Muriatic acid is a term used for hydrochloric acid, used often in masonry cleaning applications to remove the most difficult stains and deposits from rock. Because of the dangerous characteristics of muriatic acid, it is generally used as a cleaning agent of last resort, after other cleaners and less-potent acids have been tried.
Muriatic acid is very effective at cleaning rock and other masonry surfaces, but it is also one of the most dangerous chemicals available for purchase by the general public. It is important to spend time on safety preparations to prevent a serious accident.
Use heavy-duty, acid-resistant gloves, clothing that covers all of your skin, as well as eye, nose and mouth protection. Buy the acid in plastic bottles; metal containers have been known to get eaten away by the acid they are meant to contain. Have a generous supply of baking soda and water available to serve as a neutralizing agent. Plan on having a source of ventilation to deal with the toxic fumes, and make sure there are no open flames nearby. Cover any foliage or other areas that need protection before starting the cleaning.
Prepare a solution of 10 parts water to 1 part muriatic acid in a plastic container. Add the acid to the water, and not the water to the acid. The latter is likely to cause a reaction in which the acid splashes out of the container and onto you. Never mix muriatic acid with any other acid or chemical.
Use an inexpensive plastic sprayer (you will have to throw it away afterwards) or a brush to apply the acid to the rock for cleaning. Allow the acid solution to set on the rock surface for 15 seconds and then begin to rinse it off with water while brushing it with a stiff masonry brush. Repeat as necessary for stubborn stains. Do a final rinse with an 8-to-1 ratio of water and ammonia solution, followed by generous amounts of water.
Proper Cleanup and Disposal
Neutralize any leftover acid with baking soda or lime, making sure you are in a well-ventilated area. Throw away the plastic sprayer after rinsing it thoroughly. Never dispose of muriatic acid down drains or directly in the trash. It will corrode any pipes and containers it comes in contact with. You may also contact a local hazardous materials disposal or recycling center for proper disposal.