I've used plain old blackboard chalk now for thirty years and it's always worked for me. Here in Australia we get lots of variation in climate, sometimes three or four seasons in a day (it seems)This can cause a lot of variation in the grip of the peg, as can bouncing in the boot of the car in its case. My case also always contained a clothes peg which makes a very variable mute depending on where on the bridge you clamp it. You can be really quiet for rehearsing in motel rooms or just a bit dampened for sessions with quiet singers