United Scientific Supplies RST15-H2 2-Hole Rubber Stopper, #15 Size is a black stopper that is made of solid natural rubber which stays pliable over a long period of time. These stoppers will withstand temperatures from –25 to 70°C (–13 to 158°F). Each stopper has its size marked in raised numerals. They are ideal for use with non-aggressive or concentrated solutions. Low sulfur content.