158
5.
SECTIONING
The glass itself occasionally is a source of difficulty, but only occasionally. Poor glass does not fracture smoothly and straightly when freebroken, and fractures do not properly follow their score lines. One
generally has warning of impending trouble in that the cutting tool does
not behave properly in making good score lines. Such glass which is difficult, or perhaps impossible to handle, presumably was improperly annealed, so that random stresses exist within it. It should not be difficult,
though, in most places in the world, to obtain well-annealed glass.
Various efforts have been made to refine and standardize aspects of
knife making. Thus, Andre (1962) has detailed the construction of a
rather elaborate apparatus which incorporates a protractor so that score
marks can be precisely placed on a strip of glass at any desired angle.
Tokuyasu and Okamura (1959) studied some of the characteristics of free
diagonal breaks approaching a corner, and described a two-step method
of breaking the glass which presumably produced a relatively slowly
developing fracture. Fahrenbach (1963) has described a device employing
a fulcrum and two pressure points to break glass, and emphasized
that it is desirable to have the pressure points as widely separated as
possible, more widely separated than is possible with ordinary glazier's
pliers. Also he emphasized damping the developing fracture by breaking
the glass on a rubber-coated platform. The LKB-Produkter Ab., Stockholm, is manufacturing and selling a rather complex device which incorporates a jig and a mechanism for accurately scoring a glass strip, as
well as a fulcrum and a pressure point system that is damped for breaking
the strip in a standardized way. Mr. Algy Persson, an engineer associated
with the latter company, has made an extended study of glass breaking,
and in conversation with the present writer has emphasized the importance of slow-damped fractures.
What has been said in this section relates to the use of "soft" glass
for knife making. At various times investigators have attempted to use
"hard" glasses such as Pyrex and Tempax for this purpose. Such glasses
prove very hard to break consistently into reasonably shaped knives. The
ordinary glazier's scoring tool does not work satisfactorily with such
glasses. Though scratches can be made with a diamond scribe, these prove
not to be very effective fracture guides. Fracture surfaces, when they are
achieved, are not particularly flat, and are apt to be severely checked.
In effect, it has proved to be impossible so far to devise a reasonably
consistent technique of reproducibly making good-looking knives with
such glasses. Even when usable knives are obtained, their advantage seems
5.
SECTIONING
The glass itself occasionally is a source of difficulty, but only occasionally. Poor glass does not fracture smoothly and straightly when freebroken, and fractures do not properly follow their score lines. One
generally has warning of impending trouble in that the cutting tool does
not behave properly in making good score lines. Such glass which is difficult, or perhaps impossible to handle, presumably was improperly annealed, so that random stresses exist within it. It should not be difficult,
though, in most places in the world, to obtain well-annealed glass.
Various efforts have been made to refine and standardize aspects of
knife making. Thus, Andre (1962) has detailed the construction of a
rather elaborate apparatus which incorporates a protractor so that score
marks can be precisely placed on a strip of glass at any desired angle.
Tokuyasu and Okamura (1959) studied some of the characteristics of free
diagonal breaks approaching a corner, and described a two-step method
of breaking the glass which presumably produced a relatively slowly
developing fracture. Fahrenbach (1963) has described a device employing
a fulcrum and two pressure points to break glass, and emphasized
that it is desirable to have the pressure points as widely separated as
possible, more widely separated than is possible with ordinary glazier's
pliers. Also he emphasized damping the developing fracture by breaking
the glass on a rubber-coated platform. The LKB-Produkter Ab., Stockholm, is manufacturing and selling a rather complex device which incorporates a jig and a mechanism for accurately scoring a glass strip, as
well as a fulcrum and a pressure point system that is damped for breaking
the strip in a standardized way. Mr. Algy Persson, an engineer associated
with the latter company, has made an extended study of glass breaking,
and in conversation with the present writer has emphasized the importance of slow-damped fractures.
What has been said in this section relates to the use of "soft" glass
for knife making. At various times investigators have attempted to use
"hard" glasses such as Pyrex and Tempax for this purpose. Such glasses
prove very hard to break consistently into reasonably shaped knives. The
ordinary glazier's scoring tool does not work satisfactorily with such
glasses. Though scratches can be made with a diamond scribe, these prove
not to be very effective fracture guides. Fracture surfaces, when they are
achieved, are not particularly flat, and are apt to be severely checked.
In effect, it has proved to be impossible so far to devise a reasonably
consistent technique of reproducibly making good-looking knives with
such glasses. Even when usable knives are obtained, their advantage seems