Introduction to glass

Glass is the name given to all amorphous bodies that are obtained by lowering the temperature of a melt independently of its chemical composition and the temperature range of solidification, which as a result of the gradual increase of viscosity adopts the mechanical properties of a solid body.

Glass is melted at a temperature between 1000 and 2000° C.

The microscopic structure of glass is comparable to that of a liquid in which the individual constituents form an irregular network without a long range order. Glass is also the name given to a cooled melt.

Raw Materials

  • Silica (70 to 72%)
  • Lime (10%)
  • Soda (14%)
  • Oxide/aluminia/magnesia (5%)

The substances are introduced in the form of quartz sand, soda and lime. 5% oxides such as magnesium and aluminium oxide are added to this mixture. These additives improve the physical and chemical properties of the glass. 



Main Glass Groups

· Soda lime glass
· Lead glass
· Boro silicate glass

Main Glass Products

· Flat glass (for architectural or automotive applications)
· Glass containers/glass tubes
· Special glasses
· Glass fiber


Float Glass process

The float process refers to the manufacturing process for flat glass. This process came into general use in the 1960s. Saint-Gobain installed its first factory equipped with float technology in Pisa, in 1965. The theory: as the molten glass ribbon comes out of the oven, it is floated on a bath of liquid tin. Result: the glass does not need polishing or smooth grinding. It is cut directly on the production line.

The main characteristics of glass are transparency, heat resistance, pressure and breakage resistance and chemical resistance.

Mechanical Properties

2500 kg/m3

A 4mm thick pane of glass weighs 10kg/m2

470 HK

The hardness of float glass is established according to Knoop. The basis is the test method given in DIN 52333 (ISO 9385).

Compression resistance
800 - 1000 MPa

The compression strength defines the ability of a material to resist a load applied vertically to its surface

Modulus of elasticity
70 000 MPa

The modulus of elasticity is either determined from the elastic elongation of a thin bar, or from bending a bar with a round or rectangular cross section.

Bending strength
45 MPa

The bending strength of a material is a measure of its resistance during deflection. It is determined by bending tests on glass plate using the double ring method according to DIN EN 1288-5.

The main characteristics of glass are transparency, heat resistance, pressure and breakage resistance and chemical resistance.

Thermal Properties

Thermal conductivity

Thermal conductivity determines the amount of heat required to flow through the cross sectional area of the float glass sample in unit time at a temperature gradient.

Tempering & Softening
approx. 600°C

Contrary to solid bodies of crystalline structure, glass has no defined melting point. It continuously transforms from the solid state to the viscous plastic state. The transition range is called the transformation range and according to DIN 52324 (ISO 7884), it lies between 520°C and 550°C. Tempering and bending require a temperature of a further 100°C.

Specific heat
0.8 J/g/K

The specific heat (in joules) defines the amount of heat required to raise the temperature of 1g of float glass by 1K. The specific heat of glass increases slightly the temperature is increased up to the transformation range.

Thermal expansion
9.10-6 K-1

There is a difference in the expansion behaviour of a body under the effect of heat between linear expansion and volumetric expansion. With solid bodies, the volumetric expansion is three times that of linear expansion. The temperature coefficient of expansion for float glass is given according to DIN 52328 and ISO 7991.

The main characteristics of glass are transparency, heat resistance, pressure and breakage resistance and chemical resistance.

Transformation range
520 - 550°C

Optical Properties

Glass has several strong points concerning optical properties:

  • It can be produced in large and homogeneous panes
  • Its optical properties are not affected by ageing
  • It is produced with perfectly flat and parallel surfaces
Refractive index
n = 1.52

If light from an optically less dense medium (air) meets an optically denser medium (glass), then the light ray is split at the surface interfaces. The measure of deflection determines the refractive index. For float glass, this refractive index is n=1.52.

The main characteristics of glass are transparency, heat resistance, pressure and breakage resistance and chemical resistance.

Technical properties

Chemical resistance against

Water = class 3 (DIN 52296)

Acid = class 1 (DIN 12116)

Alkaline = class 2 (DIN 52322 and ISO 695)

The surface of glass is affected if it is exposed for a long time to alkalis (and ammonia gases in damp air) in conjunction with high temperatures. Float glass will also react to compounds that contain hydrofluoric acid under normal conditions. These are used for treating glass surfaces.

Wear tests

Abrasion tests

(DIN 52347 and ISO 3537) The scattering of light the transmission of directed light striking the surface is evaluated.

Light scatter

increase for float glass is approx. 1% (after 1000 abrasion cycles). The permitted light scatter increase for vehicle safety glass (windshield) is 2% in Europe (ECE R43) and the USA (ANSI Z 26.1).

Sand trickling process

(DIN 52348 et ISO 7991). For this diagonal impact abrasion test, 3kg of sand with a 0.5/0.71mm particle size are trickled onto the surface to be tested, which is inclined at 45 y from a height of 1600mm. Measurement of wear is the reduced luminous density (according to DIN 4646 part 2).

Reduced luminous density

for float glass is approx. 4cd/m2lux.

Micro scratch hardness

for float glass is approx. 0.12N (Mar resistance-test).

Tempered Monolithic Glass

The "Securit" tempering process was discovered in Saint-Gobain's laboratories in 1929, in the course of research undertaken at the request of the automobile industry. The process, which consists of strengthening the glass by very fast blast cooling (from 600 to 300° C in a few seconds), is used to make automobile, building, and specialty flat glass. Tempered glass is also a safety glass. A violent impact shatters tempered glass into many tiny non-cutting fragments.


Generally, all windows except for the windshield.

Type of glass

Clear, tinted and deep tinted glass

Standard thickness: 3, 4, 5mm

Spectral data: See table below for average values given for a thickness of 3.15mm.

Tolerances depend on production and measurement methods. For detailed information and other thicknesses, please contact Saint-Gobain Sekurit.

TL/RL: Light transmission / reflection, type of light A, 2°, daylight viewing 380-780 nm
TE/RE: Energy transmission / reflection, Parry Moon, mass 2, 280-2500nm
TIR/RIR: Infrared transmission / reflection, Parry moon, mass 2, 780-2500nm
TUV: UV transmission, Schulze, ISO 9050, 280-380 nm

For more information on the thermal performances of Saint-Gobain Sekurit glazing, see the chapter: Thermal comfort.

Physical and chemical properties

  • Density 2500 kg/m3
  • Hardness 470 HK
  • Modulus of elasticity 70 000 MPa
  • Bending strength after tempering 100-120 MN/m2
  • Specific heat 0.8 J/g/K
  • Thermal conductivity 0.8 W/mk
  • Temperature coefficient of expansion 9.10-6 K -1

Laminated glass

In 1909, a French chemist Edouard Benedictus, invented laminated glass and called it "Triplex". The process bonds two sheets of glass using a sheet of transparent plastic, producing a safety glass. If the glass is broken by an impact, the plastic retains the fragments. The process is used for automobile windshields but can be also applied for laminated sidelites, laminated backlites and roofs.

Applications Mandatory for Windshields and available for the rest of the car.
If struck or impacted, the glass cracks in the shape of a spider's web.



Material properties

  • Hardness 470 HK
  • Index of refraction 1.52
  • Temperature resistance At least 96 Hours at 90°C

More generally, all laminated glass passes all legal and OEM requirements concerning durability against heat, UV, moisture,…. 

The manufacturing process for laminated glass

Automotive windows are usually made from laminated glass for windshields and tempered glass for sidelites and backlites. Laminated glazing can also be used for sidelites and backlites, mainly to improve safety and security.