Muriatic acid, or hydrochloric acid as it's also known, is an extremely harsh acid that's sometimes used in the industrial world. Homeowners sometimes use it to clean and etch aluminum surfaces.
Place the aluminum item in your kitchen sink, if it's small enough to fit without any splashing, or a large metal tub for bigger items. Pour the muriatic acid directly on the aluminum surface and watch for the acid to start removing the top layers from the aluminum.
As soon as it reaches the color you want, pour water over the top. The water dilutes the acid and should stop the etching process. Rinse the aluminum thoroughly with water, before handling.
Always wear protective gear when working with muriatic acid. Wear goggles to cover your eyes, rubber gloves over your hands and even a rubber apron to protect your clothes and body.