A glass-ceramic is a glass that has been deliberately crystallised by a two-stage heat treatment, first to form a dense cloud of tiny nuclei and then to grow crystals from them, so the finished material is mostly crystalline with a little glass between. The crystals can be chosen to expand very little when heated, which gives the material great thermal-shock resistance. A rebuilding settlement can use borosilicate glass and recognise unwanted crystallisation, but a true glass-ceramic needs a lithium-bearing batch and exact temperature schedules that cannot be sourced here.
Part of Advanced Ceramics
Glass-ceramics are materials made by melting and forming a glass, then heating it so that a controlled fraction of it crystallises into fine interlocking grains.
Controlled Crystals Replace Uncontrolled Stress
Ordinary glass breaks in a sudden heat change because the surface and the interior expand by different amounts (see Thermal Properties and Annealing). A glass-ceramic reduces the problem in two ways:
- The crystals themselves can have very low or even slightly negative expansion, which cancels the expansion of the glass matrix.
- Millions of tiny crystals stop cracks, because a crack must change direction at each grain boundary (see Grain Boundaries), so strength and toughness rise.
The usual route is: melt, form the shape while still glass, cool to make a normal glass, then reheat to a nucleation temperature to seed crystals, then to a higher temperature to grow them. A small nucleating additive, standard examples being titanium or zirconium oxide, is needed to seed them, and the commercial low-expansion materials are lithium aluminosilicates.
The Version A Settlement Already Meets Is An Accident
Annealing warns that glass held too long at elevated temperature can crystallise at the surface, becoming cloudy, rough and opaque. Flux Materials lists the same problem, called devitrification, when there is too little flux or cooling is too slow. This is uncontrolled crystallisation: the crystals are large, unevenly distributed and often weaken the piece. It is the opposite of a glass-ceramic, but it shows the mechanism is real and why control is the entire point.
What Is Practical And What Is Not
- Use borosilicate glass. Glass Types for Different Uses gives borosilicate as the glass for flasks and beakers that will be heated, and Glass Properties for Blowing puts its working furnace need at 1,100-1,250 °C.
- Avoid devitrification in ordinary glass by correct flux and by not lingering in the crystallising range.
- Do not attempt a true glass-ceramic yet. It needs a lithium source, a nucleating agent and nucleation and growth temperatures held to a few degrees. Without a source for those temperatures and times, this article writes no schedule.
- Test, if you try. A crude crystallised glass can be assessed by plunging a heated sample into cold water. That is a harsh test and a poor one, and it should be done behind a shield.
Common Mistakes
- Mistaking cloudy, rough glass for a good ceramic — it is probably devitrified and weaker.
- Assuming thermal-shock-proof means indestructible — a glass-ceramic is still brittle.
- Overheating the shape — a glass-ceramic held too hot can slump or crack as crystals grow.
Thermal-shock tests can throw shards
Never test a glass sample by dropping it hot into water without a shield and eye protection. Fragments travel.
What’s Next
- Learn next: Composite Materials — combining materials in another way
- Combine with: Glass Types for Different Uses — the practical thermal-shock glass