Fermentools is the only manufacturer that produces real glass fermentation weights specifically designed to fit the wide-mouth Mason jar, measuring 2 ¾ inches in diameter and 10mm thick.
Other companies do not have glass weights, or they have them in odd sizes. If your weight is too small, you have to use more than one to get the job done. People come up with some creative things to keep their food submerged, but nothing works as effectively as a heavy glass weight.
Not only are the glass weights needed to keep the ferment below the brine level, but they also can be used for displacement. For instance, if you make a small batch of something that does not fill up the jar, you can simply use extra fermentation weights to occupy the void left in the top of the jar. Since a small ferment will only off gas a small amount, this practice fills the extra space with glass, leaving only a small amount of oxygen remaining to be displaced by your ferment.
Fermentation Weights are the single most expensive component of the kit, but they are extremely convenient to have on hand.
The perfect addition to your 6 or 12 pack order. There will be times when additional weights will be needed. For instance, when making beet kvass, you will only have a few sliced beets on the bottom, and you may need a few weights to cover them all. Another example is when you have not filled up the jar as high as you may like, simply add more weights to occupy space in the jar, displacing oxygen. Or when you have some especially buoyant produce. Extra weights may not be needed all the time, but they are always a nice item to have around. Buy in this quantity for $6.75 each. If you are already buying a 6 or 12 pack kit, this can be included in the same box, saving you money on postage.