last time i cleaned my colleagues mug whilst he was on holiday(15 yrs ago) he came back to work, saw it, and dropped it on the floor. The replacement i had to buy him (deer with a target on – Gary Larson???) is still in use. Looks like a sooty stove chimmney to me but as he says it gets boiling water in it several times a day so it MUST be alright….
back in my student days I invented the self cleaning mug. I found one that would collect brown stains only up to a certain number of layers (2/3 day’s worth) at which point the stain would fall off and be absorbed by the tea.
one day the handle fell off and its magical properties were left in shatters across the kitchen floor. a sad day.
Once mine gets a few thou on it it gets cleaned with burtex.(a kind of plastic wire wool we use for polishing titanium). And the spoon stays in the mug always. Daren’t leave it lying around. Wife goes mad when I drink a brew at home with the teaspoon in.
Never understood why ppl do this as “The tannin stains inside the teapot does not improve the taste of tea, but it gives a bitter taste.” so your mug not improved by being stained, it just looks grim and disgusting.
Tbh the only reason I don’t give it a good clean is that it’s a thermos mug and it’s not meant to be submerged in water. When I finish work I rinse it with boiling water, dry it off and put it in my man bag. Before I start work I again rinse it in boiling water then make a brew