Glass Tips & Terminology

We have included some of the most common glass & window related terms you may encounter. For more inclusive, additional references - Glasslinks has an excellent listing of additional resources at: Feel free to contact us with any questions you may have.

Air Infiltration: The amount of air leaking in & out of a building through cracks in walls, windows & doors.

Annealing: In the manufacturing of float glass, it is the process of controlled cooling done in a lehr to prevent residual stresses in the glass. Re-annealing is the process of removing objectionable stresses in glass by re-heating to a suitable temperature followed by a controlled cooling.
Annealing Lehr: An on-line, controlled heating/cooling apparatus located after the tin bath & before the cooling conveyor of a float glass production line. It's purpose is to relieve induced stress from the flat glass product to allow normal cold end processing.

Argon: An inert, nontoxic gas used in insulating windows to improve the insulating value of sealed glass units.
Autoclave: A vessel that employs high temperature & heat. In the glass industry, used to produce bond between glass & PVB or urethane sheet, creating a laminated glass product.

Bed or Bedding: In glazing, the bead of compound or sealant applied between a lite of glass or panel and the stationary stop of sight bar of the sash or frame. It is usually the first bead of compound or sealant applied when setting glass or panels.

Beveling: The process of edge finishing flat glass to a bevel angle.

Borosilicate Glass: A high silicate glass that has at least 5% boron oxide.

Bow (& Warp): A curve, bend or other deviation from flatness in glass.

BTU: An abbreviation for British Thermal Unit - the heat required to increase the temperature of one pound of water one degree Fahrenheit.

Bubbles: In laminated glass, a gas pocket in the interlayer material or between the glass & the interlayer. In float glass, a gaseous inclusion greater than 1/32" in diameter.

Buck: Wood frame surrounding a window unit.

Bullet Resistant: A multiple lamination of glass or glass and plastic designed to resist penetration from medium-to-super-power small arms and high-power rifles.

Butt Glazing: The installation of glass products where the vertical glass edges are without structural supporting mullions.

Caulk: The application of a sealant to a joint, crack or crevice. A compound used for sealing that has minimum joint movement capability; sometimes called low performance sealant.Chipped Edge: An imperfection due to breakage of a small fragment from the cut edge of the glass . Generally this is not serious except in heat absorbing glass.

Cullet: Broken glass, excess glass from a previous melt, or edges trimmed off when cutting glass to size. Cullet is an essential ingredient in the raw batch in glass-making because it facilitates melting.

Condensation: The accumulation of water vapor from the air's humidity on any cold surface whose temperature is below the dew point, such as a cold window glass or frame that is exposed to humid indoor air. Low conductivity or warm edge spacers reduce condensation.

Curtain wall: An exterior building wall which carries no roof or floor loads and consists entirely or principally of metal or a combination of metal, glass, and other surfacing materials supported by a metal frame.

Cut Sizes: Glass cut to specified width & height (or length).

Cutting: Scoring glass with a diamond, steel wheel, or other hard alloy wheel & breaking it along the score. Other methods of cutting glass include water jet & laser.

Deflection (Center of Glass): The amount of bending movement of the center of the glass lite perpendicular to the plane of the glass surface under an applied load.

Distortion: Alteration of viewed images caused by variations in glass flatness or inhomogeneous portions within the glass. An inherent characteristic of heat-treated glass.

Double Glazing: Any use of 2 lites of glass, separated by an air space, within an opening, to improve insulation against heat transfer and/or sound transmission. In insulation glass units, the air between the glass sheets is thoroughly dried & the space is sealed, eliminating possible condensation & providing superior insulating properties.

Double Strength: In float glass, approximately 1/8" (3 mm) thick.

Edgework: Grinding the edge of flat glass to a desired shape or finish.

Egress Size: Make size of a window unit that is sufficient to allow a minimum square foot sash opening to meet BOA or local code requirements.

Elastometric: (adj) Having the property of returning to it's original shape & position after removal of load. (n) an elastic rubber-like substance.

Emissivity: The measure of a surface's ability to emit long-wave infrared radiation.

Etch: To alter the surface of glass with hydrofluoric acid or other caustic agents. Unintentional permanent etching of glass may occur from alkali and other runoff from surrounding building materials.

Facade (Face): The whole exterior side of a building that can be seen in one view. - usually the front.

Fenestration: Any glass panel, window, door, curtain wall, or skylight unit on the exterior of a building.

Flat Glass: A general term that describes float glass, sheet glass, plate glass and rolled glass.

Float glass: Glass formed on a bath of molten tin. The surface in contact with the tin is known as the tin surface or tin side. The top surface is known as the atmosphere surface or airside.

Flush Glazing (Pocket Glazing): The setting of a piece of glass or panel into a four-sided sash or frame opening containing a recessed "U" shaped channel without removable stop on three sides of the sash or frame and one channel with a removable stop along the fourth side.

Frame Opening: The width & height measurement in inches, of the opening into which the replacement window will be installed.

Fully Tempered Glass: Flat or bent glass that has been heat-treated to a high surface and/or edge compression to meet the requirements of ASTM C1048, kind FT. Fully tempered glass if broken will fracture into many small pieces (dice) which are more or less cubical. Fully tempered glass is approximately four times stronger than annealed glass of the same thickness when exposed to uniform static pressure loads. Outside of North America is is sometimes called "toughened glass".

Gas Filled Units: Insulating glass units with a gas, other than air, in the air space to decrease the unit's thermal conductivity (U-value) or to increase the unit's sound insulating value. (i.e.: Argon)

Glass: A hard brittle substance, usually transparent, made by fusing silicates, under high temperatures, with soda, lime, etc.

Glass Clad Polycarbonate: One or more pieces of flat glass bonded with an aliphatic urethane interlayer to one or more sheets of extruded polycarbonate in a pressure/temperature/vacuum laminating process.

Glazing: (n) A generic term used to describe an infill material such as glass, panels, etc. (v) The process of installing an infill material into a prepared opening in windows, door panels, partitions, etc.

Grids: (also called Muttins or Grilles) Colonial square & diamond patterns between the glass.

Heat Absorbing Glass: Glass that absorbs an appreciable amount if solar energy.

Heat-Resistant Glass: Glass able to withstand high thermal shock, generally because of a low coefficient of expansion.

Heat-Strengthened Glass: Twice as strong as annealed glass; therefore, it is able to resist slightly stronger impacts. Produced in a similar manner to tempered glass (slower cooling than tempered). Heat strengthened glass is not considered safety glass & will not completely dice as will fully tempered glass.

Heat Treated: Term used for both fully tempered glass & heat-strengthened glass.

Insulated Glass: unit Two or more pieces of glass spaced apart and hermetically sealed to form a single-glazed unit with an air space between the glass. (Commonly called IG units.)

Interlayer: Any material used to bond 2 lites of glass and/or plastic together to form a laminate.

Jamb: Thickness of the wall from the inside surface of the house to the outside surface of the house.

Laminated Glass: Constructed by bonding a tough polyvinyl butyral (PVB) plastic interlayer between two pieces of glass under heat and pressure to form a single piece. Can be made of any kind of glass, but is most typically made of annealed, heat-strengthened, or tempered glass.

Lehr: A long, tunnel-shaped oven for annealing glass, usually by a continuous process.

Lite: Another term for a pane of glass used in a window. Frequently spelled "light" in the industry, but often spelled "lite" in text to avoid confusion with light as in "visible light".

Low-Emittance (low-E) Coating: Microscopically thin, virtually invisible, metal or metallic oxide layers deposited on a piece of glass and sealed in an insulating glass unit to reduce the U-Factor. The radiant energy (heat), i.e. long wave infrared, is in effect reradiated back toward its source.Make Size: The tip-to-tip width & height measurement in inches of the finished window.

Mica: Mica is fire proof, infusible, incombustible and non- flammable and can resist temperatures of 600-C to 900-C,depending on the type of mica. It has low heat conductivity, excellent thermal stability and may be exposed to high temperatures without noticeable effect.

Mullion: A horizontal or vertical member that supports and holds such items as panels, glass, sash, or sections of a curtain wall.

Muntin: Horizontal or vertical bars that divide the sash frame into smaller lites of glass. Muntins are smaller in dimensions & weight than mullions.

Obscure Glass: (see patterned glass)

Oriel: A double hung window unit made with one with one sash larger than the other.

Patterned Glass: One type of rolled glass having a pattern impressed on one or both sides. Used extensively for light control, bath enclosures and decorative glazing. Sometimes called "rolled", "figured" or "obscure" glass.

Picture Window: Window unit with a stationary sash that does not operate.

Polished Wire Glass: Wired glass that has been ground & polished on both surfaces.

Primer: A coating specifically designed to enhance the adhesion of sealant systems to certain surfaces, to form a barrier to prevent migration of components, or to seal a porous substrate.

Projection: The distance that a bay, bow or garden window extends outward. Measured from the exterior of the building to the furthest extension of the window.

Reflective Glass: Glass with a metallic coating to reduce solar heat gain.

Rolled Glass: Glass formed by rolling, including patterned and wired glass.

Rough Opening: The opening in a wall in which a window or door is to be installed.

Rub: A series of small scratches in glass generally caused during transport by a chip lodged between 2 lites.

R-value: The thermal resistance of a glazing system expressed ft2/hr/oF/Btu (m2/W/oC). The R-value is the reciprocal of the U-value. The higher the R-value, the less heat is transmitted throughout the glazing material.

Sandblasted Finish: A surface treatment for flat glass obtained by spraying the glass with hard particles to roughen one or both surfaces of the glass.

Sash: The portion of the window that holds the glazing, usually the portion of the window that operates.

Score: To penetrate the surface of a lite of glass by means of a cutting device, e.g. a cutter, along a predetermined line in order to produce a lite of glass of a specific size and/or shape.

Scratches: Any marking or tearing of the surface appearing as though it had been done by either a sharp or rough instrument.

Seeds: Minute bubbles in float glass less than 1/32" in diameter.

Slider Window: A horizontally opening window. In a 2 section - has 2 operable sash. 3 section has 1 stationary & 2 operable sash.

Sloped Glazing: Any installation of glass that is at a slope of 15 degrees or more from vertical.

Solar Control Glass: Tinted and/or coated glass that reduces the amount of solar heat gain transmitted through a glazed product.

Spandrel: The panel (s) of a wall located between vision areas of windows which conceal structural columns, floors, and shear walls.

Storm Door: A panel or sash door placed on the outside of an existing door to provide additional protection from the elements.

Storm Window: A glazed panel or sash placed on the inside or outside of an existing sash or window as additional protection against the elements.

Tempered Glass: Flat or bent glass that has been heat-treated to a high surface and/or edge compression to meet the requirements of ASTM C 1048, kind FT. Fully tempered glass, if broken, will fracture into many small pieces (dice) which are more or less cubical. Fully tempered glass is approximately four times stronger than annealed glass of the same thickness when exposed to uniform static pressure loads.

Tinted Glass: Glass with colorants added to the basic glass batch that gives the glass color, as well as, light and heat-reducing capabilities.

Transmittance: The ability of the glass to pass light and/or heat, usually expressed in percentages (visible transmittance, thermal transmittance, etc.)

Transom Window: Window unit with a stationary sash, usually installed over a door.

U-Factor (U-Value): A measure of the rate of non-solar heat loss or gain through a material or assembly. The lower the U-Factor, the greater a window's resistance to heat flow and the better its insulating values.

United inches: Total of the width & height of the frame opening expressed in full inches.

Weather-stripping: A material or device used to seal the opening between sash and/or sash and frame.

Weeps (or Weep Holes): Drain holes or slots in the sash or framing member to prevent accumulation of condensation & water.

Window Unit: A non-load bearing device to fill a hole in a wall, consisting of both frame & sash members.

Wire glass: Rolled glass with a layer of meshed or stranded wire completely imbedded. Available as polished glass and patterned glass. Approved polished wired glass is used as transparent or translucent fire protection rated glazing.  The wire restrains the fragments from falling out of the frame when broken.