CWCT FACETS

08.01 Glass types

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Categories: Glass & Glazing

Introduction
Glass for use in buildings comes in many forms.  However the basic product for all architectural glazing is plain annealed glass.  This is then modified and processed to produce products that satisfy the many and diverse functions that glass can now satisfy.

Annealed glass can be modified at the primary stage of manufacture when it is first formed from a melt of the raw materials.  At this stage the glass may be:

  • Tinted
  • On-line coated
  • Patterned

Following primary manufacture glass may be processed to further modify its performance and to produce assemblies of glass.  Glass processing
includes:

  • Toughening
  • Laminating
  • Bending
  • Coating
  • Printing
  • Assembly of insulated glazing units

This Section deals with the production of different types of glass.  Section 08.02 covers the use of glass as a glazing material.
 


Annealed glass
Annealed glass is untreated glass manufactured from soda lime silicates, which can be cut or scratched with a wheel and snapped.  It is the least expensive and most readily available type of glass.  The term includes float glass, sheet glass and patterned glass manufactured as follows:

  • Float glass is manufactured by pouring molten glass continuously from a furnace onto a large shallow bath of molten tin, where it floats, spreads out and forms a level surface (over ninety per cent of glass is manufactured by the float process);
  • Sheet glass is manufactured by the flat drawn process and has natural fire-finished surfaces but, because the two surfaces are never perfectly flat and parallel, there is always some distortion of vision and reflection;
  • Patterned, figured and cast glass products are manufactured by the rolling process in which semi-molten glass is squeezed between metal rollers (polished plate glass is produced by grinding, smoothing and polishing the surfaces of cast glass).



Float glass
Float glass is manufactured by pouring molten glass at a temperature of 1500oC onto a bath of molten tin. A ribbon of hot glass at  600oC comes off the bath, image.  Its thickness depends on the rate of pour.  The glass is cooled gently to avoid the stress concentrations that can arise from rapid cooling.  The cooling takes place on a lehr that is tens of metres long until the glass reaches room temperature, image.

Glass making is a continuous process with a float line producing glass without interuption for many years.  When changing from the manufacture of one thickness of glass to another there is a transition during which glass of intermediate thickness is produced.  Some of this glass will lie within the tolerances for manufactured glass given in BS952.  Glass that is out of tolerance is broken up to form cullet and melted down with raw materials to form the molten glass that is poured onto the bath at the head of the float line.

Float glass is cut into 'jumbo' sheets measuring 6m x 3m and shipped in loads of 25 tonnes to glass processors.

Float glass is available in thicknesses of 3, 4, 5, 6, 8, 10, 12, 15, 19 and 25mm.  The majority of glass produced is 4mm thick for domestic glazing.  The thickest glasses are only produced occasionally and then held in stock.
 


Patterned glass
Patterned glass is produced by the rolling process. Glass in a semi-molten state is rolled between patterned rollers to produce an embossed effect on the glass, Image.  Patterned glass is used for decoration and to provide some obscuration for reasons of privacy.

Patterned glass is available in a number of patterns and in thicknesses of 3, 4, 5, 6 and 10mm.
 


Polished plate glass
Polished plate glass is manufactured by rolling semi-molten glass between plain rollers to produce cast glass.  This is then ground smooth and polished to give a visually acceptable surface and minimal optical distortion through the glass.

The production plant for cast glass is much smaller than that for float glass, image.  Most flat glass is produced by the float process but he cast process is economic for small batches of specialist glasses where the melt is modified to produce tinted or low iron glasses.

Polished plate glass is normally available in thicknesses of 3, 4, 5, 6, 10, 12, 15, 19 and 25mm.
 


Wired glass
Wired glass is made by sandwiching an electrically welded steel wire mesh between two layers of molten glass in one continuous rolling process, image; it is supplied in rough cast (obscured) or polished (transparent) form.  It is commonly known as 'Georgian wired glass' and 'Polished wired glass'.
 


Toughened glass
Thermally toughened glass is formed by heating and then rapidly cooling (quenching), annealed glass in a toughening oven, image.  Differential cooling and hardening across the thickness of the glass generates a compressive stress in the surface layer of the glass, which is balanced by a tensile stress in the core of the pane. Depending on the skill of the operators and type of furnace, a small degree of optical distortion may occur, due to sag between the supports, during toughening.

Glass is toughened either to produce a safety glass, a glass of higher strength or both, see Section 08.02 and Section 10.01.

Toughened glass is available in thicknsses of 4, 5, 6, 10, 12, 15 and 19mm.  Sheet sizes are limited by the size of toughening oven.  The largest size available in the U.K. is 3100 mm x 2410 mm.  However most tougheners can only work up to 2000 mm x 1200 mm.

Toughened glass may be heat soaked to reduce the risk of failure due to nickel sulfide inclusions in the glass, Section 08.02.
 


Heat-strengthened glass
Heat-strengthened glass is formed by heating annealed glass and then cooling it under controlled conditions.  Heat-strengthened glass offers some of the strength of toughened glass but a reduced risk of failure due to nickel sulphide inclusions because of the reduced tensile stress in the glass.   It is produced in the same ovens as toughened glass and subject to the same size limitations.
 


Laminated glass
Laminated glass is formed by bonding together two or more panes of glass using a plastic interlayer. Any of the above forms of glass may be used, in any combination.  A common interlayer material is poly-vinyl-butyral (PVB), which is used in multiples of 0.38 mm thickness.
 


Glass properties
Glass has the following mechanical material properties regardless of whether it is annealed or toughened;

  • Young's modulus    = 72 x 109 N / mm2
  • Density    = 2500 kg / m3

Glass strengths are dealt with in Section 10.01.

The reflection and transmission properties of glass are dealt with in Section 09.02.