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.

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 like obsidian were also commonly used.

Modern glass knives were once the blade of choice for the ultrathin sectioning required in Transmission Electron Microscopy because they could be manufactured by hand and were superior in most ways to 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 quickly relegated glass knives to a second-rate status. However, some labs still exclusively use glass knives because they are several thousand times less expensive than diamond knives. A common practice is to use a glass knife only to cut the block which contains the sample to near the location of the specimen to be examined. Then, the glass knife is switched out with a superior diamond blade for the actual ultrathin sectioning. This has the added benefit of extending the life of diamond blades as they are only used when their superior performance is critical. However, this practice of facing with a glass knife brings the risk of glass shards becoming embedded in the sample, thus damaging the diamond knife during sectioning.

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-inch wide strips of 1/4 inch thick plate glass, which is cut into 1-inch 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.

Kitchen knives

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.Fact|date=October 2008

In Popular Culture

* Glass knives are the weapon of choice of the antagonist Dmitri "Raven" Ravinoff in the novel "Snow Crash", due to the fact that they are undetectable by security systems and reputed to be molecule-thin at the edges.

References