When I was in junior high school. metal welding
While sitting in class
Just thinking about a problem
Why teachers always say please see the blackboard
But what board is green?
Is there some misunderstanding? metal welding
In fact, at the beginning of the 19th century, people used large slabs of stone.
Connected to a blackboard
Let teachers write on the class
The name “blackboard” only appeared in 1815. metal welding
In the 18th century
Because paper and ink are more expensive
So cheap and durable rewritable stone
Has become a substitute
When students were in class
Will bring a black slate to attend class
These “small blackboards” are the predecessors of large blackboards in classrooms. metal welding
So the first time
The blackboard is indeed all black
The blackboard at that time was made of slate
In rural areas
The raw material of blackboard is wooden board
People mix egg whites with scorched potatoes
Paint the wood board dark
Later, the blackboards were all made of wood. metal welding
Blackboard at that time
Is not adulterated “blackboard”
In the 20th century, the blackboard began to undergo cosmetic surgery.
For teachers who are staring at the blackboard all day long
Green chalkboard is softer
More eye-friendly
So “true blackboard”
Gradually replaced by a green blackboard. metal welding
CNC Machining Service & CNC Machining parts
aluminium profile
aluminium profile for cnc
aluminium fabrication
aluminium deep processing