Melting Point

The melting point (or melting temperature) describes a fundamental change of state in matter: when a crystalline solid is heated to this characteristic temperature, its ordered lattice collapses and the substance turns liquid. Throughout the entire melting process the temperature stays constant — the energy supplied flows solely into the so-called heat of fusion, not into any further warming. This makes the melting point a substance-specific and highly precise property that is central to analytics, metallurgy and precious-metal processing.

The 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 (from solidus to liquidus temperature), because their components bring different lattice bonds to the mix.

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

The melting point depends on pressure: for most metals, higher pressure raises it slightly. Standard figures always assume normal pressure (1,013.25 hPa).

Melting points of the precious metals compared

The melting points of the main precious metals and related metals differ considerably and have a direct impact on melting, alloying and processing:

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

Significance for alloys

When two or more metals are melted together, an alloy forms whose melting point differs from that of the individual components. Particularly important 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 karat): besides gold it typically contains silver and copper; depending on the alloy the melting range lies at roughly 880–950 °C — well below the melting point of pure gold.
• 750 gold (18 karat): 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 about 893 °C (against 961.8 °C for fine silver).

This behaviour is of practical importance to goldsmiths, refineries and recycling plants: in the melt value calculator the material value is worked out on the basis of fineness — independently of the specific melting point of the alloy.

Relevance to authenticity testing

The melting point is an indirect security feature: tungsten forgeries of gold bars are, in theory, exposed by tungsten's high melting point (3,422 °C), since tungsten stays solid at gold's melting temperature. In practice, though, this is detected instead by density testing, X-ray fluorescence or ultrasound testing, because an actual melt test would destroy the bar.

For everyday checks, non-destructive methods are recommended; the coin weight checker helps to verify coins for authenticity from their 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 matters greatly for process planning. Higher melting temperatures mean:

• Higher energy input during melt-down
• Special alloys needed for crucibles and moulds (e.g. graphite, ceramic)
• Longer cooling times

Gold and silver can be recycled especially economically thanks to their comparatively low melting points — a factor that favours precious-metal recycling over 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 ranges from around 962 °C (silver) to almost 2,000 °C (rhodium), stays exactly constant for pure metals and becomes a melting range for alloys. For practical work in goldsmithing, refining and precious-metal testing it is indispensable background knowledge — from the melt value calculator through alloy planning to forgery detection.