An astute winemaker contacted me with a very pertinent question. Tim, I did the Master Vintner Pinot Noir yesterday and was kind of surprised that there was no Bentonite. I thought that you had thought this was a good thing in kit winemaking. If not needed, please let me know. Robb H
Robb, Excellent catch, sir. Yes, the kits I worked with previously had an addition of the fining agent, Bentonite, on day one. It's a common technique, done for a few reasons. But before we explore those, let's look at what Bentonite is. Bentonite, named after the Cretaceous Benton Shale near Rock River, Wyoming is a type of clay, technically siliceous Montmorillonite, a smectite (clay mineral) formed when volcanic ash is washed of all of its larger particles until only a fine, soft, powder-like material is left. Because of its of absorbency and colloidal properties it's useful in a number of areas, like drilling mud, kitty litter and wine and beer fining. It's particularly good at binding to unstable colloidal material and proteins, pulling them out of suspension. A colloid is a mixture in which one substance of microscopically dispersed insoluble particles is suspended throughout another substance. To understand this, picture your wine as a solution of all kinds of goo from the inside of a grape. There's sugars, tannins, flavor and aroma compounds and a lot of gums, pectins, proteins and colloids. These float around with a weakly positive electrical charge on them, kind of like what happens when you walk across a carpet in sock feet and get a static shock when your touch the light switch.
Bentonite Clay
Bentonite also has an electrical charge, but it's not only very strong, it's negative. Opposites attract, so bentonite particles sweep through the wine and when they encounter a bit of goo they bind straight to it, locking it in an electrical embrace. Together the two are too heavy to stay in suspension and they fall out onto the bottom of the vessel, packing down into a layer of clay and bonded proteins and colloids. Since the juices used in some kits are not completely protein stable they need to be hit with a strong fining agent like Bentonite to keep them from being cloudy after fermentation. The easiest way to get it in was to put it in on day one and leaving it in until primary fermentation is over. There's also a small benefit to yeast health, as the particles of Bentonite act as anchor points for yeast cells to bud from, helping ensure a quick start to fermentation. The juices used in Master Vintner Kits, on the other hand, are extremely protein stable. During crush, press and processing we make sure that unstable colloids and proteins are taken out of solution. They don't need any Bentonite, and where required, they have balanced yeast nutrients, which ensure that the wine ferments completely dry. Because we stabilize them in the winery before putting them in the kits, our wines clear brilliantly and beautifully, with minimal winemaking steps. So there you have it: Robb, you successfully demonstrated Vandergrift's First Precept of Home Winemaking (Read The Instructions!) and seem willing to follow the second (Follow the Directions, Omitting No Detail However Small, Making No Change, However Slight)--good job! If you're as much of a Pinot Noir fan as I am, you're going to love your wine.
Bentonite, named after the Cretaceous Benton Shale near Rock River, Wyoming is a type of clay, technically siliceous Montmorillonite, a smectite (clay mineral) formed when volcanic ash is washed of all of its larger particles until only a fine, soft, powder-like material is left. Because of its of absorbency and colloidal properties it's useful in a number of areas, like drilling mud, kitty litter and wine and beer fining. 
The juices used in Master Vintner Kits, on the other hand, are extremely protein stable. During crush, press and processing we make sure that unstable colloids and proteins are taken out of solution. They don't need any Bentonite, and where required, they have balanced yeast nutrients, which ensure that the wine ferments completely dry. Because we stabilize them in the winery before putting them in the kits, our wines clear brilliantly and beautifully, with minimal winemaking steps.