Various types of pencil leads made of graphite, the soft, slippery, crystalline form of carbon that is widely used for writing & as a lubricant. Indeed, a recalcitrant metal clothing zip may be corrected by applying a few strokes of a graphite pencil. The lubricating nature of graphite is due to its molecular structure. The carbon atoms are arranged in arrays of flat hexagons, forming flakey plates which slip easily across each other.
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