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Re: cleaning calcium deposit off an old Imari bowl

Posted By: Bill H
Posted Date: Jun 14, 2010 (11:53 PM)

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Hi Patti & Ernest,

Vinegar and other household calcium removers will not harm 'hard paste' (highly vitrified porcelain) because when properly fired the body and decorative enamels essentially transform into a form of glass inside the kiln. If the piece is 'soft paste' (incompletely vitrified pottery) the acid in these liquids will eat away the unbonded calcium, creating quite a mess. Chlorine-based bleaches will do the same damage to soft-paste dishes, though hydrogen peroxide bleach is safe for them.

Determining whether you have true porcelain or pottery can be a chore. A rule of thumb is that crackle-finish ware probably is pottery. Also, pottery doesn't usually ring like porcelain. Further, highly vitrified porcelain and its glaze can't be scratched with a steel blade, but be careful and find an inconspicuous spot on the bottom if you wish to test the piece in this manner.

I cautiously mention that some but not all types of fine wood-sanding pads use an abrasive that will not scratch glass or hard-paste porcelain, but they will remove gilt decoration, so use them only as a last resort. These pads typically have only one side of the foam rubber square impregnated with abrasive. I use a brand called Norton 3X #320 Ultra Fine Grit. Again they should be used only with hard-paste porcelain and only on vitrified surfaces, never on gilt-decorated areas. I use them only to get rid of some of the more resistant patches of calcium after applying vinegar or another household calcium removal solution.

Best regards,

Bill H.

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