Reverse alloy

Reverse alloy calculation starts from a target purity (e.g. 750‰ for 18 ct gold) and determines the exact quantities of fine metal and master alloy components (copper, silver, palladium, zinc) to mix to achieve the desired final weight. This is the daily calculation for every foundry and jewellery workshop in Switzerland.

The base formula: Fine metal quantity = Final weight × Target purity. Master alloy quantity = Final weight − Fine metal quantity. For a typical 750‰ yellow gold alloy, the master alloy is composed of 12.5% silver and 12.5% copper. For white gold, palladium or nickel replaces copper. For rose gold, copper dominates (up to 20%).

Master alloys aren't always pure: a workshop may receive a gold lot at 500‰ and want to raise it to 750‰. In this case, the quantity of pure fine metal (999‰) to add must be calculated. The reverse calculation accounts for the starting purity and the target purity, optimizing consumption of fine metal — the most expensive component.