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A glass knife is a knife with a blade composed of glass. The cutting edge of a glass knife is formed from a fracture line, and is extremely sharp.

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. Diamond knives are also extremely sharp, and the edge lasts much better than glass, but they are extremely expensive.

History
Beginning in the Stone Age, glass knives (and other tools, such as arrowheads) were produced through a process known as knapping or lithic reduction. Although such bladed tools were often made of stone, naturally occurring glasses such as obsidian, natural volcanic glass, were also commonly used.

From the 1920s through 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 ground edges.

Modern use
Modern glass knives were once the blade of choice for the ultrathin sectioning required in transmission electron microscopy because they can be manufactured by hand and are sharper than softer metal blades; the crystalline structure of metals makes it impossible to obtain a continuous edge with the sharpness of broken glass. 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 several thousand times 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, used only when its superior performance is critical. Obsidian can be used to make very sharp knives; obsidian surgical scalpels are available commercially. All these blades are brittle and very easily broken if not used with care.

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 1 in strips of 1/4 in plate glass, which is cut into 1 in 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.

In popular culture

 * Glass knives are the weapon of choice of the antagonist Dmitri "Raven" Ravinoff in the novel Snow Crash because they are undetectable by security systems and reputed to be molecule-thin at the edges, sharp enough to penetrate bulletproof vests.