Bentonite, also known as Montmorillonite, is a clay that is used as a fining and clarifying agent in winemaking.  It has a net negative charge that attracts fine particles and pulls them out of the wine.  Bentonite is often added to wine kits on the first day;  the advantage to adding it at this time is that the active fermentation that follows will churn the bentonite particles around in the juice, so that it can do its job.  If bentonite is added later in the winemaking process, it needs to be stirred up ever so often.

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