How do you remove mortar off of brick?

If the mortar which has dried upon the brick is a Portland Cement mortar there is no sure fire way to remove it. As a professional mason I tell my customers that we can reduce the degree of mortar staining but seldom can we eliminate it. We use Muriatic Acid mixed with water 1:10. That's 1 part muriatic acid to 10 parts water. You can buy Muriatic acid at most Home Improvement stores as it is commonly used in Swimming Pools. Wear chemical proof gloves and eye goggles. Use a mason's sponge, a stiff nylon brush and a good "putty" knife. Soak the dried mortar with the 1:10 mixture using the sponge. Scrub with the nylon brush. Use the putty knife to carefully scrape away anything larger than an "film". If this does not result in the removal increase the strength of the acid gradually but do not exceed 1:5. The acid will burn the brick if it is used too strong or left to sit on the brick too long. Have a bucket of clean water with a mason's sponge close at hand to neutralize the acid once your scrubbing has been completed. Be careful where the acid and water mixture is allowed to "run off". It will kill plants and harm most surfaces. Consult local codes before you attempt this method to determine where you can safely dispose of the mixture.