I am an inspector with NCDOT and field test concrete and other materials. We use vinegar to clean up our test instruments after the cement has dried. They are made of glass and porcelain is essentially a glass product. I have also taken concrete off of large porcelain signs by using a metay putty knife and rubber hammer to loosen the large clumps, being careful to stay off the porcelain. Then working using plastic instruments to get the rest off.
Good Luck Rich B
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