Pencil leads

Source In the 16th century, a violent storm uprooted an English oak tree to reveal a freak geological anomaly: the only large-scale deposit of solid graphite ever found on Earth. The deposit was located in Borrowdale, a valley in the Lake District. Locals quickly realized the dark substance left remarkably permanent marks, and its discovery completely transformed the way people wrote. Before the Borrowdale storm, people wrote and drew with styluses made of literal lead or silver (a technique called silverpoint). These metal tools produced faint lines and required specially treated paper. The newly discovered Borrowdale substance, which locals mistook for a form of lead and dubbed “plumbago” (Latin for lead ore), was soft, smooth, and required no special surface. This mistaken identity is why the core of a pencil is still called “lead” today. Because the Borrowdale graphite was incredibly pure and solid, it could be sawn directly into sticks. However, pure graphite is brittle and…