HEAT TREATED GLASS
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.
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.