Wine, as I’ve often written, is a complex chimerical soup. Wine naturally contains lots of different amino acids and some of those amino acids are in long-chain form and actually are proteins. Because proteins are pretty big molecules (as molecules go), they sometimes are so big they can’t be dissolved in the wine as a liquid anymore and are actually visible to the naked eye. If your wine has enough of these proteins your wine might be so cloudy as to have a visible “protein haze.” Now, amino acids and proteins in wine aren’t dangerous or bad for you, but they might keep your wine from being the sparklingly clear liquid you’d like it to be. This is where fining (or treating) with bentonite comes in.
Bentonite is a naturally-occurring clay that has been used in winemaking for centuries. It comes in powdered form and is rehydrated in water or wine before being added as a slurry to wine. The clay particles swell up in the liquid and adsorb the positively-charged protein molecules. Then the big protein-bentonite agglomeration gets so big and heavy that it falls (or flocculates) to the bottom of the vessel, hopefully leaving clear wine above a layer of sediment. The winemaker is then able to siphon off, or rack, the clean wine from the lees.
Since bentonite can also adsorb some aroma and flavor compounds, it’s important to add the minimum amount of bentonite needed to do the job. If you are just looking for immediate visual clarity, just add the amount that gives you that result. If, however, you want to make sure that the wine is “heat stable” and will not throw a protein haze with increased time or temperature, you may need to add a little more than would make it visually clear. In the commercial wine lab you’ll typically add increasing amounts of bentonite slurry to a sample until, when filtered and “cooked” in an incubator (say at 80 °F/27 °C for 24 hours) no protein haze is visually apparent. Then the indicated amount is added in the cellar.
Now to get to your more detailed question — is sodium or calcium bentonite preferred and what are the differences between the two additives? Since bentonite is a naturally-occurring clay that is mined from deposits in the earth (Wyoming, for instance, is a famous source of a very pure vein) other minerals and ions can come “attached” to it. Most folks I talk to say that sodium bentonite and calcium bentonite are interchangeable in winemaking. In fact, often a bentonite product you might buy off the shelf won’t even specify if it’s “calcium” or “sodium” bentonite, it makes that little of a difference.
Some people say they prefer calcium bentonite because it forms more compact lees, but I also know if you have too much calcium in your wine you could form calcium tartrate crystals instability, which can’t be fined out by seeding with potassium bitartrate. I’ve also read that sodium bentonite can leach small amounts of sodium ions into wine and indeed can cause an increase in a few mg/L . . . but at the end of the day wine will always be a “low sodium” beverage and not enough of a nutritional concern. The research I’ve done shows the dosage rates and application are similar enough to not warrant special instructions. As with any additive, the most important thing is that bench trials be done so you can use the minimum bentonite to perform the task you want done. This is to save money, of course, but most importantly to save you from unnecessarily stripping aromas, flavors and anything else from your wine if too much is added. The maximum dose before aroma stripping becomes obvious is 1 g/L in whites, and about half that amount in reds.