Bentonite is a special type of very fine clay of the aluminium-silicate type - that is used in winemaking as a clarifier. While deposits of bentonite are found in various parts of the world, there are a few deposits including the one from which the clay is named, Fort Benton in Montana, USA, that are most suitable for wine stabilisation. Bentonite is a natural substance formed from fine volcanic dust and consists of miniscule plates consisting of silicon, magnesium and aluminium.
Bentonite has very high water absorption properties that allow it to expand to almost 20 times its original size when hydrated, and the plates are negatively charged. Wine proteins have a slight positive charge, and as opposites attract, they bind electro-statically to the negatively charged bentonite plates. This attracting charge along with hydrogen bonding causes suspended particles in the wine to cling to it as it settles to the bottom of the tank.
There are several advantages to using Bentonite, as it is very effective in taking out yeast, tannins and other stubborn protein-based particles that may want to linger after fermentation. This results in a wine with a clear appearance and a radiant colour. It also helps to reduce the occurrence of certain off-flavours, as well as reduce the wine's ability to oxidize.
Bentonite is relatively easy to use - you start by mixing it with water into slurry, which will have the consistency of a thin, watery cement mix. The slurry then needs to set for about an hour so as to allow the Bentonite granules to swell and become saturated. A dose of the slurry mix is then stirred into the wine.
To make your Bentonite treatment more effective - the colder the wine is - the stronger the Bentonite static charge. At room temperature Bentonite is usually adequately effective, but by chilling the wine down to around 6-7°C the Bentonite strength is considerably enhanced. Also, stirring the wine several times after the Bentonite has been add, will give the Bentonite more time to attract particles before settling.
Bentonite fining does have a few disadvantages - as some aroma and flavour molecules are not protected from the attractive forces of the bentonite plates. Therefore, its use to fine out unwanted proteins can also produce an unintended loss of varietal character in the wine. So winemakers always conduct small scale laboratory trials to determine the minimum amount of bentonite required to achieve protein stabilisation - (i.e. the loss of sufficient proteins so that a heated wine will not go hazy).