Pure gold is used (rarely) when jewellery is purchased for investment & not for adornment, as it tends to be vulnerable to scratching. Mostly, it is usually mixed or alloyed, with other metals. Not only do they harden it, but influence the colour ; white shades are achieved by alloying gold with silver, nickel or palladium ; red alloys contain mainly copper. A harder alloy is made by adding nickel or a tiny percentage of titanium.
The proportion of gold in jewellery is measured on the carat (or karat) scale. The word carat comes from the carob seed, which was originally used to balance scales in Oriental bazaars. Pure gold is designated 24 carat, which compares with the "fineness" by which bar gold is defined.
Pure gold Gold alloys Caratage Fineness % Gold
24 1000 100
22 916.7 91.67
18 750 75
14 583.3 58.3
10 416.7 41.67
9 375 37.5

The most widely used alloys for jewellery in Europe are 18 and 14 carat, although 9 carat is popular in Britain. Portugal has a unique designation of 19.2 carats. In the United States 14 carat predominates, with some 10 carat. In the Middle East, India and South East Asia, jewellery is traditionally 22 carat . In China, Hong Kong and some other parts of Asia, sometime, "chuk kam" or pure gold jewellery of 990 fineness (almost 24 carat) is used. In many countries the law requires that every item of gold jewellery is clearly stamped with its caratage
Colours of Gold

Pure (24 carat) gold is a deep yellow colour (an orange shade of yellow) and is soft and very malleable. 
"Coloured Gold" can be obtained in a range of  shades: green (actually a green shade of yellow), pale yellow, yellow, deep yellow, pink/rose and red. There are also white golds and even unusual coloured golds such as 'purple gold'. They all have different metal-alloys in them & range in gold content from 8 to 22 carats (33.3% - 91.6% gold) .
Almost all conventional, coloured carat golds are based on gold-silver-copper mixtures.
Gold is yellow and copper is red, the only two coloured pure metals. All other metals are white or grey in colour. The addition of a red colour to yellow, as everyone knows, makes the yellow pinker and eventually red. The addition of a white makes the yellow colour paler and eventually white. This principle of mixing colours is the same in carat golds. Adding copper to gold makes it redder and adding silver, zinc and any other metal makes gold paler. Thus, we can understand that lower carat golds, because we can add more alloying metals, can have a wider range of colours than the higher carat golds.
Thus at 22 carat (91.6% gold), we can only add a maximum of 8.4% of alloying metals and hence can only obtain yellow to pink/rose shades. At 18 carat (75.0% gold) and lower, we can add 25% or more alloying metals and hence get colours ranging from green through yellow to red, depending on the copper: silver plus zinc ratio. Thus at any given caratage we can vary the colour by varying the copper: silver plus zinc ratio. This can be demonstrated in the following table:

Effect of copper: silver ratio on colour
Type
Gold % wt
Silver %
Copper %
Colour
22 ct
91.6
8.4
-
Yellow
91.6
5.5
2.8
Yellow
91.6
3.2
5.1
Deep yellow
91.6
-
8.4
Pink/rose
18 ct
75.0
25.0
-
Green-yellow
75.0
16.0
9.0
Pale yellow, 2N
75.0
12.5
12.5
Yellow, 3N
75.0
9.0
16.0
Pink, 4N
75.0
4.5
20.5
Red, 5N
14 ct
58.5
41.5
-
Pale green
58.5
30.0
11.5
Yellow
58.5
9.0
32.5
Red
9 ct
37.5
62.5
-
White
37.5
55.0
7.5
Pale yellow
37.5
42.5
20.0
Yellow
37.5
31.25
31.25
Rich yellow
37.5
20.0
42.5
Pink
37.5
7.5
55.0
Red

Apart from copper, all other alloying metals to gold will tend to whiten the colour and so it is possible to make carat golds that are white in colour. White golds for jewellery were developed in the 1920's as a substitute for platinum.
Additions of any white metal to gold will tend to bleach it's colour. In practice, nickel and palladium (and platinum) are strong 'bleachers ' of gold ; silver and zinc are moderate bleachers and all others are moderate to weak in effect.

Special Colours of Gold: Blue, Black and Purple !
These can be accomplished by one of two techniques: formation of special gold metal compounds (intermetallic compounds) OR by a surface coating or patination. Both approaches can yield attractive colours but they do have some disadvantages over normal carat gold alloys.
1. Intermetallic compound colours 
a] Purple gold (also known as amethyst or violet gold)
When gold and aluminum are alloyed in a certain fixed ratio, they form a gold intermetallic compound (AuAl2)  ; one atom of gold to two atoms of aluminum. This compound has an attractive purple colour. In terms of composition, this compound is about 79% gold by weight and hence is hallmarkable as 18 carat gold. 
However, purple gold tends to be very brittle & cannot be easily worked on & also tends to tarnish easily. 
An alternative approach to making jewellery with purple gold decoration is to physically vapour deposit (PVD) the two metals, gold and aluminum, in the correct ratio onto a carat gold substrate. Such processing can be done by a number of PVD techniques (patented) such as sputtering. 
b] Blue gold
The intermetallic compound formed between gold and indium, AuIn2, gives rise to a clear blue colour and that between gold and gallium, AuGa2, to a bluish hue.
2 Colours by surface coatings and patinas
c] Black gold (grey - black & brown)
There are several ways of obtaining a black colouration on carat golds. 
3 techniques:
- Electrodeposition of, for example, 'black' rhodium or ruthenium
- Plasma Assisted Chemical Vapour Deposition (PA-CVD) of amorphous carbon
- Controlled oxidation of carat golds containing cobalt or chromium.
d] Blue gold
A blue patina can be produced on gold alloys by oxidation treatments. 
Oxidation of gold alloys containing 25% iron or arsenic is also reported to yield a bluish colour.
e] General comment
Many of these coatings will be vulnerable to rubbing or abrasion and so should be protected where possible.

NICKEL SAFE 
Our jewellery conforms with the European Directive restricting the use of Nickel in jewellery. 
Nickel can cause skin irritation and should therefore be kept to a minimum in jewellery and other products that come into contact with the skin.
"Nickel safe", serves as a confirmation the this piece conforms to the European legislation. 
Nickel has been quantitivly banned in the U.K., and other countries are expected to follow soon. Please beware of cheaper-imitations.
What is nickel?
Can nickel affect my health?
Nickel allergy that can occur in some people who repeatedly come into contact with nickel. 
Nickel is a metallic element and in its pure state, is a hard silvery-white metal. Nickel has properties that allow it to be combined with other metals to form mixtures called alloys. It is used in some jewellery alloys (esp. for white golds) to increase the strength and hardness as well as make the jewellery resistant to corrosion and heat.
The most common adverse health effect associated with wearing jewellery that contains nickel is a type of skin irritation called allergic contact dermatitis. A person can develop an allergy to nickel following repeated contact with nickel found in jewellery. Sweat from the skin acts as a corrosive to metals, which allows nickel in jewellery to be in direct contact with the skin. Once a person is sensitised to nickel, any further contact with nickel will cause an allergic reaction. Nickel sensitivity is usually persistent for life. Allergic contact dermatitis produces a red, irritating and itchy rash that may persist for several days. In severe cases blisters may form. This rash will develop about two days after exposure. While it is more common in women, there has been a slow increase in the number of men experiencing nickel allergy. This increase is thought to be a result of use of nickel-containing jewellery especially associated with recent fashion trends such as body piercing. You are also likely to come into contact with nickel by wearing nickel-containing jewellery such as earrings, body rings or studs, watches, rings and bracelets.

The best option is nickel-safe jewellery, from AANANTA (India). Example :
REGISTERED TRADE-MARK OF LEGOR Srl.

1. IDENTIFICATION OF THE MATERIAL
Products
Product Code WD481CW (14-18KT WHITE ALLOY)
Manufacturer
Leg.Or S.r.l. Via San Benedetto,
14/34, 36050
Bressanvido Z.I.
Vicenza, ITALY
Tel. +39 0444 660666 Fax. +39 0444 660677
2. COMPOSITION
We hereby declare that the alloy mentioned above is Nickel Safe according to our tests,
for 14-18Kt applications.
 But please be aware, that even though a particular alloy has
passed the Nickel Release test, it does not mean it will pass for all jewelry made with
this alloy. According to the European Directive, it is the responsibility of the jewelry
manufacturer to also test one of their pieces to establish compliance for European
certification and permission to market the same in Europe.
Bressanvido October 2nd 2005. Leg.Or S.r.l.
(Technical Director)
Dr. Andrea Basso

 

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