In the late 18th and early 19th century, such "slateboards" were commonly used in schools in the United States and other countries. These small pieces of slate would be bound in a wooden frame to help strengthen the board and keep them from cracking. In those days paper was expensive and hard to come by, so these mini slate blackboards provided a good substitute.
At some point in time, however, these slate boards began to be used in a brand new way. A geography teacher working in Scotland is reported to have taken the slates from the students and hung them all on the wall. He then used this to make-shift blackboard to write out geography information which all the students could read at once. A revolution in blackboard usage had begun.
By the 1850's, virtually all schoolhouses included a blackboard along with their other staples: a wood burning stove and benches for the students to sit on. Still, however, our modern chalkboards were not in common usage.
As technology progressed, the old pieces of slate finally began to be replaced by chalk. The soft limestone chalk was easier to use on the boards, and easier to clean as well. The old rag erasers have been erased by new felt chalkboard erasers, which are able to absorb more of the chalk dust and keep it out of the air. The boards themselves are no longer made of slate, but instead are a steel sheet with a porcelain enamel.
( Magnetic Chalkboard Paint )