Use a wire wheel first to work off as much of the mortar as possible. Once that is done, try using a 10% solution of muriatic acid to clean. When working with muriatic acid, remember these important safety tips: Use eye protection, gloves, and body protection as the acid could burn you. Work in a well-ventilated area so vapors do not toast your lungs. When mixing the solution, always add the acid to the water and not the other way around. If your first round of using muriatic acid doesn’t work, try doing it again or try using a stronger concentration.