Climbers (and weightlifters and gymnasts) use chalk to dry any moisture on their hands. When you’re sweating, as you might while climbing, weightlifting, or doing gymnastics, your hands get wet. When your hands are wet, they can get slippery. Chalk sucks up any moisture on your hands, so your skin is dry (but could be left with a powdery coating.)
The chalk used in most gyms starts off as limestone and dolomite and is processed into magnesium carbonate (this is a name you may see on packaging.) Most magnesium carbonate is white in color, but some brands sell colored chalk if you’re feeling fancy. Chalk is sold in a few forms: block, loose, and liquid.
Block chalk is a chunk or small brick of compressed chalk; you break off smaller pieces as needed (usually by stepping on the brick to smash a piece off.) The benefit to this option is that you can choose how big you want your pieces to be, some people like to roll a big chunk around their hands to apply the chalk, others prefer a fine powder. Speaking of a finer powder…
Loose chalk is pre-powdered (though some brands offer different levels of chunkiness, so it’s not necessarily a fine powder.) You may put loose chalk into a chalk bag so you can dip your hands into it. Loose chalk can be put into a chalk ball, a porous mesh ball, that you can roll around your hands to release a limited amount of chalk.
Liquid chalk is the least messy option. It is a mix of chalk and alcohol that you apply to your hands. The alcohol dries off quickly, leaving just the chalk behind. It’s like hand sanitizer: the alcohol in sanitizing gels dries quickly, and tends to dry out your skin rather than leave any moisture.
When using block or loose chalk, you’ll need to store it in a bag or bucket. You’ll dip your hands in, roll them around in the chalk (or roll a large chunk around in your hands,) remove your hands from the chalk, and shake off the excess. Shaking off the excess is very, very important.
Less is more is the best advice here. You need just enough chalk to soak up any extra moisture, but any more powder on your skin can reduce your ability to grip. About Sports reports that there are no studies to support the use of chalk in rock climbing. There is actually one study out of University of Birmingham that found that chalk may actually reduce a climber’s grip because the powder reduces friction between skin and the holds, plus your skin can grip better with a little bit of moisture. Still, many climbers use chalk and swear by it.
Your local climbing gym likely rents out chalk and chalk bags so you can try it out before you buy it. You’ll need to figure out if it actually improves your climbing and if you like the feel of using it. You may be able to figure out which, if any, of the types of chalk will benefit you the most in your climbing.