This site: https://www.businessinsider.com/why-most-blackboards-are-actually-green-and-not-black-2017-11#:~:text=The%20color%20change%20came%20in,likely%20to%20survive%20the%20journey states that chalkboards are often green because manufacturers started using a green porcelain coating on steel, making the boards more durable and lighter. But what made the porcelain green? I couldn't find an answer online.
Thank you! Also, if this is not the right place to ask, please let me know.
A random walk thru a Google search suggests the following possibilities, or a combination of all.
But what made the porcelain green?
It looks better in pastel shades and is a popular colour in darker shades. It also happens to be the peak of the eye colour sensitivity curve.
The green was made with some kind of tinted material, and some claim it started in the 30's and became dominant in the 60's.
The coating materials and the substrates also changed over the years from porcelain to anodized aluminum and powder-coated paint or tinted silicon dioxide (Carborandum) with many variations in between.
With improvements in chalk adhesion and dry erasing, the surface texture of the substrate and coating roughness make a big difference in write, read and erasability for contrast ratio and retention of chalk.
My rule of thumb when I attended uni. was a high static friction but low dynamic friction. We had a special test to make the chalk stutter if held loosely and dragged at right angles.
Our EM prof. Dr Wolfgang Borner could fill up 8 boards worth every class with things like a derivation of Gauss's Law.
