TECHNICAL SPECIFICATIONS

HEAT TREATED GLASS

1. Introduction
2. Process
3. Properties

1. INTRODUCTION

In the Mappae Clavicula (a 9th century book) there is a description of unbreakable glass. It was understood at this early stage that glass could be toughened and made stronger by Quenching in hot oil. Similarly ‘Prince Rupert’s Drops’ were produced by dropping molten gobs of glass into water. The result is a teardrop shaped piece of glass, the head of which is strong enough to withstand heavy blows with a hammer.

However if the fine tail is snapped off, the complete teardrop explodes with a surprising amount of energy. This phenomenon occurs due to the outer ‘skin’ of the drop immediately solidifying on contact with the water while the centre cools at a slower rate. Consequently, the centre of the drop is put into tension and pulls inwards on the already hardened outer surface, which is now in compression. Breaking the tail releases the tension, which dissipates through the compressed outer surface. This is a classic demonstration of the principles involved in the toughening process.

In 1879 De la Bastie took this principle further by quenching the glass in a bath of linseed oil and tallow. The resulting product however was closer to what is today termed ‘heat strengthened’, rather than fully toughened. Both these methods had severe bowing problems which Siemens tried to overcome by quenching the glass between two cast iron blocks. It was not until 1928 that Reunies des Glaces in France invented the vertical electric furnace where large sheets of glass could be processed with minimal bowing. Pilkington (U.K) followed quickly with their process of quenching by blowing air on both sides of the glass simultaneously.

In essence the process remains the same today and although vertical furnaces are still used, almost all architectural glass is produced on horizontal furnaces.

2. PROCESS

Toughened Glass

The cut-to-size glass sheets are fed from the loading conveyer into the furnace where it oscillates back and forth on ceramic rollers until it reaches approximately 620°C.

Processing from the furnace the glass moves into the quench where it is rapidly cooled by blasting both sides with air.

This ‘snap cooling’ or quenching induces compressive stresses to the glass surface while the centre remains in tension. Although the physical characteristics remain unchanged the additional stresses created within the glass increases its strength by 4-5 times that of annealed glass of equal thickness.

Recommended for doors side panels and low lites glass balustrades, shower and bath screens, pool fences and glass walled squash courts. It is also used in automotive, marine rail and land transport as well as furniture applications.

3. PROPERTIES

  • Up to five times stronger than annealed glass of the same thickness
  • Designated Grade A safety glass as per AS/NZS 2208
  • In the event of breakage, the panel will fracture into relatively small harmless particles
  • Greater resistance to thermal stress when compared to annealed glass (can be subjected to temperatures ranging from 70° C to 290° C).

Because of its mechanical strength it is ideal for creating a ‘total vision’ concept in all glass assemblies, foyers and entrance ways.

 

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