Glass knife

Glass knife

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A glass knife is a knife
A knife is a cutting tool with an exposed cutting edge or blade, hand-held or otherwise, with or without a handle. Knives were used at least two-and-a-half million years ago, as evidenced by the Oldowan tools...

 with a blade composed of glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

. 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
Electron microscope
An electron microscope is a type of microscope that uses a beam of electrons to illuminate the specimen and produce a magnified image. Electron microscopes have a greater resolving power than a light-powered optical microscope, because electrons have wavelengths about 100,000 times shorter than...

, 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.


Beginning in the Stone Age
Stone Age
The Stone Age is a broad prehistoric period, lasting about 2.5 million years , during which humans and their predecessor species in the genus Homo, as well as the earlier partly contemporary genera Australopithecus and Paranthropus, widely used exclusively stone as their hard material in the...

, glass knives (and other tools, such as arrowheads) were produced through a process known as knapping or lithic reduction
Lithic reduction
Lithic reduction involves the use of a hard hammer precursor, such as a hammerstone, a soft hammer fabricator , or a wood or antler punch to detach lithic flakes from a lump of tool stone called a lithic core . As flakes are detached in sequence, the original mass of stone is reduced; hence the...

. Although such bladed tools were often made of stone, naturally occurring glasses such as obsidian
Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.It is produced when felsic lava extruded from a volcano cools rapidly with minimum crystal growth...

, 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
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....

 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
Transmission electron microscopy
Transmission electron microscopy is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through...

 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
Diamond knives
A diamond knife is a very sharp knife whose blade is made from diamond. The cost is very high; diamond knives are used for scientific applications where an extremely sharp and long-lasting edge is essential.-Diamond knives for ultramicrotomy:...

, 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. 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. 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.


Glass knives can be produced by hand using pliers
Pliers are a hand tool used to hold objects firmly, for bending, or physical compression. Generally, pliers consist of a pair of metal first-class levers joined at a fulcrum positioned closer to one end of the levers, creating short jaws on one side of the fulcrum, and longer handles on the other...

 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 inches (25.4 mm) strips of 1/4 in plate glass, which is cut into 1 inches (2.5 cm) squares. The glass square is then scored across the diagonal with a steel or tungsten carbide
Tungsten carbide
Tungsten carbide is an inorganic chemical compound containing equal parts of tungsten and carbon atoms. Colloquially, tungsten carbide is often simply called carbide. In its most basic form, it is a fine gray powder, but it can be pressed and formed into shapes for use in industrial machinery,...

 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
    Snow Crash
    Snow Crash is Neal Stephenson's third novel, published in 1992. Like many of Stephenson's other novels it covers history, linguistics, anthropology, archaeology, religion, computer science, politics, cryptography, memetics, and philosophy....

    because they are undetectable by security systems and reputed to be molecule-thin at the edges.