How to prevent calluses from writing on chalkboard?

A bit of stupid question:

I just finished my first semester teaching (I lead two recitation sections for pre-calculus twice a week). It went well, but I've noticed a sizable callus has formed on my right middle finger on the left side between the nail and first knuckle. This is from how I hold the chalk, I believe. The callus is rubbing against my finger when I write now and causing a bit of pain.

How do I prevent this callus? I don't have much experience with writing on the chalkboard so I imagine this is from holding the chalk incorrectly. Maybe someone knows secrets to the chalkboard that I am not aware of?