Combining inventive design with timeless style, Bodum's Assam double wall glass is perfect for serving everything from hot drinks to cold brews. Made of heat-resistant borosilicate glass, and individually mouth-blown by expert artisans, this double wall glass features excellent insulation properties. Hot drinks stay warm longer, while the glass remains cool to touch and cold drinks stay cool without any messy condensation., , How the Insulation Works:, A silicone vent at the bottom of each glass equalizes pressure within the walls of the glass with the outside air pressure. As a result, the glass self-adjusts to the temperature of the liquid. Only permeable to air, not liquids, whether your beverage is hot or cold, the vent never allows fluids into the interior of the glass layers.