Once your sake has been pressed and it is sitting in a secondary container in most cases you will want to increase its clarity prior to pasteurization and bottling. This can be done with different methods and to various degrees. The simplest method and least effective is to simply rack the sake off the lees several times keeping only the clearest of sake. This relies on the yeast and other matter suspended in the sake to naturally settle to the bottom. Given time, however, this method works well and leaves the sake with its natural color, taste and aroma. Sake made this way is Muroka (無濾過), unfiltered sake.
To take it to the next level, finings can be added. Finings are substances that are added to capture organic compounds in the sake. Finings can drag the organic compounds down to the bottom or hold on to them while being filtered from the sake. Two of these fining substances are powdered activated charcoal and bentonite.
The highest degrees of clarity are obtained through the use of activated charcoal. A fine black powder of activated charcoal is mixed into the sake creating an ink black sake. This sake is then forced through filters to remove the charcoal, leaving it lighter and brighter. This is the chief method used in commercial production. However, filtering is difficult on the small scale practiced by most homebrewers. Bentonite is more practical for the homebrewer.
Bentonite can be hard to work with but with a little know-how it is pretty easy. It is best to use a blender and ONLY add the bentonite AFTER adding the boiling water and turning the blender on. The mix ratio is one tablespoon bentonite to one cup of boiling water. This is used to make a slurry. Begin with the boiling water in the blender and the blender on. Slowly add the bentonite to the boiling water as the blender continues blending. After all the bentonite has been added let the blender continue for 2 minutes. Then take it out and let it set for an hour for the bentonite to fully absorb the water.
The slurry will look similar to a creamy split pea soup and after setting for an hour will have a smooth feel. There will be some sandy material that settles to the bottom of the slurry but this can be ignored. It is not needed.
Two to four tablespoons of this slurry is added per gallon of cold sake, 50F. The sake could be room temperature but cold works best. Stir the slurry in smoothly making sure you disperse it evenly. Stir it a few times over the next 3 to 6 hours to give it lots of time to attract yeast, proteins and the likes. Add it after the second to the last racking before your final racking for bottling.
Bentonite can affect the taste just as charcoal filtering can. You can try it both ways and see which you prefer or use it only when you want that effect.