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. If you fill your jars up to the neck, one weight will perfectly cover your ferment. You can use more than one weight if the jar isn't completely full or if you want to displace more brine to bring the level up to the neck of the jar. With more active ferments, you might not want to fill the jar into the neck to avoid overflowing brine. Then you will probably want to use two weights. Glass does not leach anything into your food. Dishwasher safe, boiling water safe, oven safe, microwave safe. Avoid sudden temperature changes. Plastic free packaging. International shipping is $5 extra to Canada.