Glossary of Terms

Glossary of Terms

Float glass

High quality, transparent flat glass manufactured by means of the float tank procedure that is, floating molten glass on a "tin-bath" at extremely high temperature.

Anti-reflective glass

Surface-coated glass which minimises light reflectance and appears therefore to show virtually no visual reflection

Acid-etched glass

Satin-like, translucent glass manufactured by acid-etching one surface of the glass


A basic form of edge working, by removing the sharp edges of cut panes of glass.


A decorative form of edge working, where the edges of a glass pane are ground and polished smoothly at an angle.

Body-tinted glass

Transparent float glass with a consistent colour throughout its depth

CE Marking

When the European marking is officially in force, any glass product that is used in construction and buildings, which is sold within the European Union, must bear the label. This label can be stamped onto the product, the packaging or the accompanying commercial documents.


Computer controlled cutting. Sizes are inputted into a computer and then the glass is cut automatically.

Counter sunk

A hole drilled through the glass so that when a screw or bolt is inserted the head of the fixing is flush/level with the surface of the pane. The fixing must be isolated from the glass by nylon or soft lining material; there must be no glass-to-metal contact.

Curtain walling

Non-load bearing, typically aluminium, facade cladding system, forming an integral part of a building’s envelope.

Curved glass

Glass, which is curved in form, produced by heating it to its softening point, so that it takes the shape of the mould. Annealed and laminated glass is available in curved form.

Double Glazed Unit (DGU)

Two panes of glass, separated by a cavity and hermetically sealed in a factory, to provide thermal and/or acoustic insulation.


Emissivity is a surface characteristic of a material. It is the relative ability of a surface to absorb and emit energy in the form of radiation. Low-emissivity (Low-E) coatings reduce the normally relatively high surface emissivity of the glass. The coatings are mainly transparent over the visible wavelengths but reflect long wave infra-red radiation towards the interior of the building. The result is greatly reduced heat loss


Abbreviation or symbol for “Solar factor” according to BS EN 410 (formerly abbreviated to SF (Solar Factor) or TT (Total Transmission)).

Heat Soak Test (HST)

This is an additional form of heat-treatment, which is carried out after the thermal toughening process in order to reduce the risk of spontaneous breakage of toughened glass in service due to nickel sulphide inclusions (NiS)

Heat-strengthened glass

Glass which has been heat-treated in order to increase its mechanical strength and resistance to thermal breakage. It has fracture characteristics similar to that of ordinary annealed glass and is not classed as a safety glass.

Heat-treated/heat treatment

A generic term for glass that has been heat-strengthened or thermally toughened in order to increase its mechanical strength and resistance to thermal breakage.

Horizontal toughening

A thermal toughening process whereby the glass is toughened horizontally and supported by rollers.


The term applied to the material used in laminated glass to bond the glass leaves together. It can be either PVB, cast-in-place resin or intumescent.

Laminated glass/laminate/laminating

Two or more sheets of annealed or heat treated glass are separated by one or more plastic inter layers (normally PVB) and subjected to heat and pressure, in order to ensure perfect adhesion between constituent elements.

Light Transmittance (T)

The proportion of the visible spectrum that is transmitted through the glass (formerly abbreviated to LT (Light Tranmittance)).


Generic term for the various loads, where relevant, exerted on a structure or elements of a structure including wind loads, snow loads, imposed loads for example those associated with accidental human impact, and dead loads such as self weight.

Low iron

Referring to extra clear glass, which has reduced iron oxide content in order to lessen the green tinge inherent to ordinary clear float glass.

Nickel sulphide inclusion (NiS)

A rare but normal impurity arising from the manufacture of the glass is present in all glass that can, in certain circumstances, lead to spontaneous breakage of thermally toughened glass in service.


Notches and cut-outs are processes whereby areas of glass are removed from a sheet of glass, such an operation might be used where hinges or handles are required on frameless glass doors (there are limitations that apply and advice should be sought).

Point load

An imposed concentrated load acting on a square contact area of 50mm sides. Most often associated with balustrading and guarding applications and also to glass used in floors.

Polished edge

A worked, smooth, bright surface to the edge of the glass with either a flat or profiled finish.

PVB (Polyvinyl Butyral)

The plastic interlayer incorporated into laminated glass in order to ensure good adhesion and the mechanical and safety breakage characteristics of the glass.

Radius corner

Either an internal or external rounded cut edge on or within a pane of glass.


The section of the frame surround which forms an angle into which the glass is placed and held.


An optical phenomenon, generally noticed in reflection, caused by contact between glass and rollers in the horizontal toughening process.

Safety glass

Glass which either must not break or must break safely.


Enamelling the surface of a sheet of glass, either partially or completely, by means of a silk-screen and thermal toughening

Self-cleaning glass

Property of glass with a hydrophilic or hydrophobic coating. The coating harnesses the dual-action of UV light and rain (or water) to break down organic dirt and reduce the adherence of mineral material. The glass stays cleaner for longer and is easier to clean

Shading coefficient (SC)

The ratio of the g-Value (SF or TT) of a glass relative to that of 3mm clear float glass (0.87) and is used as a performance comparison. The lower the shading coefficient number, the lower the amount of solar heat transmitted. The short wave shading coefficient is the direct transmittance (T) of the glass as a factor of the solar factor or total transmittance (g) of 3mm clear float glass (T ÷ 0.87). The long wave shading coefficient is the internally re-radiated energy that the glass has absorbed as a factor of the g-Value (SF or TT) of 3mm clear float glass. It is determined by subtracting the direct transmittance (T) from the (g) of the subject glass and then dividing by the (g) of 3mm clear float glass (g-T ÷ 0.87).

Silicone seal

Where the edges of double-glazed units are unframed and exposed to direct sunlight, they are sealed with silicone for UV resistance.

Snow load

An imposed load exerted onto a structure or element of a structure by formation of snow.

Solar factor g

The percentage of total solar radiant heat energy transmitted through glazing (the sum of energy transmitted directly and energy absorbed and re-emitted to the interior).

Solar heat gain

Solar radiant heat, transmitted or remitted by glazing into a building, contributing to the build-up of heat.

Spacer bar

Generally an aluminium or plastic bar along all edges of a double-glazed unit, filled with desiccant, which separates the two panes of glass and creates a cavity.


Small fragments of glass that are ejected from the surface of a laminated glass sheet when the opposite surface is impacted.

Spandrel or spandrel panel

Glass cladding panels used in non-vision areas of a facade, commonly in curtain walling. They generally comprise an enamelled or opacified glass to conceal building structure elements such as the edge of floor slabs.

Spectrophotometric performance/properties

The collective term for the transmittance, absorptance and reflectance properties of glass of solar radiant heat and light energy.

Spateduttered coating/co

An advanced metallic coating is applied to the glass "off-line" or after the float glass manufacturing process, by a technique called magnetically enhanced cathodic sputtering under vacuum conditions.

Stepped-edge unit

The edges of the double-glazed unit are not flush. One pane is larger and overlaps the other, to enable their use in roof glazing for example.

Stock size

Manufactured glass products are available in standard sheet sizes: jumbos (PLF), lehr end sizes (LES) and standard stock sizes (SSS).

Structural glazing

Glass acting as a structural support to other parts of the building structure, for example glass fins. It can also refer to glass that is fixed by means of bolted connectors where the glass is not acting as a structural element.

Structural sealant glazing

An external glazing system where the glass is bonded to a carrier frame without mechanical retention.

Thermal break

A type of metal frame that incorporates an isolating material of low thermal conductivity located between the inner and outer parts of the frame in order to reduce the rate of heat loss through the frame.

Thermal stress

The term used to describe the internal stresses created when a pane is subjected to variations in temperature within the body of the glass. If the temperature differentials in the glass are excessive, the glass may crack. This is referred to as thermal breakage or fracture.

Thermally insulating glazing

Double or triple glazed units provide thermal insulation.

Toughened glass

Glass that has been subjected to a controlled heating and cooling process, in order to significantly increase its resistance to mechanical and thermal stress. Through the thermal toughening process, the glass attains its safe-breakage characteristics

Tinted interlayer

A coloured pvb interlayer between two or more panes of glass.


Transmitting light but obscuring clear vision.


This is a measure of the rate of heat loss of a building component. It is expressed as Watts per square metre, per degree Kelvin, W/m2K.


This is a measure of the rate of heat loss through the frame element of a window or door or other frame element. It is expressed as Watts per square metre, per degree Kelvin, W/m2K. It takes no account of the glass or spacerbar/edge seal (if an IGU is installed).


This is a measure of the rate of heat loss through a single pane of glass or an IGU. It is expressed as Watts per square metre, per degree Kelvin, W/m2K. It is also know as a “centre pane” and takes no account of the spacerbar/edge seal (if it is an IGU) or any framing material it is installed into.

Uniformly distributed load (UDL)

Pressure exerted uniformly across a pane of glass, for example a wind load.

UV transmittance

The percentage of solar energy in the form of ultra-violet radiation transmitted by glazing.


This is a measure of the rate of heat loss through the “whole window”. It is expressed as Watts per square metre, per degree Kelvin, W/m2K. It takes account of all elements forming the window, glass, spacerbar/edge seal (if it is an IGU) and the framing material itself.

Vertical glazing

Glazing which is either true vertical, or within 15° either side of true vertical.

Visible spectrum

Part of the electromagnetic spectrum, with wavelengths from approximately 380nm to 780nm, to which the human eye is sensitive. The combined wavelengths of the visible spectrum result in “white light”.

Vision area

Areas of a facade which allow vision from the interior to the exterior.


Refers to the reduction of the thermal bridging effect around the perimeter of double-glazed units by replacing the conventional aluminium cavity spacer bar with a low heat-conductive thermally insulating cavity spacer

Wind load

The pressure, positive or negative, acting on an external surface of a building caused by the direct action of the wind. Commonly expressed as KN/m2.

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