Not sure about its affect on porcelain, but I know that many car detailers struggle to get concrete off of vehicles' exterior paint surfaces. Many use Muratic Acid, but I know that you have to be very careful not to get it on your skin, in your eyes, or breath the stuff. I'm not sure if it would damage porcelain, but I imagine it would work as you can use it (carefully) on a car's painted surface.

Good luck.

Mac