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Instead of Drano, have you tried soaking it overnight in plain old white vinegar? That should take care of any calcium buildup.

I don't know what would corrode gold plating, but maybe some of our coast-dwellers could shed some light.

Once you get it clean; you can keep it that way by just rinsing it out with warm water immediately after each use.

I don't know if I'd try this with a gold-plated mp, but the rest of my mouthpieces get a ride through the dishwasher every month or so. I just put 'em over the pegs on the top rack. They come out clean as a whistle, and I've not noticed any ill effect on the silver plate.
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Joe Baker, who has found that dishwasher detergent will remove just about any organic schmutz from just about any surface.

is it safe to scrub a mouthpiece with toothpaste? I do that occasionally. It leaves the piece clean with a minty fresh flavor