Why Does New Glass Have to Cool for Hours? Cool It Fast and It Explodes

Glass contracts as it cools, and if the outside cools faster than the inside the outside sets while the interior is still shrinking, which leaves the finished object with the surface pulled tight over a core that wants to be smaller. Those locked-in stresses do not relax and can be large enough to break the object spontaneously days or weeks later, or to make it shatter from a light knock, a temperature change or nothing identifiable. The oven exists to bring the whole piece down slowly enough that the inside and outside stay at similar temperatures throughout.

The cooling follows a defined curve rather than simply being slow:

• •Hold at a temperature where the glass can still flow very slightly
• •That lets existing stresses relax away over a set period
• •Cool very slowly through the range where the glass sets rigid
• •That is the critical stage and the slowest part of the curve
• •Cool faster once the glass is fully rigid, since stress no longer locks in
• •Total time runs from under an hour to several days by thickness

The industrial form is a long tunnel with a moving belt or rollers carrying the ware slowly from the hot end to the cold end, and the temperature falls progressively along its length, so an object's journey through it traces the required curve simply by travelling. Heating is applied in zones that can be controlled separately. Length runs to many tens of metres in a container plant, since output rate and required cooling time together set how long the tunnel must be. Studio glassmakers use a small insulated box instead, controlled by a programmer, and load it at the end of a working session.

Checking whether the cooling worked is done optically and the method is elegant. Glass under internal stress changes the behaviour of light passing through it, splitting a beam into two components travelling at slightly different speeds, and viewing the object between two polarising filters reveals that difference as coloured fringes. Stress-free glass appears uniformly dark, and stressed glass shows bright patterns whose colour and spacing indicate how much stress is present and where. The instrument is simple enough to sit on a bench in any glassworks and is used routinely on samples, and the same technique is used on plastics and on transparent models of engineering structures.

The required time depends on thickness far more steeply than intuition suggests, which explains why heavy glass is expensive. Heat leaves a thick piece only by travelling through the glass to the surface, and the time that takes scales with the square of the thickness, so doubling thickness roughly quadruples the cooling time. A thin blown vessel may take an hour and a solid paperweight several days. Large optical blanks for telescope mirrors have taken months, with the most famous example cooled over most of a year, because a stress or a variation left in such a blank ruins an object of enormous value.

Cooling the outside of glass faster than the inside locks in stresses that can break the object later with no apparent cause, so finished ware passes through a tunnel that holds it where stress can relax and then cools it very slowly through the range where it sets rigid. Time scales with the square of thickness, so a paperweight takes days and a telescope blank has taken months.