Borosilicate glass filtering flasks. Heavy-walled flasks feature a heavy-duty rim and bolt neck ideal for vacuum filtration. Graduations are printed in durable white enamel. United Scientific flasks deliver an unbeatable combination of thermal shock resistance and physical strength. Borosilicate glass can endure quick temperature changes, and therefore exhibits high resistance to thermal shock. (Thermal shock is cracking because of rapid temperature change). The glass withstands freezing, microwave heating, and boiling temperatures.