Heat of Fusion / Melting
Also: Latent Heat of Fusion, Fusion Enthalpy, Latent Melting Heat
The heat of fusion is the amount of energy a substance must absorb at its melting point to transition from the solid to the liquid state without any change in temperature.
The heat of fusion (also known as latent heat of fusion or enthalpy of fusion) describes the energy that must be supplied to a substance at constant pressure and constant temperature — namely at the melting point — to convert it completely from the solid to the liquid state. This energy does not raise the temperature; instead, it breaks the crystal lattice bonds in the solid. The process is endothermic; upon solidification, the same energy is released again as the enthalpy of crystallisation.
Physical Basis
When a metal is heated, its temperature initially rises in a linear fashion. At the melting point, the temperature remains constant despite continued heat input, until the entire substance has melted — only then does the temperature of the melt rise further. The energy supplied is consumed in overcoming the intermolecular binding forces.
The specific heat of fusion $\Delta H_\text{fus}$ is given by:
Q = m · ΔH_fus
where Q is the required heat energy (in J), m is the mass (in kg) and ΔH_fus is the specific heat of fusion (in kJ/kg) of the respective material.
Heats of Fusion of Key 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 have significantly higher heats of fusion both per mole and per kilogram than gold, which explains the increased energy requirements when processing them in a refinery.
Relevance to Precious Metal Processing and Melt Value
The heat of fusion plays a practical role in the purchase, recycling and refining of scrap gold, broken gold or dental gold. The melt value of an object describes the pure material value based on the current gold price or silver price — regardless of form or mintage. Refineries and recycling operations must calculate the thermal energy input required for melting, 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 (solidus to liquidus) rather than 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 required for melting, explains the plateau in the heating curve and forms the basis for the melt value calculator. A higher heat of fusion means greater energy expenditure per kilogram — with a direct impact on processing costs in refining and recycling.