Calculating gold melt loss — yield & merma guide

At a glanceCalculating gold melt loss (known in workshop parlance as merma or Schmelzverluste) is an indispensable discipline for bench jewelers, goldsmiths, and casting foundries. Every melting operation results in a measurable mass deficit between the cold input scrap and the solidified poured ingot. This technical guide examines metallurgical causes, typical loss rates by alloy, and formulas for calculating casting yield.

Metallurgical Origins of Melt Loss

Pure gold boils at 2,856 °C and does not vaporize significantly at workshop casting temperatures (1,050 to 1,150 °C). Weight loss stems from four specific chemical and physical mechanisms:

• Volatilization of low-boiling alloying metals: In commercial karat gold (18k, 14k, 9k) and antique solders, zinc (boils at 907 °C) and cadmium have high vapor pressures and readily gasify under a direct flame.
• Oxidation and slag formation of base metals: Copper reacts with atmospheric oxygen to form copper oxides ($Cu_2O$, $CuO$), which dissolve into the borax flux and are skimmed off as glassy slag.
• Combustion of organic residues: Polishing compounds (rouge, tripoli), bench sweepings, skin oils, and gypsum residues from casting flasks burn off completely during heating.
• Mechanical trapping in crucible walls: Microscopic metal beads adhere to the porous interior of used clay-graphite or ceramic crucibles.

Typical Melt Loss Benchmarks by Alloy

The following table summarizes average melt loss percentages observed in professional Swiss jewelry workshops:

Formulas for Melt Loss and Casting Yield

To track metal inventory in your workshop log:

Melt Loss Rate (%) = [ (Initial Gross Weight − Poured Ingot Weight) ÷ Initial Gross Weight ] × 100

Casting Yield (%) = (Poured Ingot Weight ÷ Initial Gross Weight) × 100

Workshop Example: A goldsmith melts 280.00 grams of clean 18k yellow sprues using an oxy-propane torch. After solidification, pickling, and brushing, the ingot weighs 277.48 grams:

• Absolute weight loss: 280.00 g − 277.48 g = 2.52 g.
• Loss rate: (2.52 ÷ 280.00) × 100 = 0.90% (normal benchmark for 750 gold).
• Casting yield: (277.48 ÷ 280.00) × 100 = 99.10%.

Best Practices to Minimize Workshop Melt Losses

1. Pre-cleaning scrap: Ultrasonic clean and pickle bench scrap before weighing and melting to remove polishing pastes.

2. Proper crucible glazing: Coat new graphite crucibles with borax before use to seal micropores.

3. Reducing flame control: Maintain a soft, slightly gas-rich reducing flame. Excessive oxygen rapidly burns out alloyed copper.

4. Sweepings and slag refining: Never discard spent crucibles or borax slags. Annual processing of workshop sweeps (*Kehricht*) at a Swiss refiner frequently recovers ounces of pure gold.

Frequently Asked Questions (FAQ): Melt Loss

Is the lost weight pure gold?

No. The vast majority of the weight loss represents oxidized base metals (copper, zinc) and burnt organic residues. The actual loss of fine gold is typically under 0.1%.

Why does white gold have higher melt losses than yellow gold?

White gold requires higher casting temperatures (1,050–1,100 °C vs. 900 °C for yellow gold), accelerating base metal volatilization and slag formation.