Clay kettle odor

I’ve made the mistake of storing my clay water kettles on my stove top when not in use. I suspect that they have picked up an off odor from using the burners in front of them because lately I’ve noticed an oily odor coming from them. I will boil them a couple of times using fresh water each time. Can anyone suggest a cleaning solution to use after boiling and cooling? Global Tea Hut, where I purchased them, suggested using white ash to clean then. I’ve moved them to another location. Thank you.