diamond knives wet differently
than glass ones, Chapter 5.16), so one has to adjust the solution accordingly. Speaking first of glass knives, 20-40% acetone is advantageously
used with methacrylate, 10-20% with epoxy and polyester resins, and
pure water with glycol methacrylate and other highly polar plastics. Pure
water or 10% acetone is apt to be best with diamond knives except perhaps when methacrylate is being sectioned.
In general, one uses a nearly flat meniscus at the knife edge for sectioning, although this depends somewhat on the angle of the knife. Thus,
quite a highly convex mensicus may be necessary when the knife has an
obtuse angle, while a somewhat concave meniscus will work best with an
acute angle.
Considerable latitude is permissible in the shape of the meniscus. If it
gets too high, however, the block face will be wet and fluid will be carried
over the edge of the knife and down its front face. This, of course, is apt
to contaminate the edge. In any event one has to wait for the face to dry,
and it is at the very least a considerable nuisance. On the other hand, if
the meniscus is too low the sections will not glide away from the knife
edge in an unimpeded fashion. There will be some back pressure and a
tendency for sections to rumple and not flatten out properly. In extreme
cases, of course, they will pile up on each other.