Put rock salt in water, and it will eventually dissolve. If you have a big chunk of rock salt that you don’t want to use all at once, however, you’ll need to break pieces off of it. The easiest way to do this is to weaken the chunk of salt first, then pry or break pieces off that can then be smashed. If the piece of rock salt is small enough that drilling into it seems ridiculous, however, you can skip right to the smashing portion of the process. This is also a great way to work out excess stress.
Put on shatter-proof protective glasses to protect your eyes and sturdy leather or canvas gloves to protect your hands.
Clamp the piece of rock salt in a vise if it’s not big and heavy enough to stay put while drilling. Spread a sheet of heavy-duty plastic, fabric or heavy paper underneath the work area both to help reduce possible mess and to catch small bits of salt, which can then later be funneled into a container.
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Use a 3/8- or 1/2-inch bit in your power drill, with the longest shaft possible, to drill holes in the rock salt every two or three inches. Drill all over the surface of the rock salt.
Use a chisel and mallet to break off pieces of the now-weakened rock salt. Smash the chunks into smaller pieces with a hammer.
Keep pounding with the hammer until the rock salt is as finely broken up as you like.