What Temperature Should Float Glass Be Poured At?

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• Float glass starts out as a mineral mix comprised of three-fourths silica sand, with varying proportions of calcium oxide, sodium carbonate, sodium hydroxide, lime and magnesium oxide making up the other fourth. These minerals are finely ground and mixed dry, then poured into a glassmaking furnace, where they are heated to 2,730 degrees F to melt them. Natural convection currents ensure thorough mixing of the molten minerals as they turn into glass. The molten glass flows to a cooling zone where the temperature drops to 2,010 degrees F, the correct temperature for pouring.

Floating Glass

• The molten glass, about the consistency of hot syrup, pours out onto a mirror-like bed of molten tin that’s under an inert gas atmosphere. The glass, being lighter than the tin, floats on top and gains a perfectly flat and glossy fire finish. At the other end of the molten tin bed, the glass cools to 1,100 degrees F, solidifies and is drawn off by rollers as a flat ribbon about 9 feet wide. The tractive effects of surface tension keep the molten glass attached to the solid ribbon as it is drawn off. The speed of pouring and drawing off determines how thick the glass will be. Float glass making is a continuous process. Once started, a glass plant must keep on running.

Coating

• As the glass is being drawn off the molten tin bed, just past the point where it solidifies, glassmakers apply coatings by chemical vapor deposition to tailor the optical properties of the glass to specific requirements. For example, coatings can be applied to reflect ultraviolet and infrared wavelengths while letting visible light through. Although there’s only a few seconds' time span to apply coatings, there’s time enough to apply multiple different coatings.

Annealing

• The glass ribbon tends to develop internal stresses as it cools that make the glass too brittle to be used. To release the stresses, glass must be annealed by heating to 400 degrees F. It is inspected by polarized light to determine whether all the stresses have been released. The finished sheet glass is inspected by laser scanners, then is cut with diamond wheels to standard sizes or to order.