Bekhorot 22: three things to remember

A miscarried mass exempts future births from bechor status, yet conveys no impurity because the fetus is nullified by congealed tissue.
A deceased human fetus conveys impurity at warp-skein size, an animal fetus at woof-skein size, and foreign earth at sack-seal size.
Purifying an am ha'aretz's fish brine in a mikveh works for dipping, but cooking might reawaken nullified impure water.