Heat of Fusion / Melting
Also: Enthalpy of Fusion, Melting Enthalpy, Latent Heat of Fusion
The heat of fusion is the amount of energy a substance must absorb to change from solid to liquid at its melting point, without any change in temperature.
The heat of fusion (also called the melting enthalpy or latent heat of fusion) describes the energy that must be supplied to a substance to convert it fully from solid to liquid at constant pressure and constant temperature — namely at the melting point. This energy does not raise the temperature; instead it breaks apart the crystal-lattice bonds within the solid. The process is endothermic; on solidifying, the same energy is released again as the enthalpy of crystallisation.
The physical basis
As a metal is heated, its temperature at first rises in a straight line. At the melting point it stays constant despite continued heating, until the entire substance has melted — only then does the temperature of the melt rise further. The energy supplied goes into overcoming the intermolecular bonding forces.
The specific heat of fusion $\Delta H_\text{fus}$ follows from:
Q = m · ΔH_fus
where Q is the heat required (in J), m is the mass (in kg) and ΔH_fus is the specific heat of fusion (in kJ/kg) of the material in question.
Heats of fusion of major precious metals compared
| Metal | Melting point | Heat of fusion (kJ/mol) | Heat of fusion (kJ/kg) |
|---|---|---|---|
| Gold | 1,064 °C | 12.55 | 63.7 |
| Silver | 961 °C | 11.28 | 104.6 |
| Platinum | 1,768 °C | 22.17 | 113.6 |
| Palladium | 1,555 °C | 16.74 | 157.3 |
| Copper | 1,085 °C | 13.05 | 205.4 |
Platinum and palladium show markedly higher heats of fusion than gold both per mole and per kilogram, which explains the greater energy input needed when they are processed at a refinery.
Relevance to precious-metal processing and melt value
For the buying, recycling and refining of scrap gold, broken gold or dental gold, the heat of fusion plays a practical role: the melt value of an object describes its pure material value against the current gold price or silver price — regardless of shape or minting. Refineries and recycling plants must budget for the thermal energy needed to melt down the material, which depends directly on the heat of fusion of the alloy mixture.
The fineness of an alloy also affects its effective heat of fusion: alloys such as 585 gold (14 karat) melt over a temperature range (from solidus to liquidus), not at a single defined point like the pure metal fine gold (999).
In brief
The heat of fusion is a key quantity in precious-metal physics: it determines the energy needed to melt a metal down, explains the plateau in the heating curve, and underpins the melt value calculator. A higher heat of fusion means more energy per kilogram — with a direct bearing on processing costs at the refinery and in recycling.