Rigid Collodion - How To Scar, also called "Scarring Liquid", is a viscous, yellowish substance widely used in theatrical makeup special effects to simulate shapes of wounds and scars.
It's particularly popular for rendering The Joker cheek scars. It works by wrinkling the skin as it dries, leaving a puked area very similar to healed scars.
Rigid Collodion should peel off easily after one day, taking with him the top layer of dead skin (epidermis).
It's not recommended to use the Scarring Liquid several times in a row on the same area, as it will come off with living skin and slightly damage it. It's usually a safe practice to wait three days between applications.