How can I remove dried foam insulation from a ceramic sink?

I have a quarter-sized spill from some foam insulation on my ceramic bathroom sink. I initially cleaned it but a thin layer still remains. It's been there for months and is as hard as rock. I'd damage the ceramic if I'd use a metal scraper. I've tried using Goof-Off, but it did little to no good.

Any suggestions on how to clean this?


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