Melting Point

The melting point (also melting temperature) describes a fundamental change of state of matter: when a crystalline solid is heated to this characteristic temperature, the ordered lattice collapses and the substance becomes liquid. Throughout the entire melting process the temperature remains constant – the supplied energy flows exclusively into the so-called enthalpy of fusion, not into further warming. This makes the melting point a substance-specific and highly precise characteristic value of central importance in analytics, metallurgy and precious-metal processing.

Physical background

The melting point is the equilibrium point of two phases: solid and liquid coexist as long as heat is supplied. For pure, crystalline substances it is sharply defined; alloys, by contrast, melt over a range (solidus to liquidus temperature), because the components bring different lattice bonds.

Melting process (pure substance):
────────────────────────────────────────────────────
Temperature ▲
           │            ●●●●●●●● (liquid)
           │           ●
Melting    │──────────●────────────── T_m (constant)
point T_m  │        ●
           │      ●● (solid + liquid simultaneously)
           │    ●●
           └──────────────────────────────────► Time
         Heat input →
────────────────────────────────────────────────────

The melting point is pressure-dependent: increased pressure shifts it slightly upwards for most metals. Standard figures always assume standard pressure (1,013.25 hPa).

Melting points of the precious metals compared

The melting points of the most important precious metals and related metals differ considerably and have direct effects on melting, alloying and processing operations:

The low melting point of silver compared with platinum explains why silversmiths managed with simpler furnaces from antiquity, while working the platinum metals only became possible with modern high-temperature furnaces and electric arcs.

Significance for alloys

When two or more metals are melted together, an alloy is formed whose melting point differs from that of the individual components. Particularly important here is the eutectic mixture: the composition at which the melting-point minimum of a multi-component system is reached.

For precious-metal alloys:

• 585 gold (14 carat): contains, besides gold, typically silver and copper; the melting range lies, depending on the alloy, at around 880–950 °C – well below the melting point of pure gold.
• 750 gold (18 carat): melting range usually 880–960 °C.
• White gold with palladium: the palladium addition raises the melting point (up to 1,100–1,200 °C).
• Sterling silver (925): melting point approx. 893 °C (compared with 961.8 °C for fine silver).

This behaviour is practically relevant for goldsmiths, refineries and recycling operations: in the melt-value calculator the material value is determined on the basis of fineness – independent of the specific melting point of the alloy.

Relevance for authenticity testing

The melting point is an indirect security feature: tungsten counterfeits of gold bars are, in theory, exposed by the high melting point of tungsten (3,422 °C), since tungsten remains solid at gold-melting temperature. In practice, however, this is more readily proven by density measurement, X-ray fluorescence analysis or ultrasonic testing, since an actual melt test would destroy the bar.

For everyday checks, non-destructive methods are recommended; the coin weight checker helps to verify coins by weight and dimensions.

Melting point and recycling

In precious-metal refining and in the recycling of broken gold, dental gold and old jewellery, the melting point plays an important role in process planning. Higher melting temperatures mean:

• Greater energy input during melting
• Special alloys needed for crucibles and moulds (e.g. graphite, ceramic)
• Longer cooling times

Gold and silver can be recycled particularly economically owing to their comparatively low melting points – a factor that favours the recycling of precious metals compared with industrial metals with very high melting points.

In brief

The melting point is one of the most precise physical characteristics of a metal: for precious metals it lies between around 962 °C (silver) and almost 2,000 °C (rhodium), stays exactly constant for pure metals, and for alloys becomes a melting range. For practice in the goldsmith's workshop, the refinery and precious-metal testing, it is indispensable basic knowledge – from the melt-value calculator through alloy planning to counterfeit detection.