Glass knife
A glass knife is a knife with a blade made of glass, with a fracture line forming an extremely sharp cutting edge. Glass knives were used in antiquity due to their natural sharpness and the ease with which they could be manufactured. In modern electron microscopy glass knives are used to make the ultrathin sections needed for imaging. History. In the Stone Age bladed tools were made by chipping suitable stones which broke with a conchoidal fracture, a process known as knapping or lithic reduction. The same technique was used to make tools, including knives, out of obsidian , natural volcanic glass. From the 1920s until the 1940s, Dur-X glass fruit and cake knives were sold for use in kitchens under a 1938 US Patent. Before the wide availability of inexpensive stainless steel cutlery they were used for cutting citrus fruit, tomatoes, and other acidic foods, the flavor of which would be tainted by steel knives and which would stain ordinary steel knives. They were molded in tempered glass, with a cutting edge ground sharp. Modern use. While glass knives as such are no longer in general use, knives with ceramic blades made from zirconium dioxide have been  available since the mid-1980s, with a very sharp and long-lasting edge produced by grinding rather than fracturing. Modern glass knives were once the blade of choice for the ultra-thin sectioning required in transmission electron microscopy because they can be manufactured by hand and are sharper than softer metal blades because the crystalline structure of metals makes it impossible to obtain a continuous sharp edge. The advent of diamond knives, which keep their edge much longer and are more suitable for cutting hard materials, quickly relegated glass knives to a second-rate status. However, some labs still use glass  knives because they are significantly less expensive than diamond knives. A common practice is to use a glass knife to cut the block which contains the sample to near the location of the specimen to be examined. Then the glass knife is replaced by a diamond blade for the actual ultrathin sectioning. This extends the life of the diamond blade which is used only when its superior performance is critical. Obsidian, a naturally occurring volcanic glass, is used to make extremely sharp surgical scalpels, significantly sharper than is possible with steel. The blades are brittle and very easily broken. Manufacture . Glass knives can be produced by hand using pliers with two raised bumps on one jaw and a single bump between the two bumps on the opposing jaw, but special machines called "knife-makers" are used in most electron microscopy laboratories to ensure repeatable results. The glass used typically starts out as strips of plate glass, which is cut into squares. The glass square is then scored across the diagonal with a steel or tungsten carbide glass-cutting wheel to determine where the square will break, and pressure is then applied gradually across the opposite diagonal until the square breaks . This technique provides two usable knife edges, one on each of the two resulting triangles. The better the break is aligned with the diagonal, the better the cutting edge.