Glass can be naturally occurring (as obsidian) or a man-made material. It is composed of sand (silica), soda, and ash that have been melted together at extremely high temperatures in a furnace. Glass has material properties of both solid and liquid states. Scientists call it a “rigid” or “slow-flowing” liquid. Hot glass stiffens as it cools. At room temperature, glass is so stiff that it behaves like a rigid, brittle solid. Molten glass has the consistency of honey and can be formed into utilitarian objects or sculptural shapes as it cools and hardens. While it is being worked, glass must be repeatedly reheated in a furnace. Once formed, the hot object must be cooled very slowly to prevent it from shattering. Example: Vittorio Zecchin, V.S.M. Cappellin Venini & C., Trasparente, 1921-1925. Photograph by Luca Vignelli.