94
4.
EMBEDDING
One can have several blocks that would all be described as equally
hard, and perhaps all of the same plastic, and yet there can be considerable differences in the way they cut. Once again we are dealing
with physical properties that are more subtle than just the degree of
hardness. The most difficult type of block to section (sometimes really
impossible) seems not to be truly homogeneous throughout. If one attempts to make shavings with a sharp razor these commonly will have a
frosted appearance rather than being more or less glassy. It is as though
the plastic was gritty or granular. The plastic lacks the horny quality
that is highly desirable. Slicing it with a sharp razor may give the impression that it is rather plastic, and even sticking to the razor edge.
Thus it is deformed or stretched, and flows before being cut. If sections
can be obtained at all from such blocks they are almost certain to show
"chatter" marks (Chapter 5.22). Assuming that one has used a standard
and proven formulation, such blocks presumably are not uniformly
polymerized for one reason or another. Perhaps inadequate dehydration
and consequent residual traces of water may be the most common fault.
Some epoxy resins, particularly Epon, are actually hydroscopic and will
pick up water from the atmosphere if not protected. No doubt sometimes
the effects result from inadequate impregnation of the tissue block,
without all components of the plastic mixture in proper proportions, or
perhaps the polymerization was rushed. If any sections at all can be obtained from such a block, they may literally break up on the surface of
the trough fluid within a minute or two after being cut, and the parts
float away separately. Obviously, hard plastic does not penetrate throughout such a block. Of course, old chemicals may undergo deterioration
and eventually not polymerize properly. However, one does not expect
such changes to occur suddenly. If one has been continually using a
given method successfully, and a dramatic change is noted, one should
suspect an error of technique.
Undoubtedly there is some intrinsic variation in plastic components
manufactured by different companies, and even in different lots from a
single manufacturer. These almost uncontrollable variations can on
occasion result in a distressing waste of time and energy. While the experienced investigator will soon realize that something is wrong, it could
easily be that the novice would not realize that the source of his difficulty
was beyond his control. Thus, it seems reasonable to encourage the
novice to begin with two different embedments on the assumption that
at least one surely will produce satisfactory blocks.
4.
EMBEDDING
One can have several blocks that would all be described as equally
hard, and perhaps all of the same plastic, and yet there can be considerable differences in the way they cut. Once again we are dealing
with physical properties that are more subtle than just the degree of
hardness. The most difficult type of block to section (sometimes really
impossible) seems not to be truly homogeneous throughout. If one attempts to make shavings with a sharp razor these commonly will have a
frosted appearance rather than being more or less glassy. It is as though
the plastic was gritty or granular. The plastic lacks the horny quality
that is highly desirable. Slicing it with a sharp razor may give the impression that it is rather plastic, and even sticking to the razor edge.
Thus it is deformed or stretched, and flows before being cut. If sections
can be obtained at all from such blocks they are almost certain to show
"chatter" marks (Chapter 5.22). Assuming that one has used a standard
and proven formulation, such blocks presumably are not uniformly
polymerized for one reason or another. Perhaps inadequate dehydration
and consequent residual traces of water may be the most common fault.
Some epoxy resins, particularly Epon, are actually hydroscopic and will
pick up water from the atmosphere if not protected. No doubt sometimes
the effects result from inadequate impregnation of the tissue block,
without all components of the plastic mixture in proper proportions, or
perhaps the polymerization was rushed. If any sections at all can be obtained from such a block, they may literally break up on the surface of
the trough fluid within a minute or two after being cut, and the parts
float away separately. Obviously, hard plastic does not penetrate throughout such a block. Of course, old chemicals may undergo deterioration
and eventually not polymerize properly. However, one does not expect
such changes to occur suddenly. If one has been continually using a
given method successfully, and a dramatic change is noted, one should
suspect an error of technique.
Undoubtedly there is some intrinsic variation in plastic components
manufactured by different companies, and even in different lots from a
single manufacturer. These almost uncontrollable variations can on
occasion result in a distressing waste of time and energy. While the experienced investigator will soon realize that something is wrong, it could
easily be that the novice would not realize that the source of his difficulty
was beyond his control. Thus, it seems reasonable to encourage the
novice to begin with two different embedments on the assumption that
at least one surely will produce satisfactory blocks.