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To anneal glass, it is necessary to heat it to its annealing temperature, at which its viscosity, η, drops to 10 13 Poise (10 13 dyne-second/cm 2). [2] For most kinds of glass, this annealing temperature is in the range of 454–482 °C (849–900 °F) [ citation needed ] , and is the so-called stress -relief point or annealing point of the glass.
The manufacturing process for soda–lime glass consists in melting the raw materials, which are the silica, soda, lime (in the form of (Ca(OH) 2), dolomite (CaMg(CO 3) 2, which provides the magnesium oxide), and aluminium oxide; along with small quantities of fining agents (e.g., sodium sulfate (Na 2 SO 4), sodium chloride (NaCl), etc.) in a glass furnace at temperatures locally up to 1675 ...
Annealing consists in slowly cooling a glass in a controlled manner from a certain temperature at which it has begun to solidify (around 1,000 °C for silica glass or 450 °C for soda-lime glass, for example). Annealing is necessary to eliminate internal stresses in the material that may have arisen during melting (impurities, streaks, bubbles ...
Lehr (glassmaking) In the manufacture of float glass, a lehr oven is a long kiln with an end-to-end temperature gradient, which is used for annealing newly made glass objects that are transported through the temperature gradient either on rollers or on a conveyor belt. The annealing renders glass into a stronger material with fewer internal ...
The glass floats on the tin, and levels out as it spreads along the bath, giving a smooth face to both sides. The glass cools and slowly solidifies as it travels over the molten tin and leaves the tin bath in a continuous ribbon. The glass is then annealed by cooling in an oven called a lehr. The finished product has near-perfect parallel surfaces.
Tempered or toughened glass is a type of safety glass processed by controlled thermal or chemical treatments to increase its strength compared with normal glass. Tempering puts the outer surfaces into compression and the interior into tension. Such stresses cause the glass, when broken, to shatter into small granular chunks instead of ...
Borosilicate glass is a type of glass with silica and boron trioxide as the main glass-forming constituents. Borosilicate glasses are known for having very low coefficients of thermal expansion (≈3 × 10 −6 K −1 at 20 °C), making them more resistant to thermal shock than any other common glass. Such glass is subjected to less thermal ...
Temperature differences across the surface of glass panes can lead to cracks in the glass. [25] This may occur where the glass is partially shaded and partially heated from the sunlight. Tinted glass increases heating and thermal stress, while annealing reduces internal stress built into the glass during manufacturing.