Brass becomes oxidised and loses its shine over time. Despite your best attempts to clean it, the brass 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 thins your brass, so only do it when the brass is really dull and tarnished.
Fill a sink with warm water and a squirt of washing-up liquid.
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 per cent hydrochloric/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 microfibre towel to prevent water spotting.