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And here's the same mouthpiece after 30 minutes immersed in white malt vinegar. You can quite clearly see that the scale deposits have gone. What you can also see is that the mouthpiece has taken on a dull greenish hue. This is due to the action of the vinegar on the Ebonite that the mouthpiece is constructed of.
Try letting the vinegar sit on the hard water spots. If you let it sit and soak in for a few minutes instead of wiping it off right away, it'll have more time to break down the mineral deposits holding the scum in place. Let it soak in for a good 5 - 10 minutes before you try to wipe it off.