From instructables user pdelorey:

Continuing with the series I’ve been working on in aluminum, marble, and coal, I decided to create a panel in salt as well. This piece was directly machined on the ShopBot at Pier 9 during my residency there. Because there are already so many resources about ShopBot projects, for this Instructable I will focus primarily on acquiring, processing, and preparing the salt block, as well as the specific tooling and settings that I found were adequate for machining it. Here are a few good resources if you need more information regarding CAM and toolpathing for a ShopBot CNC router.

WARNING: Salt rapidly accelerates the corrosion of metal tools. Consider dust collection and cleanup procedures carefully before attempting. No matter what, spend adequate time in the cleanup steps of any machining you undertake in a similar project to prevent rust from setting in.

Full tutorial