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I deprime and tumble for about 45 minutes in walnut dust, then soak for an hour in Citric Acid, 1 teaspoon per quart of water. Get Citric Acid at Whole Foods or GNC (it's a food product so you can also wash your tomatoes in it) I suggest do them first, then the tarnished shells)

For tarnished or really dirty brass I use Citric Acid (Whole Foods or health food stores have it). A very small amount - quarter of a tea spoon of the powder in a gallon of hot water poured over a dish pan or bucket of brass. Stir or swish it around and let it sit for 10 or 20 minutes then drain off the mixture and keep it for the next batch. Just reheat to a boil and pour again. Air dry for a dull shine or tumble in walnut or other mixture of choice.