Amos, speaking as someone who has prepared and drunk cowboy, bush or campfire coffee, whatever you wish to call it, for almost 50 years, please do no promulgate the myth that eggshells are used to settle coffee grounds.

They don't settle the grounds, they counteract the bitterness that is released by boiling, as posted above. The longer you boil it, the more bitter it becomes, to the point that even eggshells from a gaggle of geese will not resurrect it. They are not a needed ammendment if the coffee is brewed correctly.

Grounds are dealt with by removing the boiled coffee from the fire and allowing the grounds to settle for a few minutes, before pouring it, or carefully adding cold water.

The best bush coffee should only be prepared by someone who views it as a sacrament and never by someone who prefers tea. There are some Arts that that only reach the sublime when a pinch of love is added to the pot. A true student of Zen and the Art of Bush Coffee can tell the difference.