Liquid acidic rinsing to the surface of the stone. This will slightly to moderately etch the stone according to the acid percentages in the rinse and the type of stone; typically a mixture of muriatic acid and water. This is done a lot to take the shine off of polished stones down to a honed finish. It also can sometimes make the stone more slip resistant. Acid washing tend to enhance the color and character more. Interior and exterior use depending on the stone and the type of acid washing.