These premium Borosilicate Glass 500 ml Boiling (Flat Bottom) Flasks are heat and chemical resistant. Fluid markings will not wear off. Bomex can be sterilized by boiling or run through the dishwasher with no fear of measuring accuracy loss. These laboratory grade 500 ml Conical Boiling (Flat Bottom) Flasks will last a life time unless dropped or shattered. Made in China. DOES NOT SHIP WITH RUBBER STOPPER. Uses Rubber Stopper Size #6.