These black stoppers are made of solid natural rubber that stays pliable over a long period of time. Ideal for use with non-aggressive or concentrated solutions. Low sulfur content. Withstand temperatures from ?25 to 70?C (?13 to 158?F). Available in solid, one-hole, and two-hole versions. Each stopper has its size marked in raised figures. All stoppers are 25mm tall. Sold as one-pound bags.