5.13.
GLASS KNIFE MANUFACTURE
155
denly applied. In a wide piece of glass the crack may slow or stop its
advance before the sheet is bisected. More powerful squeezing should
continue it, although at times it is apt to prove necessary to build up
the pressure points either by adding more tape, or by inserting slivers
of wood or cardboard (toothpicks or matches). A definite objective, however, is to develop the crack as slowly as possible. Thus it is that one can
successively divide a piece of glass into smaller and smaller rectangles or
squares until a suitably sized piece is obtained to make a knife. (Fig. 15).
The knife holders of different microtomes require knives of varying
overall shape and size. Some require simple right-angle triangles which
are best obtained by diagonally bisecting a square of suitable size. The
original Porter-Blum microtome required a rhomboidal shape, best obtained by making two diagonal breaks across a strip of glass [Figs. 15(j)
and 16(d)]. In any case, the knife edge to be used represents the intersection of a diagonal break with one of the original fracture faces of the
glass rectangle. It is necessary that there be a fairly long score mark to
establish the diagonal break. However, the score mark need not, and indeed should not, terminate too near the point of the final intersection. It
will be better if the final portion of the break which creates the knife
edge be a "free break." This can be at least 2 mm long [Figs. 15(i) and
16(a)-(c)].
In making the final diagonal break, the position of the pliers is of
some importance, and will influence the knife edge. Superior knives can
be expected if the diagonal fracture is started from the middle of the
mass of glass, rather than from the back edge. Thus, the pliers should
grip the glass at approximately the midpoint of the diagonal score, as in
Figs. 16(b) and (c), rather than near the back edge. It is possible that the
improvement can be attributed to a damping effect which reduces the
speed and violence of the fracture when it is initiated from within the
glass, rather than from an edge.
If one is making a knife of simple triangular shape by diagonally bisecting a square of glass, it probably will not matter which way the free
break deviates from the midline to intersect with a fracture face to one
side or the other of the corner. Chance, then, decides which triangle
becomes the knife, and which the discarded piece. However, in making
rhomboidal knives, one must control the deviation of the free break. If
the score mark is aimed just to one side of the corner, the deflection of
the free break can be expected to turn to that side. However, the best
knives, those with most acute angles and with straightest edges, in general
GLASS KNIFE MANUFACTURE
155
denly applied. In a wide piece of glass the crack may slow or stop its
advance before the sheet is bisected. More powerful squeezing should
continue it, although at times it is apt to prove necessary to build up
the pressure points either by adding more tape, or by inserting slivers
of wood or cardboard (toothpicks or matches). A definite objective, however, is to develop the crack as slowly as possible. Thus it is that one can
successively divide a piece of glass into smaller and smaller rectangles or
squares until a suitably sized piece is obtained to make a knife. (Fig. 15).
The knife holders of different microtomes require knives of varying
overall shape and size. Some require simple right-angle triangles which
are best obtained by diagonally bisecting a square of suitable size. The
original Porter-Blum microtome required a rhomboidal shape, best obtained by making two diagonal breaks across a strip of glass [Figs. 15(j)
and 16(d)]. In any case, the knife edge to be used represents the intersection of a diagonal break with one of the original fracture faces of the
glass rectangle. It is necessary that there be a fairly long score mark to
establish the diagonal break. However, the score mark need not, and indeed should not, terminate too near the point of the final intersection. It
will be better if the final portion of the break which creates the knife
edge be a "free break." This can be at least 2 mm long [Figs. 15(i) and
16(a)-(c)].
In making the final diagonal break, the position of the pliers is of
some importance, and will influence the knife edge. Superior knives can
be expected if the diagonal fracture is started from the middle of the
mass of glass, rather than from the back edge. Thus, the pliers should
grip the glass at approximately the midpoint of the diagonal score, as in
Figs. 16(b) and (c), rather than near the back edge. It is possible that the
improvement can be attributed to a damping effect which reduces the
speed and violence of the fracture when it is initiated from within the
glass, rather than from an edge.
If one is making a knife of simple triangular shape by diagonally bisecting a square of glass, it probably will not matter which way the free
break deviates from the midline to intersect with a fracture face to one
side or the other of the corner. Chance, then, decides which triangle
becomes the knife, and which the discarded piece. However, in making
rhomboidal knives, one must control the deviation of the free break. If
the score mark is aimed just to one side of the corner, the deflection of
the free break can be expected to turn to that side. However, the best
knives, those with most acute angles and with straightest edges, in general