Why do vessels get a better shine when tea-leaves are soaked overnight in it before washing?

The other day I dumped consumed tea leaves into a brass utensil because I needed the tea-pan urgently. The vessel with the tea leaves didn't get around for cleaning until the next day. Ergo, the tea leaves soaked overnight in dirty water. When finally the vessel was cleaned (as in hand-washed with utensil cleaner), the quality of shine was exceptional.


Piqued, I repeated the tea-soaking in another vessel — and was rewarded with the same high gleam. This vessel was made of aluminium.


Why do vessels get a better shine when tea-leaves are soaked overnight in it before washing?


