To obtain the best ultrathin sections in ultramicrotomy and cryoultramicrotomy, a high quality knife is of paramount importance. Glass knives were introduced for ultramicrotomy, in the manually broken form, in 1950. Since the introduction of mechanical knifemakers, the production of glass knives has become a standardized laboratory routine. In conventional ultramicrotomy different knife angles are used to cover different specimens and embedding media. The 50° knife broken with a small deviation of the score from the diagonal of the square gives a long useful edge, especially if damping during breaking is used. The 50° knife is, therefore recommended as the knife for routine ultramicrotomy.
The demands of cryoultramicrotomy initiated a re-evaluation of the concepts of glass-knife preparation. The need for a sharp (45° real angle) knife for the preparation of ultrathin cryosections became evident. The "balanced break" concept of producing sharp knives from perfectly flat-sided squares greatly facilitates cryosectioning routines. Knives produced with a short score ("long free break") potentially have the sharpest edges and give the thinnest sections.
Sevéus, Lahja and Tarras-Wahlberg, Cecilia
"The Glass Knife in Ultramicrotomy,"
Scanning Electron Microscopy: Vol. 4
, Article 13.
Available at: https://digitalcommons.usu.edu/electron/vol4/iss1/13