Every workday, I send Gwen off to her job with a small stainless thermos full of coffee. It’s basically impossible to clean this thing effectively, so instead, I just fill it with very hot water and shake it up.
Today, when I dumped out that water, these bits of film came out. They’re jet black, very smooth, very thin, very uniform, and very brittle—I was reminded of a cheap plastic bag that had been left in the sun for a long time.
Though I was initially incredulous, it turns out this was the husk of coffee residue in the thermos. Most of it got shucked off (some is still in there). I’ve never seen anything like it.
I get similar buildup in my thermos. You’re right, it’s hard to clean. A bottle brush would do it, but I don’t have one. I occasionally mix baking soda with boiling water and let it stand overnight.