Sorry, don't know what to tell you... maybe someone else will. Oh, just thought of something - have you tried CLR? It's sold (in Canada, anyhow) all over for lime (and rust) deposits and is supposed to be safe on porcelain sinks.

You might try white vinegar.

I would use CLR or Lime away. I think it would have to soak for a LONG time before it would harm the glaze. Most glazes fired in a kiln are quite sturdy.

tanyag is right about most glazes being fairly sturdy but what makes a big difference is wheher its a high fire glaze or a low fire glaze. one way you can usually tell the difference is the color of the clay. the picture below shows three different types of clay. the red on the left is earthenware it cannot be high fired so any glaze you see on this colored clay will be low fire glaze. the other two are the same type of clay stoneware. as you can see there are two different color tones the middle picture is high fired while the right picture is done at a low fire. as you can see the high fire has a darker almost gritty look to it aposed to the clean white of the low fire piece. the only time you will find high fire pieces that are white is if they are porcelain and if that is the case you will have payed almost double the money and it should have been marked as such. that being said high fire glaze will hold up to far greater punishment then low fire because it is set at temperatures of 2000 deg plus so there is very little you can do to damage it. i hope this helps.
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