154
5.
SECTIONING
These are known as "Glass-Run Pliers, Model SM-610." Their illustration shows a much greater curvature than I have had experience with,
however, and damping would present more of a problem.
Cameron (1956) demonstrated that glass that was "free-broken" without
scoring could be expected to yield fracture faces which were remarkably
smooth. On the other hand, fractures following score lines invariably are
somewhat contoured, although this can be minimized by damping the
glass so that the fracture develops slowly [Fig. 14(A)-(C)]. Thus it is
that a fracture face created by a free break is much to be preferred as the
front face for a glass knife, although it is not strictly necessary. Although
investigators have realized this for a number of years, Cameron's technique of breaking glass was so wasteful that originally it was not commonly employed. However, it slowly has become apparent that refinements in the technique can eliminate most of the waste. What immediately
follows is a description of current practices of knife manufacture in my
own laboratory.
We begin with a rectangular piece of glass. It can be square, or perhaps
a broad strip as in Fig. 15(a). If a free break is started at right angles to
one edge, and almost exactly in the middle
of that edge, it can be expected to hold its course reasonably straight and true across the glass,
and remain at right angles to the starting edge [Fig. 15(c)]. The key to
success is to divide equal masses of glass. One cannot estimate or measure
this accurately if the glass is not very nearly rectangular to begin with.
To initiate a break, a small score about J inch long is made, intersecting with the middle of one edge as in Figs. 15(a), (d), and (g). The
modified glazier's pliers are then placed so that the fulcrum is immediately
below the score mark, and about £
mcn
m
from the edge of the glass
[Fig. 15(b)]. The pliers are then slowly squeezed so that force is not sud(a,d,g) Small scores, about \ inch long are made intersecting one long edge of the
glass at right angles.
(b) Modified glazier's pliers (f) are positioned with the fulcrum exactly below
the
score mark, and with the lips about | inch in from the edge of the glass.
(c,e,h) Slowly increasing pressure "free breaks" glass as slowly as possible into nearly
perfect identical rectangles.
(i) Suitably small rectangles are scored diagonally at 45° to make knives. The score
does not actually intersect what is to be the front face of the knife, so that the last
part of the break is a "free break" (see Fig. 16).
(j) A completed knife of a shape suitable for a Porter-Blum microtome.
5.
SECTIONING
These are known as "Glass-Run Pliers, Model SM-610." Their illustration shows a much greater curvature than I have had experience with,
however, and damping would present more of a problem.
Cameron (1956) demonstrated that glass that was "free-broken" without
scoring could be expected to yield fracture faces which were remarkably
smooth. On the other hand, fractures following score lines invariably are
somewhat contoured, although this can be minimized by damping the
glass so that the fracture develops slowly [Fig. 14(A)-(C)]. Thus it is
that a fracture face created by a free break is much to be preferred as the
front face for a glass knife, although it is not strictly necessary. Although
investigators have realized this for a number of years, Cameron's technique of breaking glass was so wasteful that originally it was not commonly employed. However, it slowly has become apparent that refinements in the technique can eliminate most of the waste. What immediately
follows is a description of current practices of knife manufacture in my
own laboratory.
We begin with a rectangular piece of glass. It can be square, or perhaps
a broad strip as in Fig. 15(a). If a free break is started at right angles to
one edge, and almost exactly in the middle
of that edge, it can be expected to hold its course reasonably straight and true across the glass,
and remain at right angles to the starting edge [Fig. 15(c)]. The key to
success is to divide equal masses of glass. One cannot estimate or measure
this accurately if the glass is not very nearly rectangular to begin with.
To initiate a break, a small score about J inch long is made, intersecting with the middle of one edge as in Figs. 15(a), (d), and (g). The
modified glazier's pliers are then placed so that the fulcrum is immediately
below the score mark, and about £
mcn
m
from the edge of the glass
[Fig. 15(b)]. The pliers are then slowly squeezed so that force is not sud(a,d,g) Small scores, about \ inch long are made intersecting one long edge of the
glass at right angles.
(b) Modified glazier's pliers (f) are positioned with the fulcrum exactly below
the
score mark, and with the lips about | inch in from the edge of the glass.
(c,e,h) Slowly increasing pressure "free breaks" glass as slowly as possible into nearly
perfect identical rectangles.
(i) Suitably small rectangles are scored diagonally at 45° to make knives. The score
does not actually intersect what is to be the front face of the knife, so that the last
part of the break is a "free break" (see Fig. 16).
(j) A completed knife of a shape suitable for a Porter-Blum microtome.