Glass knife

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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.

[edit] History

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 a 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.

[edit] Manufacture

Glass knives are produced by a special pair of pliers with two raised bumps on one jaw and a single bump in the direct center of the two bumps on the second jaw. The glass to be used typically starts out as a 2 × 2 × 1 inch cuboid. The pliers are placed on the square's diagonal and pressure is applied slowly and evenly until the piece breaks. The glass can be scored with a razor blade across the diagonal to ensure that the block breaks properly. This technique usually leaves only one usable blade edge on one of the two resulting blocks. The better the break is aligned with the diagonal, the better the cutting edge.

Specialized machines can also be used to manufacture glass knives more quickly and with increased regularity. These machines often involve a stage for the glass to sit in and a lever arm which is depressed manually to break the glass.