Sometimes it's a good thing to not clean the grinder...
my discovery:
One day I neglected to clean the grinder after grinding up some cloves.
By the next time I made coffee, I'd forgotten and didn't notice that the
brown powder residue in there wasn't coffee. But the coffee I ground
up with it sure did make a nice pot of "Clove Coffee". I've made it on
purpose (with one or two cloves per pot) several times since.
I'm thinking cardamom might be nice to try too.
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