Annealing vs toughening

The statement “annealing stained glass makes it stronger” appeared on the internet some time ago.  Of course, without annealing there is no glass, it would simply crumble.  Annealing is the process of allowing the glaseous state to be achieved.

I think the statement is more about the difference between annealed and toughened/tempered glass.  In summary, it relates to the amount of stress within the glass.  Well annealed glass has less stress than inadequately annealed glass and so is more stable.  Toughening is a process that balances stress and tension in the glass.

The processes are for different purposes and follow different processes. 

Annealing

Annealing of glass is a process of slowly cooling hot glass to relieve residual internal stresses introduced during manufacture. Annealing of glass is critical to its durability. Glass that has not been properly annealed retains thermal stresses caused by rapid cooling, which decreases the strength and reliability of the product. Inadequately annealed glass is likely to crack or shatter when subjected to relatively small temperature changes or to minor mechanical shock. It even may fail spontaneously from its internal stresses.

To anneal glass, it is necessary to soak it at its annealing temperature. This is determined mathematically as a viscosity of 10¹³ Poise (Poise is a measure of viscosity). For most soda lime glass, this annealing temperature is in the range of 450–540°C, and is the so-called annealing point or temperature equalisation point of the glass. At such a viscosity, the glass is too stiff for significant change of shape without breaking, but it is soft enough to relax internal strains by microscopic flow. The piece then heat-soaks until its temperature is even throughout and the stress relaxation is adequate. The time necessary for annealing depends on its maximum thickness. The glass then is cooled at a predetermined rate until its temperature passes the strain point (viscosity = 10^14.5 Poise), below which even microscopic internal flow effectively stops and annealing stops with it. It then is safe to cool the product to room temperature at a rate limited by the thickness of the glass.

At the annealing point (viscosity = 10¹³ Poise), stresses relax within minutes, while at the strain point (viscosity = 10^14.5 Poise) stresses relax within hours.  Stresses acquired at temperatures above the strain point, and not relaxed by annealing, remain in the glass indefinitely and may cause either immediate or delayed failure. Stresses resulting from cooling too rapidly below the strain point are considered temporary, although they may be adequate to promote immediate failure.

But annealed glass, with almost no internal stress, is subject to microscopic surface cracks, and any tension gets magnified at the surface, reducing the applied tension needed to propagate the crack. Once it starts propagating, tension gets magnified even more easily, causing it at breaking point, to propagate at the speed of sound in the material.

In short, the aim of annealing is to relieve the stress to create a stable piece of glass. The above describes when and how that occurs.

Toughened/Tempered Glass

Toughening or tempering glass starts with annealed glass to form one type of safety glass.  This done through a process of controlled thermal or chemical treatments to increase its strength compared with normal glass. Tempering puts the outer surfaces into compression and the interior into tension. Such stresses cause the glass, when broken, to crumble into small granular chunks instead of splintering into jagged shards as annealed glass does. The granular chunks are less likely to cause injury – thus safety glass.

Toughened glass is stronger than normal glass.  The greater contraction of the inner layer during manufacturing induces compressive stresses in the surface of the glass balanced by tensile stresses internally. For glass to be considered toughened, the compressive stress on the surface of the glass should be a minimum of 69 megapascals (10,000 psi). For it to be considered safety glass, the surface compressive stress should exceed 100 megapascals (15,000 psi).

It is the compressive stress that gives the toughened glass increased strength. Any cutting or grinding must be done prior to tempering. Cutting, grinding, and sharp impacts after tempering will cause the glass to fracture.

Toughened glass is normally made from annealed sheet glass via a thermal tempering process. The glass is placed onto a roller table, taking it through a furnace that heats it well above its transition temperature of ca. 540°C (depending on the glass concerned) to around 620°C. The glass is then rapidly cooled with forced air drafts while the inner portion remains free to flow for a short time.

An alternative chemical toughening process involves forcing a surface layer of glass at least 0.1 mm thick into compression by ion exchange of the sodium ions in the glass surface with potassium ions (which are 30% larger), by immersion of the glass into a bath of molten potassium nitrate. Chemical toughening results in increased toughness compared with thermal toughening and can be applied to glass objects of complex shapes. 

This blog entry is largely based on Wikipedia

https://en.wikipedia.org/wiki/Toughened_glass

and other sources.