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Measure out 1 cup (240 mL) of vinegar into a large pot, then add 1 tbsp (17 g) of salt. Use a measuring cup add 3 cups (710 mL) of warm water to the pot. Then, stir the ingredients to combine them. If you’re cleaning a large copper piece, use a bigger pot and double the ingredients for the cleaning solution. It’s safest to put the copper into the pot before you start boiling the solution. Place the item into the pot and make sure the stains are covered by the cleaning solution. It’s okay if part of the item is not covered, as long as you aren’t trying to clean the exposed area.  It’s okay to clean more than 1 piece at a time if they all fit into the pot. For instance, you might clean 2 copper mugs at the same time or a handful of copper jewelry pieces. Turn the burner on high heat to heat the pot. Monitor the pot as it heats to boiling. After it reaches boiling, adjust the heat to keep it at a rolling boil. Stay near the pot while it’s heating to make sure it doesn’t boil over. The vinegar and salt will draw out the stains or black patina. Check the water to see if the discoloration is leaching out of the copper and into the solution. Let the item boil for up 15 minutes. If the item looks clean before you get to 15 minutes, it’s okay to turn off the pot early. After 15 minutes, turn off the burner so that the solution and the copper item will start to cool. Leave the pot to cool for 30 minutes to an hour. Don’t try to handle the copper while it’s still hot. The metal may burn your hand, so be careful.

Summary:
Combine vinegar, water, and salt in a large pot. Submerge your copper piece in the solution. Bring the solution to a boil over high heat. Watch for the stains to fade from the copper. Turn off the heat so the copper piece can cool.