Cleaning water stains on pottery
Try Bar Keepers Friend, the powder.
Make a paste and put it on the areas and let it sit. Then use a scrubby pad.
Try a paste of baking soda and warm water, leave on for an hour, then scrub gently with an old toothbrush
Vinegar diluted with a little water.
Go to the dollar tree and buy the cleaner called awesome.
The works, toilet bowl cleaner
a warm/hot water soak with Dawn and white vinegar.
Just plain Dawn & hot water, no vinegar
You will then need to soak your art pottery in tap water until the vinegar smell is gone.