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Noble metals in jewellery

Lessons 3 · 1 lessons · O. A. Qo‘ysinov, O‘. O. Tohirov, D. N. Mamatov, D. F. Aripova. Labour education (Mehnat ta’limi), Grade 9: methodical guide for teachers. Delta-Print, Tashkent, 2017. Note: this is a teacher’s guide, not a student textbook (lesson plans for the “Technology and design”, “Service” and “Basics of agriculture” directions). The course is built from the lesson-plan topics of the “Technology and design” direction; the “Iroqi” skullcap lesson comes from the “Service” direction, and lesson 25 (joining parts) is written from the topic listed in the guide’s calendar plan.
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Precious and noble metals in jewellery

Teacher’s guide: pp. 13–15
GoalKnow the precious and noble metals used in jewellery, their main properties and the meaning of the fineness mark of an alloy.
New words
noble metal · nodir metallalloy · qotishmafineness mark (parts of pure metal per 1000) · probadensity · zichlik
Explanation

Noble metals resist air, moisture and most acids, that is, they are corrosion-resistant; because they are expensive they are also called precious. Jewellery mainly uses gold, silver and platinum, and the platinum group also includes palladium, rhodium, ruthenium, iridium and osmium. Gold is yellow, very soft and ductile, has a density of about 19.32 g/cm³, melts at about 1064 °C and does not oxidise in air; very thin gold leaf, “zarvaraq”, is made from it. Silver is white and conducts heat and electricity best of all metals (density about 10.5 g/cm³, melts at about 961 °C), but it tarnishes under the action of sulfur compounds in the air. Platinum is grey-white, very heavy (density about 21.45 g/cm³), melts at about 1768 °C and is one of the most chemically resistant metals. Pure gold and silver are soft, so items are usually made of alloys; the fineness mark (proba) tells how much pure metal is in 1000 parts of the alloy, for example 585-fineness gold has 585 g of pure gold in 1000 g of alloy. Copper has long been used in crafts: it is reddish, soft, conducts heat and electricity well and turns green in damp air.

Worked examples
The “585” mark on a ring means that of every 1000 parts of the alloy 585 are pure gold and the rest are additions such as copper or silver.
A silver spoon darkens in a cupboard: sulfur compounds in the air form a dark film on the surface. Clean it together with an adult, with a soft cloth and a suitable product.
Class activity

Property cards: the teacher hands out cards with metal names (gold, silver, platinum, copper) and properties (yellow, best heat conductor, very heavy, turns green in damp air). Groups pair the cards and justify the answer. No chemical tests are done.

Practice
1
Why are noble metals called “noble”?
2
Which three metals are mainly used in jewellery?
3
How many grams of pure gold are in a 200 g casting of 750-fineness gold?
4
Why are jewellery items made of alloys and not of pure gold?