The term “blackboard” is a misnomer, as blackboards range in color from brown to green to blue and even red. Black, however, is the predominant color of slate, the smooth stone from which the classic blackboard is made.
The first blackboards were called slates. Unlike the wall-mounted blackboards of today, slates were small, wood-framed panels. Students carried them to and from school, writing on them with pieces of chalk or “slate pencils” made of soft stone.
Slate is what geologists call a metamorphic rock. It began as fine mud, deposited layer by layer in shallow seas. Buried by other sediments, the layers are fused and hardened by immense pressure, as beds of volcanic rock push against continental plates. Over millions of years, this pressure produces a stone that is dense, durable and easily quarried.
The mud from which slate formed was the habitat of microscopic organisms that died and decomposed, leaving behind carbon that gives most slate its black hue. But other colors of slate abound. Iron compounds in mud oxidize over time, yielding slate that may be green, red or purple.
The word “slate” derives from the French “esclater,” meaning to splinter or break. Slate is especially suited for blackboards, because it splits easily into thin sheets. This is because the pressure exerted on slate over millions of years caused flat minerals in the stone to align with one another, like leaves floating in a pond.
Like slate, chalk begins as a marine sediment. As a result, quarriers often find these two varieties of stone in close proximity to one another. Science writer David B. Williams calls slate and chalk “the peanut butter and jelly of the geology world” because they are abundant and go together so well.
Today, blackboards are rarely made from slate. Inexpensive materials like wood and plastic composites may be covered with compounds that furnish a slate-like writing surface at a fraction of the cost of quarried stone. Consumers can even purchase spray paints impregnated with grit that will turn any surface into a blackboard.