Three thermal properties decide how a material behaves in a fire or a furnace: its melting point, how well it conducts heat, and how much it expands when heated. A steel member 10 m long that warms by 40 °C grows about 4.8 mm, and a material that is heated unevenly or joined to one that expands differently cracks long before it reaches its melting point. Metals conduct heat well and ceramics poorly, which is why ceramics are good insulators but are vulnerable to thermal shock.
Part of Materials Science Principles
Thermal properties describe how a material responds to heat: the temperature at which it melts or softens, the rate at which heat flows through it, and the change in size it undergoes when its temperature changes.
Melting Point Reflects How Strongly The Atoms Are Bonded
The temperatures below come from articles elsewhere in the corpus and the Advanced Materials article. They span a wide range, and that spread decides what can hold what.
| Material | Melts at | Source in this corpus |
|---|---|---|
| Tin | 232 °C | Bronze Making |
| Aluminium | 660 °C | Advanced Materials article, Common Mistakes table |
| Copper | 1,085 °C | Copper Properties |
| Iron | 1,538 °C | Crucibles |
| Alumina | 2,072 °C | Aluminum Smelting |
Pure metals melt at a single temperature. Alloys melt over a range from the solidus to the liquidus, and that range is what Phase Diagrams draws. Glass has no melting point at all and softens gradually: Glass Properties for Blowing puts the soda-lime softening point at about 700 °C.
Do not trust colour for aluminium. The Advanced Materials article warns that it melts at 660 °C with no visible colour change, so heat treatment of aluminium needs a measured temperature.
Conductivity Depends On Free Electrons
Metals carry heat mostly by free electrons, which is why a good electrical conductor is usually a good thermal one. Aluminum Smelting gives aluminium 61% of copper’s conductivity at about 30% of the weight. Alloying lowers conductivity sharply because foreign atoms scatter the electrons, so bronze and austenitic stainless steel conduct heat far worse than pure copper or aluminium. Ceramics and glass have no free electrons and conduct heat mainly by atomic vibration, so they insulate.
One practical result is a hot spot. A pan with a thin stainless wall heats unevenly because heat cannot spread sideways, which is why cookware pairs it with a copper or aluminium base.
Expansion Is Proportional To Length And Temperature Rise
A bar changes length by ΔL = α × L × ΔT, where α is the expansion coefficient of the material.
Worked example: take α for steel as 12 × 10⁻⁶ per °C. A rail 10 m long is 10,000 mm. Heated by 40 °C it grows by 12 × 10⁻⁶ × 10,000 mm × 40 = 4.8 mm. Aluminium expands about twice as much as steel, so the same bar in aluminium would grow roughly 9.6 mm.
Expansion works for you and against you:
- For you. Tire Shrinking fits an iron tire on a wheel by heating it until it is slightly too large, then letting it contract tight.
- Against you. A joint between two metals with different coefficients stresses the weaker one every time the temperature changes. A glazed pot cracks when body and glaze shrink by different amounts as they cool.
Thermal Shock Is Uneven Expansion Inside One Part
When a ceramic or glass part is heated or cooled quickly, the surface and the interior sit at different temperatures, and so want to be different sizes. The outer layer pulls on the inner one, and because ceramics are weak in tension they crack. Annealing describes the same mechanism for glass: the surface cools and contracts first, leaving the interior in tension. The fixes are the same everywhere: heat and cool slowly, make walls uniform in thickness, avoid sharp internal corners, and use a material with low expansion. Glass Types for Different Uses gives borosilicate as the thermal-shock-resistant glass, and Glass-Ceramics pushes the idea further.
Common Mistakes
- Assuming a high melting point means thermal-shock resistance — alumina melts at 2,072 °C and can still crack in a cold draught if it is heated unevenly.
- Ignoring expansion in long runs — rails, pipes and bridge decks need gaps or bends.
- Joining dissimilar materials rigidly — allow slip or use a flexible joint.
What’s Next
- Learn next: Refractory Ceramics — materials chosen for melting point and shock resistance
- Combine with: Failure Modes — what repeated heating does to a part