Window glass

Roman window glass
MFA Acc. #: 01.8231

Description

Cross brooch
MFA Acc. #: 2006.148

A transparent, flat, glass sheet used for glazing windows. Window glass has been made by several techniques. These include Crown glass (17th century), Cylinder glass (mass produced in the 1830s), Float glass (developed between 1953 and 1957) and Insulated glass. Currently most window glass is made by the float process in which a continuous ribbon of glass is flowed onto a molten tin bath. The top surface of the glass is subjected to nitrogen under pressure to obtain a polished finish. After forming, the glass is usually annealed for the removal of stresses and to increase the glass's hardness and durability. In general, window glass has a higher magnesium oxide and sodium oxide content than container glass, and a lower silica, calcium oxide, and aluminum oxide content.[Surface treatments, coatings or lamination may be applied to improve the chemical durability.

• Annealed glass also called float glass that has not been heat-strengthened or tempered is annealed glass. Annealing float glass is the process of controlled cooling to prevent residual stress in the glass and is an inherent operation of the float glass manufacturing process. Annealed glass can be cut, machined, drilled, edged and polished.
• Heat-strengthened (HS) glass has been subjected to a heating and cooling cycle and is generally twice as strong as annealed glass of the same thickness and configuration. HS glass must achieve residual surface compression between 3,500 and 7,500 PSI for 6mm glass, according to ASTM C 1048. HS glass has greater resistance to thermal loads than annealed glass and, when broken, the fragments are typically larger than those of fully tempered glass and initially may remain in the glazing opening. Heat-strengthened glass is not a “safety glass” product as defined by the various code organizations. This type of glass is intended for general glazing, where additional strength is desired to withstand wind load and thermal stress. HS glass cannot be cut or drilled after heat-strengthening and any alterations, such as edge-grinding, sandblasting or acid-etching, can cause premature failure.
• Fully tempered glass is approximately four times stronger than annealed glass of the same thickness and configuration, and residual surface compression must be over 10,000 PSI for 6mm, according to ASTM C 1048. Please contact Guardian for thicker glass standards. When broken, it will break into many relatively small fragments, which are less likely to cause serious injury. The typical process to produce tempered glass involves heating the glass to over 1,000 degrees F, then rapidly cooling to lock the glass surfaces in a state of compression and the core in a state of tension as shown in the diagram. Tempered glass is often referred to as “safety glass” because it meets the requirements of the various code organizations that set standards for safety glass. This type of glass is intended for general glazing, and safety glazing such as sliding doors, storm doors, building entrances, bath and shower enclosures, interior partitions and other applications requiring superior strength and safety properties. Tempered glass cannot be cut or drilled after tempering, and any alterations, such as edge-grinding, sandblasting or acid-etching, can cause premature failure.
• Laminated glass consists of sheets bonded together using a plastic interlayer using heat and pressure. The glass and interlayers can be supplied in a variety of colors and thicknesses. Laminated glass can be broken, but the fragments will tend to adhere to the plastic layer and remain largely intact, reducing the risk of injury. Laminated glass is considered “safety glass” because it meets the requirements of the various code organizations that set standards for safety. Heat-strengthened and tempered glass can be incorporated into laminated glass units to further strengthen the impact resistance. Hurricane resistance, bomb blast protection, UV protection, sound attenuation and ballistic or forced-entry security concerns are all primary uses for laminated glass.
• Insulating glass refers to two or more panels of glass sealed around the edges with an air space between, to form a double- or triple-pane unit. Commonly referred to as an “IG unit,” insulating glass is the most effective way to reduce air-to-air (convection) heat transfer through the glazing. The air space may be filled with an inert gas (Argon, Krypton) to further reduce heat transfer and increase comfort within a building. When used in conjunction with low-E and/or reflective glass coatings, IG units become effective means to conserve energy and comply with energy codes.
• Patterned glass contains an inlayed pattern or texture for use in privacy and decorative applications. The majority of patterned glass is used in bath and shower enclosures. It provides figure obscuration while permitting transmission of large amounts of light. Patterned glass is produced by pressing semi-molten glass between rollers to imprint a distinct pattern or by using an acid etching process that partially dissolves parts of the glass leaving an aesthetically pleasant pattern. There are many varieties of patterned glass that provide varying degrees of obscuration.
• Coatings can be applied to the surface of float glass to modify its appearance and improve its properties, such as altering the reflective and transmissive properties to provide energy efficiency or minimize UV light.

Many plastic alternatives to window have also been developed. For a comparison of the properties of transparent plastic boards, please see glazing comparison.

Synonyms and Related Terms

float glass; flat glass; plate glass; annealed glass; tempered glass; insulating glass; sheet glass; vitre (Fr.); vidraça (Port.)

Physical and Chemical Properties

Glass	Melt Temperature C	Extrusion Temperature C	Density g/ml	RI	Visible light transmission (%)	Haze (%)	Shrinkage %	Water absorption %	Hardness Shore D	Flex. Mod. Gpa (Bending stiffness)	Tensile strength Mpa	Izod impact J/m	Thermal Expansion X10-5/C	Fire Resistance (LOI) %	Link
Soda-lime glass	1040	<600 (working pt)	3.4-2.5	1.513-1.523	99	variable	slight expansion until >500C	0.001	6-7	72	7-90	Izod impact J/m	0.3-0.6	Non-flammable	Physical properties of glass

Resources and Citations

• G.S.Brady, Materials Handbook, McGraw-Hill Book Co., New York, 1971 Comment: p. 363
• Dictionary of Building Preservation, Ward Bucher, ed., John Wiley & Sons, Inc., New York City, 1996
• Thomas C. Jester (ed.), Twentieth-Century Building Materials, McGraw-Hill Companies, Washington DC, 1995
• Wikipedia: Glass (Accessed Nov. 29, 2005 and Aug. 2023)
• Climaguard: Tech Guide