Egg shells and even whole eggs, plus a pinch of salt were used in boiled coffee, Norwegian or Swedish coffee or as many call it "Cowboy Coffee" boiled in a big graniteware coffee pot over a campfire (or nowadays a grill).
I was introduced to it in 1956 when I went to Minneapolis to attend baking school (Dunwoodie) and lived with a Norwegian family.
They made coffee that way.
I'm sure coffee was made that way on my grandpa's farm but by the time I was a child in the '40s, there were more sophisticated and modern brewers. My grandpa loved gadgetry - no question where I got the trait.