I’ve noticed crystals forming inside my Zinfandel bottles. Do you have an explanation?

Dear Wine Wizard, I have been making red Zinfandel wine at home with juice concentrate. I‘ve noticed crystals forming inside and they stick to the bottle. When you shake the bottle they disappear, but when you let it stand for a few minutes they come back and stick to the bottle again. I am stumped — do you have an explanation? Carol Otway Port Coquitlam, British Columbia The Wine Wizard replies: Ahhh yes, as I lean back in my trusty desk chair and tent my fingers like a wizened wine psychiatrist, I can picture in my mind’s eye exactly what you describe. The small crystals can look like tiny grains of quartz sand or small shards of glass; some are free-floating and others form a layer of plaque on the sides of your bottles. It seems as if you are experiencing what I call “tartrate fall out,” or in slightly-more scientific terminology (i.e. geek speak), a nucleation and subsequent visible precipitation of once-soluble potassium bitartrate ions. Take two Campden tablets and call me in the morning . . . next.