Materials Science and Precious Metals
What is examined
Materials Science and Precious Metals tests whether you can choose a metal or alloy for a job and predict how it behaves under the torch, the rolling mill and the polishing lathe. Expect questions on fineness, colour, density, melting range, hardness, work hardening and the difference between a parent metal and its solder.
Core rules
- Gold fineness is parts per thousand: 24 ct = 999, 22 ct = 916, 18 ct = 750, 14 ct = 585, 9 ct = 375. Carat for gold is 1/24 by mass; gemstone carat is 0.200 g and is a different unit.
- Sterling silver is 925; Britannia silver 958; fine silver 999. Platinum jewellery is usually 950, alloyed with iridium, ruthenium or cobalt; it is denser and much higher melting than silver or gold.
- Pure metals are soft and ductile. Alloying and cold work raise hardness and strength but lower ductility. Annealing restores ductility by recrystallisation; overheating causes grain growth and a coarse, orange-peel surface.
- Densities (g/cm³, approximate): fine gold 19.3, fine silver 10.5, copper 8.96, platinum 21.45. Density helps with identification but cannot distinguish two alloys of similar density.
- Melting points: fine silver about 961 °C, fine gold 1064 °C, copper 1085 °C, platinum about 1768 °C. An alloy usually melts over a range, and its solidus is below the pure metal's melting point.
- White gold is not platinum. Nickel-white gold can trigger nickel allergy; palladium-white gold and rhodium plating are common alternatives. Silver tarnishes through sulphur compounds; gold is resistant but its alloying metals can tone.
Worked example
An 18 ct yellow gold ring weighs 4.80 g. Fine gold content = 18/24 = 0.750; 4.80 g × 0.750 = 3.60 g. The article should be marked 750 (or 18 ct where carat marking is permitted). If the same design were made in 14 ct, gold content = 4.80 × 0.585 = 2.81 g, and the remainder - mainly silver, copper and zinc - is deliberate alloy, not dirt or a casting fault. The lower carat alloy is generally harder and less dense, so the same ring may feel lighter and wear differently.
Traps
Treating 18 ct as 18% gold; confusing gold carat with gemstone carat; assuming all white metals are platinum; quoting a single melting point for an alloy; forgetting that solder melts below the parent metal; using density alone to 'prove' a metal; ignoring nickel allergy and hallmarking rules.