Screechy chalk

Factual Questions
1

This term, I’ve been teaching a class in a classroom with chalkboards; I readily adapted my whiteboard style to the new environment by using coloured chalk.

What I’ve noticed, though, is that while the usual white chalk only screeches rarely, the coloured chalk screeches more often. And it varies—my red, light blue, and yellow chalk are pretty good, but I have a green that makes noise about every other word, and a brown that is simply unusable due to screeching constantly.

Why does chalk screech, in general? Why do some colours do it significantly more than others? Is there some way to hold the chalk that will make it screech less?

I found a slightly related question in the archives—Why is the sound of fingernails scraping a blackboard so annoying?—but that one really answers the question of “why is the screech painful”, not “why does the screech happen in the first place”.

2

Probably a stick-slip phenomena occurring at the frequencies of the sounds. The different coulors of chalk having different properties with regard to the stick-slip.

Less likely to screech if the diredtion of motion drags the chalk stick rather that pushing it toward the slate at a steep angle.

3

I had a prof. that insisted that breaking the chalk in half helped.

::shrug::

Worth a try.