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Amorphous Materials: Oxide Glasses and Glass-Ceramics

An amorphous solid has no repeating pattern. Oxide glass is a random net of corner-sharing SiO₄ units (network former). Soda (Na₂O) and lime (CaO) are modifiers: they break links and lower the softening temperature. Glass has no sharp melting point; it softens slowly through the glass transition. A glass-ceramic is glass heated in a controlled way so that millions of tiny crystals grow inside it, giving strength and low thermal expansion.

🎬 Step-by-step story

  1. This is silica as a crystal. Grey dots are silicon, blue lines are Si–O–Si links. The hexagon pattern repeats everywhere.
  2. Now it becomes glass. The same dots and links, but the pattern is bent out of shape. Rings have different sizes. No repeating pattern. This is amorphous.
  3. Add soda (Na₂O). The yellow Na⁺ ions break links in the net. A broken net is looser, so the glass softens at a lower temperature and is easier to shape.
  4. Heat a crystal bar (green) and a glass bar (red). The crystal stays stiff and then melts at one sharp point. The glass droops slowly as it gets hotter.
  5. Glass-ceramic: reheat the glass in a controlled way. Tiny crystal grains (orange) start from seeds and grow inside the glass. The material becomes tougher and handles heat shocks.
  6. Free play. Use the soda slider and the heat-treatment slider to see how the network changes.

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

🤔 Common doubts, cleared

What exactly is repeating in a crystal that glass does not have?

The same unit (a hexagon here) repeats in order across the whole crystal. Watch the pattern at step 0 then at step 1.

If glass has the same atoms as a crystal, why is it different?

Only the arrangement differs: same dots and links, but the rings differ in size and the pattern is bent. This is amorphous.

Why do glass makers add soda?

Soda breaks links in the network. A weaker net softens at a lower temperature, so less fuel is needed and the glass is easier to shape.

Why does glass not melt at one temperature?

Its bonds differ from place to place, so weak places soften first and strong ones later. The red bar droops slowly while the green crystal bar melts suddenly.

How can heating glass make it crystal?

If glass is held at the right temperature, atoms get just enough freedom to arrange. Seeds form and crystals grow (orange). This is how glass-ceramics are made.

What happens if I add a lot of soda?

Too much soda breaks too many links; the glass would dissolve in water. Lime is added to keep it durable. Try the slider in free play.

Oxide glasses: how the network is built

A glass is an amorphous solid: its atoms have no repeating pattern. In oxide glasses the basic net is made of corner-sharing tetrahedra, such as SiO₄. Oxides play three roles:

Pure silica glass is very heat-proof but needs about 1700 °C or more to shape. Soda-lime glass (about 70% SiO₂, 15% Na₂O, 10% CaO) is cheap and easy to shape: windows and bottles. Soda alone would make glass that dissolves in water, so lime is added to make it durable. Borosilicate glass (silica with about 13% B₂O₃) expands very little when heated, so it is used for lab glassware and ovenware. Lead glass bends light strongly and shines (crystal glass). Small amounts of metal oxides give colour: cobalt blue, iron green, chromium green.

Making glass: melt the mix at about 1500 °C, shape it while soft, then anneal (cool slowly) to remove stress.

Glass transition: no sharp melting point

A crystal has one melting point. Glass does not. When heated, glass becomes softer and softer over a range. Below the glass transition temperature (Tg) it is a hard, brittle solid; above Tg it becomes a thick, sticky liquid; then it flows more easily as it gets hotter. This slow change is why glass blowers can shape it at leisure.

Why? In a crystal, all bonds are the same, so all break together. In glass the links differ from place to place, so weaker places soften first. Adding modifiers like Na₂O breaks more links and lowers Tg. This is also why glass has the same properties in all directions and breaks with curved surfaces.

Glass-ceramics

A glass-ceramic starts as ordinary glass and ends up part-crystal. First the glass is made and shaped easily. Then it is given a controlled heat treatment in two steps: a lower temperature where tiny seeds (nuclei) form, helped by nucleating agents such as TiO₂ or ZrO₂; and a higher temperature where crystals grow from the seeds. In the end 50% to 95% of it is crystal, with millions of crystals smaller than a thousandth of a millimetre, and a glassy skin between them.

Compared with glass: stronger and tougher (a crack has to wind its way round crystals), and much better at withstanding sudden heating (thermal shock). Lithium aluminosilicate glass-ceramics hardly expand at all when heated. If the crystals are very small, the material can even stay transparent.

Uses: cooktop panels, cookware, telescope mirror blanks, dental ceramics, and fire-door windows.

Key formulas and definitions

Worked examples

1. A glass is 72% SiO₂, 14% Na₂O, 9% CaO and the rest other oxides. What percentage is "other"? Which are network modifiers?

72 + 14 + 9 = 95, so other = 5%. Na₂O and CaO are the modifiers; SiO₂ is the former.

2. Why is a lab beaker made from borosilicate and not soda-lime glass?

Borosilicate expands very little when heated, so sudden hot water does not set up large stress. Soda-lime glass expands more and can crack.

3. Why is soda added to silica when making window glass?

Soda breaks links in the network and lowers the softening temperature, from about 1700 °C for pure silica to about 1500 °C or less for soda-lime glass. This saves energy and makes shaping easier.

4. How does a glass-ceramic become tougher than glass?

It contains many tiny crystals. A crack that starts in the glass runs into a crystal and must bend or stop, so it needs more energy to break.

Common mistakes

Practice quiz

1. Which is a network former?
2. Adding soda to silica glass:
3. A glass-ceramic has:
4. Lab glassware is mostly:
5. Glass differs from a crystal because it:

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

What is the difference between crystalline and amorphous solids?

Crystalline solids have atoms in a repeating pattern and a sharp melting point. Amorphous solids, like glass, have no repeating pattern and soften slowly.

Is glass a solid or a liquid?

A solid. Its atoms are frozen in a random arrangement. It does not flow noticeably even in old windows.

What is a glass-ceramic used for?

Cooktop panels, heat-proof cookware, telescope mirror blanks, dental restorations and oven windows, because it is tough and hardly expands when heated.

Where this is taught

Japan高校（専門学科）1〜3年Ceramic Chemistry

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