I tried "sunning" my cup this past week for the first time ever (I've used it for a year and a half now, and usually just clean it with soap and baking soda at the end, with only water during my period). I sat it on my windowsill for four days (three of which were very sunny), giving it a quarter-turn each day so all sides got direct light, and the difference is astounding (with the way it looks now, I actually wouldn't hesitate to show it to someone! It looks very nice!).
But the weird part is, at the end of each day my cup had a sort of oily/greasy feel to it; sort of similar to oily skin. I'm not really concerned because it washed right off, and doesn't appear to have damaged the cup in any way, but it was weird that it kept reappearing each day. I did have my cup sitting right next to a plant, so is it possible that the plant was emitting...something...that was landing on my cup? Any ideas?
Btw my cup is a small MCUK, old style (the more yellowy kind).