Tempering glass is expensive, but we think it's worth it. We have seen many glass fermentation weights that had air bubbles crack while in use. These weights were designed to be the very best on the market for fermenting in wide mouth Mason jars. They are 68.5mm (2.7 inches) in diameter and 18.5mm (.73 inches) thick. They weigh 6 ounces each (170 grams). The weights are cut from sheets of high quality tempered soda-lime glass. Mason jars themselves are made of untempered soda lime glass. Most competing discs are not tempered, which is much cheaper to produce but less safe. Air bubbles and other imperfections in lower quality glass cause them to crack during use.