Sponge
They’d erase slowly and deliberately, postponing the start of the class. Keeping an important balance of being slow enough not to annoy the class bullies, and fast enough not to annoy the teacher.
Erasing should have been done before class, so that even a wet blackboard had time to dry. But, no one wanted to be called teacher’s pet. We just waited for them to call out one victim to erase the board.
If the blackboard was erased with a wet sponge, it was hard to see what’s written on its glistening surface. And, after the wisdom dried, it was also almost un-erasable with a dry sponge.
If we used a dry sponge, there was inevitably a cloud of chalk. Sometimes, I admit, created on purpose. The teachers knew they could never dress black.