I've been using my cup 6 months ago, everything works PERFECT =D
I use a boiling stainless steel casserole (not highest quality) and I get always the same when I finish boiling my cup (4 minutes boiling) let me tell you:
There is a thin white layer on the casserole walls where the water was boiling. And also there are small spots in the bottom of the casserole.
The only way to remove them is rubbing/scrarping hard with a brand new scourter, water and dish-washing soap.
The water in my zone is very hard (I live on a sea town)
I wish to know if anybody knows what's that white layer and specially that spots on the bottom!
I'm afraid about them, maybe they can be any kind of fungus or stainless steel degrade?
Another hypotesis: the thin layer is water minerals (like calcium) and the spots are the points where the air go up when boiling so there the thin layer don't appear.
Thanks in advance!!
NEW: I've taken photos today! So you can see it better:
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Thank you everybody for your super-fast answers! Calcium in water seems to be the problem...
I tried to boil the cup with mineral water but it left a harder mineral layer.
I've clean the casserole with vinegar (as you suggested me) and it WORKS! I tell you how I do it: When I finish boiling the cup I get it out of the casserole. Then I add some vinegar to the boiled water, and leave it to rest a few hours. Then I wash it with clean water and the calcium has gone.
Thank you!! =D