Heliodor

Gold and platinum are from the few metals that can be found in their pure state, uncombined with other elements. Gold is typically found in veins or placers as rounded nuggets or as fine specks of dust. While 40% of the world’s gold production comes from South Africa, gold can be found almost all over the world.

Gold can be refined from its impurities by electrolysis. Such gold is called five-niner because it is 99.999% pure. Five-niner is too soft to be used, so normally it is alloyed with other metals to become harder. Since ancient times gold’s purity is measured in karats. Thus 99.999% (or 100%) rates 24 karats. Going down the scale we have gold of 23 karats (95.833%), 22 karats (91.666%), 18 karats (75%) and 14 karats (58.333%). Lower than 12 karats (which is 50% pure) is not considered gold, but an alloy of metals including gold. In modern times it is very commonly used the scale of 1000, by itself or in combination with the karat scale. Thus we have 999/24K, 960/23K, 916/22K, 750/18K and 585/14K.

Gold alloys are always calculated by weight. For example 18-karat gold must contain at least 18 parts gold out of 24 parts total (75%). The proportions by volume can show a startling difference.

Gold in its pure form has a rich yellow colour. The most common alloys of gold include silver and copper. Adding silver in the alloy makes the colour lighter, and adding copper makes it reddish, so changing the percent of silver and copper, gold’s colour can vary from pale green, to greenish yellow, pale yellow, yellow, deep yellow, pink, reddish pink and red.

Alloying gold also means that some of gold’s properties, such as hardness, strength and malleability, will change. In fact, as the karats lower, strength and hardness increase. But malleability, the melting range and alloy density decrease, so other metals are used in addition in the alloy in minute quantities. Some of them are zinc, silicon, iridium and cobalt.

White gold comes as a result of alloying gold with “white metals” such as palladium and nickel. Since 2000 the EU legislation prohibits the use of nickel, but it can still be found in gold alloys produced earlier.

Additions of about 10-12% palladium to gold impart a good white colour (“white” is in fact a bright silver-grey colour). But, as palladium is more expensive, many commercial palladium white gold only contains about 6-8% palladium plus silver, zinc and copper. This doesn’t have such a good white colour and is almost always rhodium plated.

White gold is also denser and so such jewellery will be heavier and also contain more gold. It is also more difficult to process as the melting temperatures are substantially higher. White gold for jewellery was developed in the 1920's as a substitute for platinum. It is very often used to enhance diamonds and other gemstones.

Special techniques, such as formations of intermetallic compounds, applying a surface coating or patination, can give gold colours like blue, purple, brown, grey or black.

Gold is widely used by science and industry, particularly in the electrical and electronic fields. It is a better conductor than any other metal except silver and copper and because it will not tarnish or oxidize, it is prized for a number of applications in which its special wear-resistance and durability are worth the price.