Gold: Alloys And Colors
An In Depth Explanation
Digging for Gold
Posted by Mycroft on January 03, 1998 at 23:41:33:
Gold is one of the most commonly used precious metals in watchmaking, specifically in cases. Of late, there have been some sporadic posts regarding white gold and of course, titanium.
This post stems from my own specific curiosity which has led to me researching the metal, digging for gold as it were in more ways than one. So, for those who are interested, here is some trivia for you.
Gold has since the dawn of time and its discovery, been treasured for its wonderfully rich colour, its weight and relative rarity. In its original form, it is a beautiful warm yellow colour that has led to its association with the sun, the Sun God, with warmth and well-being.
The word has 2 meanings: referred to diamonds and gems, it is a weight equal to 1/5 of a gram.
As a measure of gold finess, 1 carat is 1/24 part; Fine gold is 24kt, 18kt is 18/24 or * pure (75% gold).
Carat golds are alloys of gold with other metals. There are different standards throughout the world. 18kt is 1 of the most universal. The other commonly found ones are: 9, 14, 18, 22kt. Gold is very soft unless alloyed into carat gold. The alloys are often used to vary the properties and colour of gold.
PROPERTIES & CHARACTERISTICS:
Gold (Au) has a specific gravity of 19.32 (silver 10.5, platinum 21.45), Atomic weight 197.2.
Most pure metals are grey or silvery in colour except for gold and copper. When carat gold contains large amounts of copper, compared to other alloying metals (like silver & zinc), a red or rose gold is produced. Many different rose golds have been alloyed and it is difficult to match an old rose gold with modern alloys.
18 kt red gold is a subtle warm rosy colour. Yellow gold usually contains gold, silver, zinc and other metals to enhance hardness. Alloyed with silver (Electrum), it is very soft and is greenish-gold in colour. 22kt gold is a very rich yellow colour and is soft.
White gold is usually from gold, silver and nickel (see table) and sometimes palladium.
Pure gold is a rich yellow colour and does not tarnish. There is some tendency (not often though) for carat golds to discolour.
Sulphides in the atmosphere, sweat from skin, perfumes, household chemicals can all contribute to this, although only the surface is affected and can be polished away.
W/gold is often more susceptible and can be yellowed by bleach. Antibiotics can alter the properties of sweat and promote discoloration.
COMPOSITIONS OF METAL IN THE COMMON NAMES OF ALLOYS FOR WATCH CASES.
| ALLOY NAME
| BLUE GOLD 18K
| YELLOW GOLD 14K
| YELLOW GOLD 18K
| YELLOW GOLD 22K
| GREEN GOLD 18K
| RED GOLD 18K
| STERLING SILVER
| WHITE GOLD-1
| WHITE GOLD-2
| WHITE GOLD - 14Kt A
| WHITE GOLD - 14Kt B
| WHITE GOLD - 18K
(FROM: Engineering Alloys, Fifth Edition, Edition by Woldman & Gibbons)
Pure metals have very precise melting points - silver 961C, gold 1063C, platinum 1769C. Alloys however, being mixtures melt over a range of temperature. 18kt yellow gold melts typically at 895-930C.
Metals have a crystalline structure. When a metal is strained and distorted mechanically, the crystals become stressed and the metal hardens. To relieve this stress, and "relax" the structure, they must be heated to a temperature somewhat below the melting point. This is called annealing. Gold alloys must be cooled slowly after annealing.
Pure gold is very soft (Mohs hardness 2.5-3). It is commonly wrongly belived that the purer the the alloy of gold, the softer it is. In fact, there are several different types of hardness.
Ductility refers to the flow characteristics of a metal under pressure, undergoing plastic deformation in stretching, bending etc. Maleability is how the metal deforms uncer impact from a hammer or punch.
After annealing, gold is generally quite ductile (soft). Fine golds are softer than carat golds.
Deforming metals by pressure or impact causes them to harden, at differing rates dependent on the alloy. As it hardens, it becomes less deformable, and more brittle.
Most alloys containing copper will age-harden. This may be apparent over months to years.
The wear resistance is related to its surface hardness and age as well as to the actual metal. For eg, the annealed hardness of 9 & 18 kt golds are similar. 18, 14, 9 kt golds are considerably harder than 22 or 24 kt golds. Annealing softens them, and working (bending, twisting) tends to work harden metals (this explains why forged steel is much stronger than cast steel for the same alloy content).
These are most commonly caused by allergies to nickel; if so, w/gold which contains nickel, should be avoided. In addition, tight fitting, poorly ventilated bracelets worn in humid climates are invitations to fungal infections.
SOME OTHER TRIVIA:
121,000 tons of gold have been mined so far (only 15% of this is missing). 43% of known gold lies in Central Banks. Over 80% of gold produced in a year goes to jewellery and (less) watches.
I hope that this article has provided a cure for insomniacs, and some salient information for other gold diggers of minutiae.
I have to go now, my wife is on my back - she can't sleep while I'm pecking on the computer. Goodnight all!