As brass ages, it becomes oxidized and loses its shine. Despite your best attempts to clean it, it just won't return to its former brightness. Using hydrochloric acid, also commonly referred to as muriatic acid, can help restore your brass. The acid eats away at the outer dull layer, exposing a fresh, bright layer underneath. Using this method causes your brass to become thinner, so it should only be done when it's really needed.
Fill a sink with warm water and a squirt of liquid dish soap.
Dip the brass item into the soapy water. While holding the item directly above the water, use a soft- to medium-bristled cleaning brush to scrub the item clean. This removes all dirt and dust from the item.
Fill a bowl deep enough to fully submerge the brass item with a 32 percent hydrochloric or muriatic acid solution.
Submerge the brass item for one minute. Lift it from the acid and rinse it under cool running water.
Dip fine steel wool into the acid. Rub on any remaining spots on the brass. Rinse under cool running water.
Dry the brass item with a microfiber towel to prevent water spotting.