Does climbing chalk actually expire?

No, not in the way food or cosmetics do. Magnesium carbonate is a mineral, not an organic substance, so there's no biological process that causes it to spoil or become unsafe over time.

So why does old chalk sometimes feel useless?

This comes down to moisture, not age. Magnesium carbonate is hygroscopic, meaning it absorbs moisture from the air around it. Chalk that's been sitting exposed, in an open bag or a container that doesn't seal well, gradually picks up humidity and can turn clumpy or gummy. That's a storage problem, not the chalk reaching some kind of shelf life.

A bag of chalk stored properly, sealed and kept dry, doesn't lose its usefulness just from sitting around for a while.

What actually causes chalk to go bad, then?

Moisture exposure is the main one. Contamination is another, if something else has gotten mixed into the bag, though this is uncommon with a sealed container. Physical compaction into a hard block, from moisture or pressure, is really the same underlying issue as clumping, just further along.

Can you fix chalk that's already clumped?

Often, at least partially. Breaking it apart by hand can loosen the texture back up, though how well that works depends on how compacted it's become. Prevention through proper storage matters more than trying to fix it after the fact.

How should you actually store it?

Somewhere sealed and dry, rather than left open in a gym bag. A hard, resealable container limits how much humid air reaches the chalk between sessions, which is the main thing that keeps it usable over time.

The takeaway

Chalk doesn't expire the way perishable products do, but it does respond to how it's stored. Keeping it sealed and dry is what actually determines whether a bag performs the same on day one hundred as it did on day one.