Glass is a uniform material of arguable phase (where the word "phase" is used to describe either a gas, liquid, or solid), usually produced when the viscous molten material cools very rapidly to below its glass transition temperature, without sufficient time for a regular crystal lattice to form. The most familiar form of glass is the silica-based material used for household objects such as light bulbs and windows. Glass is a biologically inactive material that can be formed into smooth and impervious surfaces. Under tension, glass is brittle and will break into sharp shards. Under compression, pure glass can withstand a great amount of force. The properties of glass can be modified or changed with the addition of other compounds or heat treatment.
Most glass formulations contain about 70–72 % by weight of silicon dioxide (SiO2). The most common form of glass is soda-lime glass, which contains nearly 30 % sodium and calcium oxides or carbonates. Pyrex is borosilicate glass containing about 10 % boric oxide. Lead crystal is a form of lead glass that contains a minimum of 24 % lead oxide.
The major raw material of glass is sand (or "quartz sand") that contains almost 100 % crystalline silica in the form of quartz. Although it is almost pure quartz, it may still contain a small amount (less than 1 %) of iron oxides that would color the glass, so this sand is usually depleted before production to reduce the iron oxide amount to less than 0.05 %. Large natural single crystals of quartz are pure silicon dioxide, and upon crushing are used for high quality specialty glasses. Synthetic amorphous silica, an almost 100 % pure form of quartz, is the raw material for the most expensive specialty glasses. The most common method for glass production is using molten tin, where the molten glass floats on top of the tin, thus giving it the name "float glass".