162
5.
SECTIONING
sufficiently acute knife has at least a narrow zone sufficiently free from
defects to section reasonably well. I personally rarely reject a knife even
though its "spur" may be unduly high, and even though the edge may be
somewhat tilted. I expect to find in all but disastrously broken knives an
acute zone, near the intersection of the "fracture ridge" with the edge,
that will cut well (Fig. 17). The very best knives are not so much better
than routine ones that it makes very much difference in the practice of
sectioning. If one does have a really long length of excellent edge, the
chances are that it will be wasted anyhow as a result of contamination
before all of it can be used. A new knife can be made so easily and quickly
that this should not be regarded as a bottleneck.
5.16. Diamond Knives
Diamond knives were introduced by Fernandez-Moran (1956) who
demonstrated that industrial diamonds could be ground so that they had
satisfactory edges for ultrathin sectioning. It has become apparent that
knives with a fairly acute included angle of about 45° or even less are
best suited for most histological work. Knives with an obtuse angle of
55°-60° also can be obtained, but are recommended mainly for hard
materials, including the softer metals. The latter may also serve special
biological purposes in sectioning mineralized tissues.
An obvious advantage of diamond knives is their durability, and the
fact that once the correct tilt has been obtained, the knife can be left
undisturbed in its holder for repeated sectioning over long periods of
time. Thus it eliminates the uncertainty and inconvenience of positioning a knife every time sectioning is to begin.
Diamond knives are relatively hydrophobic, so that when the trough
is filled, the edge usually has to be wet with a sliver of wood. This means
that a higher meniscus ordinarily must be maintained with a diamond
knife than when a glass knife is used, everything else being equal.
The relatively hydrophobic character of the diamond edge as compared
to glass also means that there is a greater tendency for adhesion between
the edge and the plastic embedment as sections are being cut. Thus,
sections may not properly glide away from the knife edge, and compression effects will result. Indeed it may be relationships of this sort which
underlie apparent differences in the cutting qualities of different diamond
knives. In any event, it must be realized that cutting with a diamond
5.
SECTIONING
sufficiently acute knife has at least a narrow zone sufficiently free from
defects to section reasonably well. I personally rarely reject a knife even
though its "spur" may be unduly high, and even though the edge may be
somewhat tilted. I expect to find in all but disastrously broken knives an
acute zone, near the intersection of the "fracture ridge" with the edge,
that will cut well (Fig. 17). The very best knives are not so much better
than routine ones that it makes very much difference in the practice of
sectioning. If one does have a really long length of excellent edge, the
chances are that it will be wasted anyhow as a result of contamination
before all of it can be used. A new knife can be made so easily and quickly
that this should not be regarded as a bottleneck.
5.16. Diamond Knives
Diamond knives were introduced by Fernandez-Moran (1956) who
demonstrated that industrial diamonds could be ground so that they had
satisfactory edges for ultrathin sectioning. It has become apparent that
knives with a fairly acute included angle of about 45° or even less are
best suited for most histological work. Knives with an obtuse angle of
55°-60° also can be obtained, but are recommended mainly for hard
materials, including the softer metals. The latter may also serve special
biological purposes in sectioning mineralized tissues.
An obvious advantage of diamond knives is their durability, and the
fact that once the correct tilt has been obtained, the knife can be left
undisturbed in its holder for repeated sectioning over long periods of
time. Thus it eliminates the uncertainty and inconvenience of positioning a knife every time sectioning is to begin.
Diamond knives are relatively hydrophobic, so that when the trough
is filled, the edge usually has to be wet with a sliver of wood. This means
that a higher meniscus ordinarily must be maintained with a diamond
knife than when a glass knife is used, everything else being equal.
The relatively hydrophobic character of the diamond edge as compared
to glass also means that there is a greater tendency for adhesion between
the edge and the plastic embedment as sections are being cut. Thus,
sections may not properly glide away from the knife edge, and compression effects will result. Indeed it may be relationships of this sort which
underlie apparent differences in the cutting qualities of different diamond
knives. In any event, it must be realized that cutting with a diamond