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.

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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

Glass knives were once the blade of choice because they could be manufactured by hand and were superior in most ways to more brittle metal blades. The advent of diamond knives quickly relegated glass knives to a second-rate status. Glass knives are still used exclusively in some labs because they are a hundred or more times less expensive than diamond knives. Mostly however, when glass knives are used, it is to cut the block down to the location of the specimen to be examined in the microscope. Then, the glass is switched out with a diamond knife for the actual ultrathin sectioning.

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 other jaw. The glass to be used is a 2 × 2 × 1 inch cuboid. The pliers are placed on the square's diagonal and pressure 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 one usable blade edge on one of the two resulting blocks. The closer the break is aligned with the diagonal, the better the cutting edge.

Specialist machines can also by used for the manufacture of glass knives.

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