92
4.
EMBEDDING
the American market may help in minimizing danger. Otherwise it is
almost impossible to avoid all contact with these substances without
taking truly heroic measures. Should one develop a contact dermatitis,
probably the simplest thing for him to do would be to turn to another
embedding medium. One of the polyester resins would seem to be a
logical choice. This writer has not heard of serious difficulties with
allergic sensitization to Vestopal W, although he may have a false
sense of security since presumably there is not much industrial use of
this resin in America, and the use of it by American electron microscopists
has been limited. I have had one former student who developed an extreme sensitivity to epoxy resins, who did manage to work out a routine
for handling embedding processes within a sealed glove-box.
4.6. Desirable Characteristics of the Embedment
Satisfactory embedding plastics must reasonably satisfy a number of
diverse criteria including the adequate preservation of fine structure,
stability during electron bombardment, specimen "stainability," and
good cutting characteristics. In the paragraphs which follow we will
speak only of the latter.
In considering the features of a block which relate to its sectioning
qualities, we are concerned with physical properties such as hardness,
toughness, elasticity, plasticity, and homogeneity, which can be understood easily, but which are difficult to measure and treat completely
objectively. Furthermore, there are other factors which are much more
subtle, and relate to the wettability of the plastic and the knife, and to
adhesive forces that may exist between the two. These are difficult indeed
to evaluate except by empirical experience.
Perhaps block hardness is easiest to deal with. Good sectioning with
most of the cross-linked
plastics demands that they be quite hard, usually
so that they will chip if they are roughly trimmed with a razor blade
that has been somewhat dulled. Yet a really sharp razor blade should
be able to cut clean shavings of the plastic. Indeed, it is not practical
to use a razor blade in the coarse trimming of a good block for its edge
will be turned and nicked almost immediately (Chapter 4.19). Yet, obviously, unnecessary hardness puts an added strain on the knife edge
and has the effect of reducing the potential size of a good section that
might be obtained. In the end, the plastic formulations that have been
4.
EMBEDDING
the American market may help in minimizing danger. Otherwise it is
almost impossible to avoid all contact with these substances without
taking truly heroic measures. Should one develop a contact dermatitis,
probably the simplest thing for him to do would be to turn to another
embedding medium. One of the polyester resins would seem to be a
logical choice. This writer has not heard of serious difficulties with
allergic sensitization to Vestopal W, although he may have a false
sense of security since presumably there is not much industrial use of
this resin in America, and the use of it by American electron microscopists
has been limited. I have had one former student who developed an extreme sensitivity to epoxy resins, who did manage to work out a routine
for handling embedding processes within a sealed glove-box.
4.6. Desirable Characteristics of the Embedment
Satisfactory embedding plastics must reasonably satisfy a number of
diverse criteria including the adequate preservation of fine structure,
stability during electron bombardment, specimen "stainability," and
good cutting characteristics. In the paragraphs which follow we will
speak only of the latter.
In considering the features of a block which relate to its sectioning
qualities, we are concerned with physical properties such as hardness,
toughness, elasticity, plasticity, and homogeneity, which can be understood easily, but which are difficult to measure and treat completely
objectively. Furthermore, there are other factors which are much more
subtle, and relate to the wettability of the plastic and the knife, and to
adhesive forces that may exist between the two. These are difficult indeed
to evaluate except by empirical experience.
Perhaps block hardness is easiest to deal with. Good sectioning with
most of the cross-linked
plastics demands that they be quite hard, usually
so that they will chip if they are roughly trimmed with a razor blade
that has been somewhat dulled. Yet a really sharp razor blade should
be able to cut clean shavings of the plastic. Indeed, it is not practical
to use a razor blade in the coarse trimming of a good block for its edge
will be turned and nicked almost immediately (Chapter 4.19). Yet, obviously, unnecessary hardness puts an added strain on the knife edge
and has the effect of reducing the potential size of a good section that
might be obtained. In the end, the plastic formulations that have been