I dont know about chicken eggs but duck eggs have a very thick protective membrane on the egg that takes a good scrubbing to remove. If the eggs are very dirty before setting its best to wash them because the misting may help bacterias enter the egg. The water for misting should always be a tiny bit warmer than the egg. When we let our mama duck sit on her eggs she purposefully bathes and then sits back on the nest, especially at the end of the incubation period. Eggs should be misted 48 and then 24 hrs before they hatch. We reference Storey's Guide to Raising Ducks by Dave Holderread. He really knows his stuff, we know because our first Mama, the one who sets, is his baby. We get ducklings from his conservancy and they are the strongest and healthiest ducklings we have seen!