The bloom on duck eggs is really thick and I try not to scratch it as it protects the eggs from bateria, can you believe that the really yucky eggs hatch under momma ducks without a problem. To clean them up, I get a wet paper towel and wrap them up in it for a couple of moments. Then just use it to lightly wipe it off, leaving the egg clean and the bloom intact. I try not to use anything abrasive on eggs.