My electric water boiler has a small, yet dense bit of iron sponge, about the size of a Brussels sprout. I think it is something to prevent the water from superheating. However, I'm not sure that's the only reason, because they could also just make the inside of the boiler somewhat rougher, a solution that doesn't involve a loose element hanging around in the pot that can get lost. I think there's another reason for that loose element, but I'm not sure what that reason could be. Could there be another reason beyond superheating?

  • It could very well be there to provide nucleation points for bubbles to form and prevent superheating, yes. Commented Sep 25, 2015 at 14:49
  • @ElendilTheTall Thing is, wouldn't making the inner surface a bit rough achieve the same effect without having something that you can easily lose?
    – Nzall
    Commented Sep 25, 2015 at 14:53
  • Perhaps it's easier/cheaper to just whack a sponge in there than to rough up one side of the steel? Commented Sep 25, 2015 at 14:55

1 Answer 1


They are claimed (I don't know what research has actually been done) to prevent the build-up of limescale.

  • If the "sponge" is a cathode, then yes it indeed will prevent lime build up. What makes it an anode depends on the two metals are. The electricity from galvanization will draw alkaline ions (like lime) to the cathode.
    – Escoce
    Commented Sep 25, 2015 at 18:21

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