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General Information

General Information

Objective: To help to develop an understanding of the metals used in jewellery and their properties in order that we might sell them more effectively.
Relevant Terminology: MALLEABILITY:
The ability of a metal to be hammered and shaped without breaking or being damaged.
DUCTILITY:
The ability of a metal to be drawn into wire.
REFLECTION:
The ability of a metal to return light rays.
Metals used in Jewellery: Gold, Copper, Zinc, Aluminium, Silver, Platinum, Rhodium, Palladium & Iron.
Gold:
  • Lustrous yellow
  • Most malleable of all metals
  • Most ductile of all metals - 1oz. can be drawn into a wire 50 miles long.
  • Does not form any oxides
  • Unaffected by common acids and alkali exc. - mercury
Silver:
  • Lustrous white metal
  • Most reflective of all metals - up to 98% of all rays returned
  • Highly malleable.
  • Highly ductile
  • Will not oxidise at room temperature
  • Forms silver sulphate in industrial atmospheres and turns black
Palladium:
  • Greyish metallic lustre
  • Highly malleable and ductile
  • Does not oxidise
  • Has by and large replaced platinum due to its similar physical properties.
  • It is less expensive and has a lower melting point.
Platinum:
  • Greyish white metallic lustre
  • Highly malleable and ductile
  • Extraordinarily resistant to corrosion and chemical attack
Rhodium:
  • Widely used in the plating of white gold and silver
  • Highly reflective with brilliant white lustre
  • Becomes eight times harder when electrodeposited on metal
  • Impervious to all acids and forms of corrosion
N.B. Gold and silver are too soft to use in their pure form and are therefore alloyed with other metals to give them strength. Carat refers to the units of gold to alloy - 24ct refers to pure gold, 18ct to 18 parts gold, 6 parts alloy etc. European system uses 1000 as pure gold, 750 as 18ct etc. Alloys not only strengthen gold, but also alter its colour. See chart attached.
Rolled or Gold Filled: Usually stamped 1/10 or 1/20th 9ct gold meaning the proportion of 9ct gold to total metal content by weight. Rolled gold is produced by sandwiching a layer of base metal, usually brass, between two layers of gold. The gold is fused to the base metal by heat and pressure.
Hard Gold Plate: Has replaced rolled gold largely because it allows the manufacture of more intricate pieces in gold.
Au - Gold
Ag - Silver
Cu - Copper
Zn - Zinc		 Al - Aluminium
Fe - Iron
Ni - Nickel
Yellow Gold Alloys
ct. Value% of Metals in Alloy
24ct (1000)100% Au
22ct (916.6)91.7% Au4.1% Ag4.2% Cu
18ct (750)75% Au12% Ag13% Cu
15ct (625)62.5% Au17.5% Ag10.5% Cu9.5% Zn
14ct (585)58.5% Au15% Ag26.7% Cu
9ct (375)37.5% Au11% Ag51.5% Cu

18ct Alloys
Colour% of Metals in Alloy
White75% Au16.5% Ni5% Zn3.5% Cu
Green75% Au25% Ag
Pale Yellow75% Au16% Ag9% Cu
Normal Yellow75% Au25% Ag25% Cu
Rose75% Au4% Ag21% Cu
Deep Red75% Au25% Cu
Blue75% Au25% Fe

9ct Alloys
Colour% of Metals in Alloy
White75% Au16.5% Ni5% Zn3.5% Cu
Green75% Au25% Ag
Pale Yellow75% Au16% Ag9% Cu
Normal Yellow75% Au25% Ag25% Cu
Rose75% Au4% Ag21% Cu
Deep Red75% Au25% Cu
Blue75% Au25% Fe

The Mystery of Skin or Metal Discolouration
Gold and silver are metals which normally neither tarnish nor corrode, however, there are seven reasons why, very occasionally, gold jewellery blackens skin and skin tarnishes gold and silver jewellery.
Case 1:Skin secretions and perspiration contain amino acids and sometimes sulphides. These chemicals combine with the molecules of silver and copper in gold alloy and may stain, particularly during pregnancy.
Case 2:The outside chemical influences of living near the sea may combine with normal skin secretions to form corrosive chemical.
Case 3:Sulphur dioxide, phosphates and other air pollutants of smog laden society can also attack gold alloys, even when not being worn. The tarnished jewellery then leaves a black smudge when worn.
Case 4:Tiny particles of dust or powder which become embedded in a person's skin act as 'sandpaper' on gold jewellery. Because each of the eroded specks is smaller than the wave length of light, they have no golden sparkle. Instead, they appear as an ugly black-violet smudge.
This is called 'black dermographism' or 'crocking', but is very rare.
Case 5:An actual allergy to gold or other metals in the alloys, particularly nickel, can lead to contact dermatitis.
It affects either the skin itself, as a sort of eczema or the hair follicles. If this occurs a doctor should be consulted because the change of any hormone penicillin or other special scripts may contribute.
Case 6:Solvent based perfume and jewellery cleaners are known to cause skin or metal discolouration.
We advise jewellery cleaners to be purchased from jewellery stores.
Case 7:Where in the case of a neck chain turning a blouse or shirt black and not turning the skin around the skin black, is due to the garment being dry-cleaned in a solvent base or the detergent in which the garment been washed contains some form of sulphate or ammonia.
Solutions:In Cases 1, 2 and 3 a change to white gold or to gold of higher carat fineness should solve the problem.
In Case 4, careful and frequent scrubbing of the affected skin should provide relief. If not, a harder gold alloy may alleviate the condition.
In Case 5, a doctor can confirm the allergy in which case a logical move would be platinum. This is called 'black dermographism' or 'crocking', but is very rare.
Finally it must be remembered that all these cases are not very common and 99.9% of people can wear gold and silver jewellery without any problems.
If this problem still occurs, it is recommended to change from 9 carat to 18 carat.