These 50 mL PYREX Erlenmeyer flasks are designed with heavy duty rims to reduce chipping. Their uniform wall thickness provides the proper balance between mechanical strength and thermal shock resistance. Approximate graduations are in durable white enamel. An extra large marking space is also provided. These black rubber 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. Solid natural rubber Low sulfur content Each stopper has its size marked in raised figures Withstands temperatures -25°C to 70°C