Slate is siliceous stone, which is composed of silica and quartz particles. Slate is relatively easy to clean and manufacturers often recommend cleaning slate with very mild acidic cleaning solutions. If you use very acidic cleaning solutions, however, you can etch the surface of the slate. Etching is a very light scratch on the slate, which can change its sheen. You can repair the etching by blending it with the rest of the slate.
Wipe up the acid material as soon as possible with a water-dampened cloth. This will help prevent the acid from etching further into the slate.
Spread a thin layer of water over the slate surface. This will help control the polishing powder so that you do not etch the surface even more.
Sprinkle polishing powder over the acid etch stain. Polishing powder is a mild abrasive that will grind away the top of the slate to blend the etched area.
Rub the polishing powder over the etch stain and the surrounding area with a damp cloth. Use medium pressure and work until the etch mark disappears.
Wipe the area with a water-dampened cloth.
Dry the slate completely with a clean cloth.