Glass knife

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.

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 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; 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 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, used only when its superior performance is critical. Obsidian, a naturally occurring volcanic glass, can also be used to make sharp glass knives; obsidian surgical scalpels are available commercially. All these blades are brittle and very easily broken if not used with care.

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