Apparently this substance is also known as
bittern, a word which we already accept, possibly because it's also a type of bird. The only general dictionary I found
nigari in is Wiktionary. I'm always cautious about allowing a new word on the strength of a Wiktionary entry alone. Because Wiktionary aims to be an all-inclusive reference, some of its words are too specialised or archaic to be even rare words in our lexicon.
However,
nigari doesn't seem to be totally obscure. It is mentioned occasionally in news articles, as in this item about Tasaki pearls from
The Week (UK) in May this year:
Following a 20-minute-long bath in a nigari solution – also known as bittern, this organic brine-like coagulant is also used in the production of tofu, and its list of ingredients includes potassium ions, magnesium and calcium – anaesthetised oysters are gently prised open and held in place in a metal contraption.
Moreover there are plenty of firms selling nigari, presumably to those who want to make their own tofu.
I'll add
nigari as a rare word.
