Sodium Hydroxide used to be the active ingredient in dip tanks.
It is a common ingredient in oven cleaners.
Also called Caustic Soda.
It is a quite dangerous chemical in its pure form, normally stored as a flake.
Interesting fact, as long as your skin is dry the flake can lay on your skin with no ill effects. Don't sweat or the burn will start in a hurry. In pure liquid form it is very dangerous and most industries try and avoid it if possible.
When mixing flakes with water, put the full amount of water in a container, then slowly add the flakes. Never the other way around.