Device for breaking a glass plate so as to obtain a sharp edge
The present invention refers to a device for breaking a glass plate so as to obtain a sharp edge. The edge formed is intended to be used at a knife edge when preparing sections for light microscopy. 
     When cutting biological material embedded in paraffin or similar substances for preparing a sample suitable for studies in a light microscopy knives made of steel have hitherto mainly been used. The steel knives do however suffer from the essential disadvantage that they only in special cases permit sections of a thickness down to the range of 1 mu. This is an essential disadvantage which recently has been more significant when thermosetting resins often replace the paraffin when embedding the material for light microscopy studies. These resins give a better resolution of the tissue and could be cut in thinner sections than parrafin and are therefore preferred as an embedding material in investigations requiring an optimal resolution. The thickness range in these cases is between 0.5 and 3 mu for which the steel knives in practice are not useable. Attempts have therefore been made to replace the steel knives by glass knives of the type hitherto used in ultramicrotomes, i.e. instruments to be used for cutting a sample to be studied in an electron microscope. These glass knives ae produced by breaking a glass plate, an edge of the breaking surface being used as a knife edge. Manual production of such glass knives for light microscopy gives however very bad results and one has to produce a high number of knives of which only a minor part meet the requirements. When producing glass knives for light microscopy one has therefore started to use the type of devices for manufacturing ultramicrotomy knives which is described for instance in the Swedish Pat. No. 310768. These devices do however suffer from the disadvantage that the length of the knife edge is determined by the thickness of the broken glass plate. When producing sections for light microscopy one does however need knives of a length of up to 40 mm and the breaking of glass plates of this thickness is not possible with the type of devices described in the above cited patent, since the breaking forces thereby required would be unreasonably high. The devices described in the above cited patent could produce knives of a length up to 12.5 mm but even at this length the breaking forces are so high that the production involves considerable risks for the operator. 
     It is an object of the present invention to provide a device by means of which long glass knives can be produced in a simple, safe and reproducible way. The characteristics of the invention will appear from the claims attached to the specification. 
    
    
     The invention will now be described in detail, reference being made to the attached drawing in which: 
     FIG. 1 is a schematical side view of a device according to the invention; 
     FIG. 2 shows the breaking rule used in the device according to FIG. 1; and 
     FIGS. 3a and b show glass plates broken in the device according to FIG. 1. 
    
    
     In FIG. 1 reference 9 denotes a bottom plate on which a breaking rule 4 is displaceably arranged. The breaking rule 4 is provided with two breaking shoes 5 (FIG. 2) which are made from a relatively soft material for instance aluminum and are symmetrically arranged with a certain internal distance. A glass plate 3, 7 is clamped between the breaking rule and two breaking knobs 2. Between the breaking knobs a scribing device 1 is arranged, this device being displaceable parallel to the breaking rule so as to provide a scribed line along the rule. 
     When producing a glass knife 7 with an edge 8 (FIG. 3a) a scribing line is made by the scribing device 1, whereafter the breaking knobs are pressed downwards and the glass plate is broken towards the breaking rule along a breaking surface 6. It thereby appears that by displacing the breaking rule perpendicularly to its longitudinal extension a breaking surface is obtained which starts from the scribing line and bends towards the edge of the breaking rule. By displacing the breaking rule the desired edge angle could thus be obtained. 
     The advantage of the device hitherto described is that the breaking forces required are limited, since a glass plate of a thickness of 6 mm is preferably used. The knife obtained could however in spite of this be made three times as long as by the use of a 12.5 mm thick glass plate at an instrument in accordance with the above cited patent. Furthermore, the edge angle could easily be changed in a reproducible way. 
     Concerning the design of the breaking rule it appears that it is essential to obtain an even pressure along the glass plate and the rule has therefore been provided with two displaceable shoes which are suitably made by a soft material, for instance aluminum, for avoiding high pressures at certain points. It has furthermore proved to be advantageous to have a certain distance between the shoes since one single integral rule easily gives rise to a bending of the breaking surface along the rule.