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Float Glass

Float glass is made by heating a mixture of minerals together and floating the clear molten glass on a surface of molten tin.  In general, equivalent types also known as simple glass, annealed glass, sheet glass and plate glass.  Float glass is the most common type of glass used throughout the world.  It is commonly known as architectural soda lime silica float glass and is the most commonly used glass product in the flat glass construction industy.

Annealed glass

Annealed glass is generally given to glass that is produced by a normal mixing-heatingforming-cooling process.  There are several different methods, one of which is the float glass process, originally developed by Pilkington, and described above.  Other types of annealed glass include plate glass, sheet glass, cast glass, and crown glass.

Toughened glass

Toughened glass is made from float glass via a defined temperature cycle.  This is done by heating the glass to a high temperature and then cooling the glass in a special way.  This causes the surface of the solid glass to be under compression.  Glass is very strong under compression so toughened glass is very strong and is often difficult to break.  However, if the compressed surface is disrupted then the glass will shatter quite easily.  Toughened glass is also known as tempered glass and fortified glass. Toughened glass is used where safety is desired and/or where protection against thermal stress breakage is needed.

Heat Strengthened glass

Heat Strengthened glass is made from float glass via a temperature cycle similar to that for toughened glass.However,there are differences including lower temperatures, so the glass is not as strong.Heat Strengthened glass is used where some resistance to thermal stress breakage is needed; this resistance is higher than that for float glass but less than that for toughened glass.Also heat strengthened glass is NOT a safety glass unless contained within a laminated make-up.

Chemically toughened sodium-calcic glass

Processed by dipping into melted potassium salt at a temperature above 380 degrees Celsius for a period of 16 hours.  The ion exchange that takes place between the surface of the sodium-calcic glass and the melted potassium salt induces a residual stress which is higher than that of thermally toughened glass but of a lesser surface penetration.  Unless incorporated into a laminated make-up chemically toughened sodium-calcic glass is not a safety glass product. 

Laminated glass

>Laminated glass is made from two layers of glass that are adhered together by a plastic layer.  Two common ethods are either to use a layer of PVB (PolyVinylButyral) or to use a cold poured resin.  PVB is a sticky solid and is simply placed between two sheets of glass.  The assembly is then put into a machine and processed via a temperature and pressure cycle to give a clear three layer product. Alternatively, two pieces of glass are assembled together with a small gap between them and a seal around the whole perimeter except for a small hole at the top.  This assembly is held vertically and a liquid resin poured between the two sheets.  Once the gap is full the hole is sealed up and the pane exposed to UV light, making the resin turn solid with a high adhesion to the glass.  This is termed Cast-In-Place (CIP) lamination. The laminated structure improves the glass properties, depending upon the type and thickness of resin used.  For example, laminated glass often has resistance to human impact, so provides safety performance; alternatively it may have resistance to manual attack (‘Anti-Bandit’) or fire resistance properties.  Obviously, thick glass can be used and/or many layers can be built up to give thicker laminated glass with higher performance for safety, ‘Anti-Bandit’ or even ‘Anti-Ballistic’ performance. The most common laminated glass is two layers of 3 mm glass with a 0.4 mm layer of PVB, giving a 6.4 mm thick laminated glass.  (NB All thicknesses are nominal; often the PVB thickness is 0.38 mm).  Most laminated glass is made from sheets of float glass but some is made from toughened and heat strengthened glass.

Wired glass

Wired glass is generally made from two layers of float glass melted together with a mesh of wires between the two layers.  Wired glass can not be made from toughened glass because the temperature cycle required would cause the wires to expand and break the glass.  This glass is used for fire protection – the wires help to keep the pieces together when the glass breaks (as it will due to the heat of the fire).  Most wired glass is not safety glass.  In the late 1990’s  a wired glass was been introduced that has thicker wires in it; the wires help to keep glass pieces together upon impact.  There are special wired glasses that use laminated glass with the wire mesh between two layers of PVB – these are classed as a safety glass but tend to be expensive.

The following table gives a list of common glass types and their safety performance:

Glass Type Typical Break Pattern & Safety Performance
Float glass (including clear, tinted and reflective float glass) Breaks into sharp pieces with high risk of cutting and piercing injuries.  Generally float glass is not a safety glass.
Heat Strengthened glass. Breaks into sharp pieces with high risk of cutting and piercing injuries. Heat Strengthened float glass is NOT a safety glass unless incorporated into a laminated make-up.
Heat Strengthened glass. Breaks into small crumb-like particles with low risk of cutting and piercing injuries.Toughened glass is a safety glass.
Chemically toughened sodium-calcic glass Breaks into large shards and is NOT a safety glass unless incorporated into a laminate make-up.
Laminated glass – can be constructed with annealed, heat strengthened and fully toughened glass. Breaks into sharp pieces but these pieces are held together by the plastic layer inside the glass.
Wired glass/Laminated Wired Glass Breaks into sharp pieces but these pieces are held together by the pvb  inter layer inside the laminated version of this glass.  Wired glass is not a safety glass unless the wires are thick enough for safety performance or it has been incorporated into a laminated make-up.

Thicker float glass and thicker heat strengthened glass may be assumed to be safe in certain circumstances because of the difficulty in breaking them, but this should always be verified independently.  According to current legislation, all safety glass in the UK must be marked, including annealed glass coated with safety film. If glass is not appropriately marked then it must be assumed that it is not a safety glass, even if measurement/testing shows it is a laminated or heat treated glass.

Glass Performance Data Sequence and Terminology Defined


Abbreviation Term Meaning
V.L.T %

Ext LR %

Int LR %
Visible Light Transmission

External Light Reflection

Internal Light Reflection
The amount of useful day light penetrating the glass

The amount of light reflecting from the external surface

The amount of light reflecting from the internal surface

Solar Energy

Abbreviation Term Meaning
ET %

ER %

EA %


SHGC (g)
Energy Transmitted

Energy Reflected

Energy Absorbed

Shading Coefficient

Solar Heat Gain Coefficient
Solar energy (short wave) penetrating the glass

Solar energy (short wave) reflected by the glass

The energy absorbed & converted to heat by the glass

Literally the % of “Shade” provided by the glass

Always 87% of the Shading Coefficient

Thermal Insulation

Abbreviation Term Meaning
U-Value W/m².K Conducted Heat Gain The conducted (Long Wave) heat through the glass

A combination of external temperature(s) and energy absorbed
  • RHG is 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 ↓LOWER the relative heat gain, the more the glass product restricts heat gain.


Abbreviation Term Meaning


Classic SC
Magnetic Sputtered Vacuum Deposition

Low Emissivity

Classic Solar Control
The most widely used process to deposit coating on glass

The coatings which slow conducted heat gain in glass

The coatings which only reflect solar (short wave) energy


Abbreviation Term Meaning




Outer Pane/Inner Pane

Air Gap or Air Space

Molecular Sieve Desiccant

Spacer Bar

Primary Seal

Secondary Seal
How a sealed glass unit is referred to - 1, 2, 3, 4 surfaces

The space between the glass panes in a glass unit

Material inserted into the spacer bar absorbing moisture

The bar separating the glass panes commonly Aluminum

The material on the spacer bar edge sealing the glass unit

The material providing the unit with structural strength