In the Middle Ages, people began to find that with lime and water, huge objects could be made and recorded on dark or hard surfaces in a way similar to charcoal pens. At that time, paper was a very expensive item, and it was easy to blur the lettering on wood and rocks with a carbon pen.
Of course, after nearly a thousand years, it is difficult to find out who first thought of this idea, but at the earliest, the role of chalk was definitely not used for teaching.
To be widely used in teaching, blackboard is more important than chalk.
/kloc-before the 0/9th century (earlier AD 180 1AD), wooden boards were protected from erosion with black paint. It is also used to announce important matters, similar to the role of bulletin boards now. But the "blackboards" at that time were very small, and the purpose was not for teaching. But by the middle of19th century, universities all over the world began to flourish. The original teaching method of teacher dictation and student dictation has become more and more inconvenient because of the increase in the number of students. Therefore, in European and American countries, the original small, unfixed black bulletin board is usually enlarged for teaching purposes, which is convenient for distant students to copy the teacher's dictation.
Therefore, the slow evolution in the past two hundred years has finally enabled more students to learn enough knowledge and eliminated the illiterate and ignorant masses through convenient tools.