Both Priscilla and Pam raise additional and very valid points.

Microtomy is indeed a very delicate dance. Even with a well fixed and
processed block, the skill and experience of the operator is paramount in
any given situation. It is what makes a good histotech!
While training students to acquire microtomy skills, they almost always
pointed out that 'It looks so easy when you demonstrate it'! Why am I having
so much trouble? I always emphasized the need to understand the mechanisms
involved, to practice, practice, practice and to have the patience
necessary.
Jokingly, I also added that I had cut literally millions of sections and
now, I thought I finally had the hang of it!

The skill of balancing the following variables, in order to obtain high
quality sections, can only be acquired after much heartache, trial and
diligence.
a)    The relative hardness of the tissue and the support matrix.
b)    The relative degree of support supplied by the matrix both within and
around the tissue.
c)    The section thickness.
d)    The cutting speed, the amount of friction generated and the resultant
static charge.
e)    The influence of  the ambient environmental conditions of temperature
and humidity on the above.
f)    The quality of the knife edge.
Finally, a point that again is sometimes overlooked.
g)    The interaction between knife geometry and block.
Sections are not actually cut in the true sense, but split or wedged off the
surface of the block. There are certain combinations of knife geometry and
block which can conspire to give microchatters. The combination of knife
bevel angle( the included angle between the knife facets) and clearance
angle(the angle between the lower knife facet and the block face) is known
as the effective wedge angle (cutting angle) i.e. the angle between the
upper knife facet and the block face. This is the angle that the section has
to turn through as it is wedged(split) from the block.Generally speaking,
the harder the block and the thinner the section, the greater the effective
wedge angle.  Too great an angle can initiate microchatters. Solution, lower
the effective wedge angle by either 1) lowering the clearance angle, or 2)
change to a knife with a lower bevel angle. Sometimes both adjustments will
be necessary. If the knife is thin and already has a low bevel angle(17-24
degrees), too great a clearance angle will induce vibration of the edge or
'ringing' and microchatters. Lowering the clearance angle and cutting
slowly, should stop it. If not, then switch to a thicker blade with a higher
bevel angle(27-31 degrees) and keep the clearance angle to 3 degrees, this
is less likely to 'ring'.

As my instructor, mentor and long time friend, told me many years ago " The
best sections are always obtained by cutting as slowly and evenly as
possible".





----- Original Message -----
From: "Pamela Marcum" 
To: ; "Priscilla Delventhal"

Sent: Saturday, April 27, 2002 9:23 AM
Subject: RE: microchatter


> Pricilla makes some very good points on why microchatter occurs.  Another
> area often not considered is the fact that the block warms as it is
> sectioned and thus expands and contracts depending on how cold the block
> was and how warm it gets.  This is caused by the friction of the block
> moving across the knife.  Re-cooling the block with an ice cube or (try
not
> to do this one to avoid other artifacts) using a freezing spray must be a
> very even cooling to prevent the block from contracting with the cold
> unevenly and creating more chatter.  If the block is too cold it will
> chatter from the movement over the knife again due to the formation of
very
> cold spots in the paraffin.  WE get to attempt to figure out what is too
> cold and when the block is too warm for sectioning and this is a very
> delicate dance with the polymers in the paraffins today.
> Pam Marcum
>
>
> > [Original Message]
> > From: Priscilla Delventhal 
> > To: 
>  > Date: 4/27/2002 8:33:44 AM
> > Subject: microchatter
> >
> > Hi to all in Histo land.  I have read with interest the ever present
>
> > problem of microchatter.  We seem to be honing in on many different
>
> > techniques to help with this problem.  With everything that has been
>
> > said, no one has addressed the friction that occurs from the passing of
>
> > the knife over the paraffin block cut edge.  Faster cutting would
>
> > increase this friction and a surfactant; i.e. water or soap, etc., would
>
> > help to decrease this friction.  Also, I would think that the angle of
>
> > the knife to the block would affect this friction.
>
> > I know this doesn't solve the problem, but sometimes understanding what
>
> > is happening helps.  I found it interesting that the polymers in the
>
> > paraffin need time not cold to harden.
>
> >
>
> > Many years ago, I read an article in the Journal of Histotechnology
>
> > about a solution called Baker's softening solution.  I made this
>
> > solution up using less phenol and find that it helps with what I have
>
> > always thought to be a friction problem.  Many techs don't want to take
>
> > the time to soak their faced blocks, but I face all of my blocks and
>
> > when I can, I soak in Baker's softening solution.  If I had no time
>
> > constrictions I would do this all of the time as the sections are
>
> > beautiful.  For blocks with calcium I use the softening solution 1:1
>
> > with a decal solution.  I do cut with a slow even motion.
>
> >
>
> > This solution solved my problems with microchatter and when I cut deeper
>
> > into the block without soaking the microchatter returns.
>
> >
>
> > For those of you that would like to try this solution, here are the
>
> > directions.
>
> >
>
> > Modified Baker's Softening Solution (modified because the original
>
> > directions called for more phenol).
>
> >
>
> > 720  CC of 60% ROH
>
> > 80 CC Glycerol
>
> > 20 CC Acetone
>
> > 20 CC Phenol
>
> > Mix well.
>
> >
>
> > This stuff smells great and feels good on your hands too.
>
> >
>
> > I surface the blocks, place them face up on the ice or cold plate and
>
> > cover the surface with a paper towel soaked in Bakers solution.
>
> >
>
> > If you try this, I would appreciate hearing your thoughts on your
>
> > experience with it.
>
> >
>
> > Priscilla Delventhal - Currently in New Hampshire on a traveling
>
> > assignment.
>
> >
>
> >
>
> >
>
> >
>
> >
>
> >
>
> >
>
>
>
> --- Pamela Marcum
> --- mucram11@earthlink.net
> --- EarthLink: The #1 provider of the Real Internet.
>
>
>