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Keep in mind that store bought juice is already pasteurized so you do not need to add campden tablets to it. Also, there are two natural ways to clear your wine of impurities. a) Allow it to finish fermenting until its dry adding to the alcohol content and making it more difficult for yeast or bacteria to survive, and b) placing it in the fridge for a week before bottling. Other then this, one way to ensure survivability of the wine is to fortify it with adding Brandy. Basically Brandy+Wine=Port, and port can last up to 40 years under some circumstances not likely without considerable skill and technique but give it a try. I just started and I think using Welch's concord grape juice is an excellent idea.