Does Climbing Chalk Expire?
No. Climbing chalk essentially does not expire. Chalk is an almost completely inert chemical & stays good far beyond the expiration date on the label that is essentially only there to comply with regulations. It does not expire the same way food does. It's not going to become less effective over time, suddenly develop adverse side-effects or react in unexpected ways with other chemicals.
Magnesium carbonate for sports use does come in contact with your skin though, so some regulations need to be observed. This entirely depends on your jurisdiction. Europe tends to have the most strict cosmetics regulations so if your chalk was purchased from a European supplier, you should be good.
Chalk can absorb moisture from the environment if you keep large quantities outside of a sealed container. It's always best to store your chalk either in its original bag or an airtight container like a Nalgene bottle or Tupperware box. You want to avoid your chalk getting wet so it's a good idea to store your chalk inside a small plastic bag before throwing it in your climbing bag. Ziplock bags work great for both storing excess chalk & transporting your chalk bag.
What if your chalk does get wet? No worries! You can bake it! Putting wet chalk in the oven at 120 degrees C for a couple of hours should bake the moisture right out of it! You can also put your chalk outside on a sunny dry day.
While chalk does not expire if you keep large amounts of excess chalk in storage, we think you're doing something wrong. We encourage climbers to use as little chalk as possible or switch to alternatives to chalk powder like liquid chalk or chalk balls. A little chalk goes a long way especially if you use liquid chalk as a base layer & topic up with some chalk from a ball.
Photo by Matthew Hume