When I was at Caltech, we called our Physical Chemistry prof "Wild Bill Goddard" because he wore a cowboy hat and boots to his lectures. His course was largely conceptual, illustrated with balloon-shaped drawings of electron orbitals, drawn with 3-D projection in which the yellow and red were in the plane of the chalkboard while the pale blue projected in and out.
We all complained that we could not see the blue diagrams. "That's OK, you're not supposed to, that's why I use pale blue."
On night Sonja Benson and I snuck into the classroom to put the arm on his blue chalk. We took it down into the steam tunnels where she doused it with her hairspray, giving it a hard coating so it would not mark the board anymore.
The very instant Wild Bill saw what we had done, he cracked his blue stick in half then continued drawing orbital diagrams we were not meant to see.