Under natural conditions, the surface of molten glass will cool more rapidly than the centre. This results in internal stresses which may cause the glass sheet or object to crack, shatter or even explode some time later. The annealing process is designed to eliminate or limit such stresses by submitting the glass to strictly controlled cooling in a special oven known as a "lehr". Inside the lehr, the glass is allowed to cool to a temperature known as the "annealing point". When the glass reaches this point, the lehr temperature is stabilized for a specific length of time (depending on the glass type, its thickness, its coefficient of expansion and the amount of residual stress required) to allow stresses present in the glass to relax. This phase is followed by a period of cooling with a pre-defined temperature gradient.


A process used widely in the production of bowls, plates, ashtrays, etc., whereby the shaped glass article (which may be pre-printed) still in sheet form is placed on a stainless steel, sheet steel or cast iron mould coated with talc or powdered chalk. The temperature is increased until the glass sheet sinks into shape in the mould.

Bent glass
Flat glass that has been shaped while hot into curved shapes.

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

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

Bulletproof Glass
Armour plate glass which is more than 60 mm thick and which resists penetration by bullets.


Cast Glass
Glass produced by 'casting', in other words by pouring molten glass into a mould or by heating glass already contained in the mould until the glass melts and assumes the shape of the mould.

Coated glass (reflective glass)
Float glass coated with a very fine layer of precious metal. It allows active regulation of daylight and solar heat and glare. It absorbs a certain amount of solar heat and simultaneously reflects another and it transmits a certain amount of solar light ant at the same time it reflects another. Besides the thermal and light performance it adds a high aesthetic touch wherever applied.
The hard coating (also called pyrolytic coating) reinforces the low-e and/or the solar performance of glass. It is applied using a high-temperature and penetrates the surface of the glass, becoming remarkably durable. This last property means that processors can process those products very easily.
The soft coating is applied by projecting metal particles onto the glass using a vacuum electromagnetic process. The composition of this coating depends on the desired colour and features.


Direct Energy Transmission (DET)
Percentage of solar energy flux transmitted directly through the glass with a spectral density between 300 and 2150 nm.

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

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


The shaping or finishing of the edges of a glass surface, usually by grinding with an abrasive wheel.

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

Energy Absortion (EA)
Percentage of solar energy flux absorbed by the pane or panes making up the glazed wall. The absorbed energy is then reradiated to the outside or inside at different rates depending on the characteristics of the pane or panes of glass, wind speed, internal air speed, internal and external temperatures.

Energy Reflection (ER)
Percentage of solar energy flux reflected by the glass.

The production of a design in glass by cutting into the glass surface. Engraving methods include copper wheel engraving, diamond or tungsten point engraving, acid etching and sand blasting.


Facade (face)
The whole exterior side of a building that can be seen at one view; strictly speaking, the principal front..

Figured Glass
Patterned glass is a translucid glass made from clear or coloured cast glass that passes between two rollers located at the end of the glass furnace. The pattern is printed at high temperatures on one side of the glass by the engraved (embossing) roller. Eventually, a wire mesh can be incorporated into the glass. This glass is then gradually cooled down to the surrounding temperature.

Fibre Glass
Very fine strands of glass (normally with a high boric oxide and content) used in the form of glass wool for insulation, glass fibre for matting, etc., and also for the reinforcement of plastics. The principal production process involves blowing jets of steam or air onto molten glass as it emerges from a tank furnace through very small diameter nozzles.

Fire-resisting Glass
Flat glass with a wire inlay which helps retard shattering in the event of fire and thus delays the spread of smoke and flames. Such glazing is classified either as class G (which resists fire and prevents the spread of smoke and flames for specified periods of time) or class F (which has the characteristics of class G but also impedes the spreading of radiant heat for specified periods). New developments include pre-stressed wire-free borosilicate glass and double glazing with the inner space filled with an intumescent material which acts as a heat shield.

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

Float glass
Float glass is the product of sand and sodium carbonate heated to more than 1.500°C and running off over a bath of molten tin. The high quality of this glass has helped Glaverbel gain a solid worldwide reputation.

Float Process
A method for the production of high-quality sheet glass whereby a ribbon of molten glass is fed across a bath of heated liquid, usually molten tin, in a carefully controlled atmosphere. The process was developed by the UK firm Pilkington Brothers.


General Colour Reproduction Index RD65 (DIN 6169)
The general colour reproduction index evaluates the resemblance between the colour of objects in daylight (represented by the illuminant D65) and the colour of the same objects in daylight passing through the glass.

A homogeneous material with a random, liquidlike (non-crystalline) molecular structure. The manufacturing process requires that the raw materials be heated to a temperature sufficient to produce a completely used melt, which, when cooled rapidly, becomes rigid without crystallizing.

The removal of glass with abrasives or abrasive (grinding) wheels in order to shape, polish or otherwise finish both flat and hollow glass. Grinding processes include milling, sawing, edging and drilling.


Heat-absorbing glass
Glass that absorbs an appreciable amount of solar energy.

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

Heat-strengthened glass
Flat or bent glass that has been heat-treated to a specific surface and/or edge compression range to meet the requirements of ASTM C 1048, kind HS. Heat-strengthened glass is approximately two times as strong as annealed glass of the same thickness when exposed to uniform static pressure loads. Heat-strengthened glass is not considered safety glass and will not completely dice as will fully tempered glass.

Term used for both fully tempered glass and heat-strengthened glass.

Heat Resistant Glass
Glass which has a low coefficient of expansion and which is therefore less liable to thermal shock. Borosilicate glass is the most common type of heat resistant glass.

High Performance Thermal Insulating Glass
Thermal insulating glass to which an extremely fine, invisible coating of precious metal is applied, giving the glass considerably greater power of insulation against heat loss and cold.

High-transmission glass
Glass which transmits an exceptionally high percentage of visible light.


Infrared Lamp
An incandescent lamp working at a low filament temperature and consequently emitting relatively high amounts of infrared radiation. Infrared bulbs are usually made of borosilicate glass with molybdenum or tungsten wires.

Insulating glass unit
Two or more lites of glass spaced apart and hermetically sealed to form a single-glazed unit with an air space between each lite. (Commonly called IG units.)

Insulating, To Exclude Air Inleakage
The application of various insulating plastics and concretes (produced using grains of exfoliated vermiculite, perlite, bubble alumina, glass fibre, etc.) to prevent unwanted air from entering the furnace (see "inleakage"). For furnace crowns, asbestos-free calcium silicate plastic can be used. Insulating materials can be applied by hand-trowelling or by pneumatic gunning.

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


K- or U- VALUE
The heat transfer coefficient (k- or U- value) is the amount of heat in watts per unit of time, which is transferred through 1 sq.m of glass surface per degree Kelvin difference between internal and external temperature. The k- or U- value is calculated for the wall's surface heat exchange coefficients:
- internal : 8 W/m2K
- external: 23 W/m2K.

An oven used to process a substance by burning, drying, or heating. In contemporary glassworking kilns are used to fuse enamel and for kilnforming processes such as slumping


Laminated Glass
Laminated (or compound) glass consists of two or more sheets of glass with one or more viscous plastic layers "sandwiched" between the glass panes. The solid joining of the glasses takes place in a pressurised vessel called an autoclave. In the autoclave, under simultaneous heating of the already processed layers of glass and special plastic, lamination occurs. When laminated safety glass breaks, the pieces remain attached to the internal plastic layer and the glass remains transparent.

Light Reflection (LR)
Ratio of the light flux reflected by the glass to the incident light flux expressed by the illuminant CIE D65.

Light Transmission (LT)
Ratio of the light flux transmitted through the glass to the incident light flux expressed by the illuminant CIE D65 with a spectral density between 380 and 780 nm.

Long-Wave Shading Coefficient
Proportion of absorbed energy transfer to the inside divided by 0.87.

Low Emissivity Glass
Commonly known as "low-E" glass and often used in double and triple glazing units, this window glass has a special thin-film metallic or oxide coating which allows the passage of short-wave solar energy into a building but prevents long-wave energy produced by heating systems and lighting from escaping outside. Low-E glass thus allows light to enter while also providing thermal insulation.


Clear or coloured high quality float glass with a silver layer deposited on one side of the glass to obtain a light reflection of more than 92 %. The silver layer receives a special treatment, and is then protected by two different layers of special protective paint.

A form, normally made of wood or metal, used for shaping and/or decorating molten glass. Some moulds (e.g., dip moulds impart a pattern to the parison, which is then withdrawn, and blown and tooled to the desired shape and size; other moulds are used to give the object its final form, with or without decoration.


Opal Glass
Glass that resembles an opal, being translucent and white, with a grayish or bluish tint.

Optical Glass
The glass used in lenses and prisms, in which homogeneity, refractive characteristics and the absence of defects such as seed are of major importance. In many modern optical glasses, lead oxide has been replaced with barium oxide while lanthanum oxide is used for optical glass with low dispersion and a high refractive index.


Patterned Glass
Glass to which a pattern is applied by passing it while still soft between two rollers, one of which has a decorative pattern engraved on its surface.

Pittsburgh Process
A method for the production of sheet glass. The process was developed by the USA Pittsburgh Plate Glass Company in the 1920s. The glass is drawn from the melt and conveyed vertically through an annealing shaft around 12 metres long prior to cutting.

Plate Glass
Flat glass made by the casting or rolling of molten glass which is then mechanically ground and polished to produce a smooth and transparent sheet.

Pressed Glass
Glassware formed by placing a blob of molten glass in a metal mould, then pressing it with a metal plunger or "follower" to form the inside shape. The resultant piece, termed "mould-pressed," has an interior form independent of the exterior, in contrast to mould-blown glass, whose interior corresponds to the outer form. The process of pressing glass was first mechanized in the United States between 1820 and 1830.

The abbreviation for polyvinyl butyral. PVB is used in sheet form as a strong plastic interlayer in the production of laminated glass.


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

Relative heat gain
The amount of heat gain through a glass product taking into consideration the effects of solar heat gain (shading coefficient) and conductive heat gain (U-value). The value is expressed in Btu/hr/ft2 (W/m2).
The relative heat gain is calculated as RHG = (Summer U-value x 14oF) + (Shading Coefficient x 200). The lower the relative heat gain, the more the glass product restricts heat gain.

Rolled Glass
Rolled (or cast) glass is a translucent glass with 50-80% light transmission, depending on its thickness and type of surface. It is used where transparency of the glass sheet is not important or not desired. To produce rolled glass, molten glass pours from the melting tank over a refractory barrier (the "weir") and onto the machine slab where it flows under a refractory gate (the "tweel"), which regulates the volume of glass, and then between two water-cooled rollers. The distance between the rollers determines the thickness of the glass.


Safety Glass
Glass which does not disintegrate into sharp and potentially dangerous splinters when it is broken. Safety glass may be produced by laminating (see "laminated glass") or by tempering (see "tempering").

Sand Blasting
A method for giving glass surfaces a matt finish either for decoration or to reduce transparency. The method was invented in 1870 by an American, Benjamin C. Tilghman, who is thought to have been inspired by the effect of sand being blown against windows on the American prairies. Compressed air forces the abrasive material through the nozzle of a sandblasting gun and onto the glass surface. Although sand can be used, more effective abrasives with less toxic effects are now available. Silicon carbide is commonly used, as is electro-corundum (aluminium oxide). The glass is normally placed inside a special cabinet with arm holes, a viewing window and dust extraction facilities.

Screen Printing
A process for the decoration of glass whereby coloured ink is forced by a flexible "squeegee" through a fine-mesh screen, or "mask", (traditionally made of silk, now also made of nylon, polyester and stainless steel) onto the glass surface. A separate mask is used for the application of each colour. Considerable automation of the process has been developed, thus allowing extremely high printing speeds for even complex designs.

Sealing, Insulating Glass
Glazing units made up of two or three sheets of glass (known as "double glazing" or "triple glazing" respectively) held in a metal frame and sealed (normally with butyl) to create an airtight space of 9-12 mm between them. A dessicant substance in the airtight cavity prevents the formation of condensation. Also known as "IG".

Shading Coefficient (SC)
The shading coefficient is obtained by dividing the solar factor by 0.87, which is the solar factor of 3 mm clear float glass.

Short-Wave Shading Coefficient
Direct energy transmission divided by 0.87.

Single glazing
A single pane of glass.

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

Solar energy reflectance
In the solar spectrum, the percentage of solar energy that is reflected from the glass surface(s).

Solar energy transmittance
The percentage of ultraviolet, visible and near infrared energy within the solar spectrum (300 to 2100 nanometers) that is transmitted through the glass.

Solar Factor (SF)
· sun at 30° above the horizon at right angles to the facade
· ambient temperature equal to outside ambient temperature
· surface heat exchange coefficients
- internal : 8 W/m2K
- external: 23 W/m2K.

Solar heat gain coefficient
The ratio of the solar heat gain entering the space area through the fenestration product to the incident solar radiation. Solar heat gain includes directly transmitted solar heat and absorbed solar radiation which is then reradiated, conducted, or convected into the space.


Thin glass
Glass with a thickness of between 0.4 mm and 2.0 mm, made by the float process or drawing process and usually destined for advanced industrial applications.

Tinted glass
Glass with colorants added to the basic glass batch that give the glass color, as well as, light and heat-reducing capabilities. The color extends throughout the thickness of the glass. Typical colors include bronze, gray, dark gray, aquamarine, green, deep green, blue and black.

Tempered glass
A strong, break-resistant glass made by speeding the cooling process by immersing the molten glass in water. The resultant glass has a surface compression five to ten times more resistant to mechanical and thermal shocks than well-annealed glass.

Thermal Shock Testing
Assessing the effects on a material of rapid temperature change. In glass, the shock may derive from the external surface of glass expanding or contracting more rapidly than the interior surface as a result of heating or cooling. Any such difference may lead to cracking or shattering.

Permitting light to come through but diffusing it so objects on the other side appear vague, distorted, or imperfect.

Special process of solidification of a glass sheet in order to make it particularly resistant to breakages. The process may be physical (thermal) or chemical. In the former, the glass sheet is heated to a temperature just below its softening point and then immediately cooled by special jets of cold-air. These harden the surface of the glass, giving the inside more time to cool. This allows the external layer to crystallize into a wider lattice while the inside solidifies with greater compression than in the crystal lattice. The result is a sheet of glass which is two or three times stronger than untempered glass and which, upon breakage, shatters into tiny pieces with blunt edges (the most common applications are for automotive glass). The chemical process, on the other hand, is based on the so-called ion-stuffing technique. Different chemical elements possess different ionic radii and therefore different densities. Hence, if glass containing sodium is cooled slowly in a salt bath of molten potassium, the sodium ions will migrate from the glass to the salt, while the potassium ions will move to the surface of the glass where, due to their wider radium, they create a denser and therefore stronger surface layer (of no less than 0.1 mm). Glass sheets which have been chemically tempered are five to eight times stronger than those which have not undergone any tempering process.

Tempered glass
Float glass that has been thermally treated to give it a higher degree of mechanical and thermal resistance. The glass is first heated to more than 600° C, then cooled down quickly (full toughening or tempering) or cooled down more slowly (heat strengthening), in both cases at a carefully controlled speed. These processes put the surface of the glass under permanent compression stress, giving the glass its special characteristics.
Resistance to mechanical and thermal shocks up to 2 times (heat strengthened) or 5 times (toughened) higher than normal float glass.
Prevents glass from breakage caused by high temperature differences on the surface of the glass pane (caused by for example shadows). This is particularly important for glass types with a high energy absorption that are exposed to heat radiation.
Only for toughened glass: safety glass that breaks into small fragments without cutting edges, ensuring protection against risks of injury.
Heat strengthened: no risk of breakage caused by nickel sulphide inclusions: heat soak test not necessary.
Heat strengthened: better surface evenness and less optical distortions compared to toughened glass (particularly important for highly reflective glazings in facades).
Heat strengthened: breaks into larger fragments, with less risk of glass falling out when broken.
Toughened and heat strengthened glass can be neither shaped nor cut.
Toughened: doors, furniture, transparent (vision) glazing as well as (opaque) spandrel glazing in glass facades, any application where improved resistance to thermal or mechanical stresses is required.
Heat strengthened: single glass in spandrels, exterior and interior sheets of double glazing in spandrels, exterior sheet of solar control double glazing with a high energy absorption factor, solar control laminated glass.

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

Permitting light to come through without distortion so objects on the other side can be seen clearly.


The name of the invisible portion of the light spectrum with wavelengths shorter than 390 nanometers.

A measure of air-to-air heat transmission (loss or gain) due to the thermal conductance and the difference in indoor and outdoor temperatures. As the U-value decreases, so does the amount of heat that is transferred through the glazing material. The lower the U-value, the more restrictive the fenestration product is to heat transfer. Reciprocal of R-value.


An optical effect in flat glass due to irregularities in the surface of the glass that make objects viewed at various angles appear wavy or bent.

A shield of glass, in one or more sections, projecting above and across the dashboard of a vehicle.

Wired glass
Rolled glass having a layer of meshed or stranded wire completely imbedded as nearly as possible to the center of thickness of the lite. This glass is available as polished glass (one or both surfaces) and patterned glass. Approved polished wired glass is used as transparent or translucent fire protection rated glazing. Patterned wired glass is sometimes used as decorative glass. It breaks more easily than unwired glass of the same thickness, but the wire restrains the fragments from falling out of the frame when broken.

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