Chloe Kheng

# Glen is playing

## chemistry

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# Jewellery

Three prominent metals in the making of jewellery today are gold, silver and platinum.

## Origin

The use of gold has been far spread in many ancient cultures, as the epitome of wealth and money. Gold nuggets found in the Spanish Caves relating to Paleolithic Man have been dated to around 40 000 BC and suggest that gold was in use by humans since paleolithic times.2 The earliest known use of gold as jewellery can be dated to 3000 BC, which were found in the tombs of Queen Zer in Egypt and Queen Pu-abi of Ur in Sumeria.3

Silver is a precious metal which has also been in use since ancient times. Silver objects and ornaments found in Greece and modern Turkey (previously known as Anatolia), as well as royal tombs in Egypt, can be dated to around 4000 BC.4.

Platinum is a relatively new element, but its importance has quickly grown in the creation of jewellery and other products. Though platinum was in use by South American Indians as long as a 1000 years ago, platinum was only introduced to the rest of the world during the Spanish Conquests between the 15th and 16th century. It was only in 1735, that platinum was officially recognised as a new element.5

## Evolution

6

Though the elements itself have not changed through time, these metals are now incorporated into alloys to strengthen their properties and create more metals suitable for jewellery.

Previously, gold was used in its pure form, which, though still durable, malleable and ductile, is very soft. These days, metals like silver, copper and zinc are added to gold to reinforce it's properties and create a stronger, harder substance. Because gold is a very expensive material, adding other elements is also a way of creating types of gold which are more affordable.7 Gold is often measured in karats, which shows the percentage of pure gold in relation to the other elements. The table below shows the relation of karats to gold8:

Karats Percentage of Gold
10 41.7%
12 50%
14 75%
18 58.3%
22 91.7%
24 99.9%

'White Gold' is another example how gold has evolved. By mixing gold with varying amounts of nickel, palladium, platinum, manganese, copper, zinc and/or silver, this produces an alloy of gold with a silver finish and a higher strength and durability.White gold is also plated with rhodium to give it a lustrous finish.9

Silver is a soft metal and though it is not as reactive as metals such iron, silver does have the tendency to oxidise and tarnish over many years. Therefore, the composition of silver has evolved to include other elements which strengthen its properties. A popular alloy of silver in jewellery is sterling silver, which is an alloy consisting of 92.5% pure silver and 7.5% of another metal, usually copper. This produces a silver that maintains the same shade and sheen, but is more durable and less soft. Unfortunately, the added copper creates a silver which is even more easily tarnished, however sterling silver can be plated with rhodium to stop this.

Previously, platinum was rarely used, but now platinum is being incorporated into all sorts of purposes and it's popularity in jewellery has increased substantially. It is very rare to have 100% pure platinum jewellery as the price is very costly - pure platinum costs about twice as much as pure gold. The highest percentage of pure platinum is usually 95% and platinum is alloyed in varying amounts with other platinum group elements and metals such as copper and cobalt. Platinum has evolved in the frequency of use since its discovery and likewise with gold and silver, has alloys to strengthen it's properties and create more affordable variations.

Amounts of gold, silver and platinum are also mixed with each other to produce a greater alloys for jewellery. For example, platinum-sterling, in which platinum is used to replace a considerable amount of the copper in normal sterling silver.10

## Manufacturing & Chemical Composition

Gold:

• Symbol: Au (comes from the Latin word, 'aurum')
• Atomic number: 79
• Belongs to group 11 in the periodic table
• Atomic mass: 196.96655 amu
• Transition metal

Gold atom electronic configuration:
11
Gold is a metal that can be formed at the earth surface (exogenetic) or deep within the earth (endogenetic), therefore gold can be found in nuggets or in other minerals. Gold is also most normally found in it's native state, that is, they appear in nature as metals, either in pure form or as an alloy. One type of gold is alluvial gold, which refers to the small gold particles found in areas such as riverbeds and streams. Gold can be collected in this way through panning or found as gold nuggets, melted down, purified and then shaped to create gold jewellery.12 When extracting gold from ores there are specific processes used such as carbon-in-pulp, electrowinning and smelting. Electrowinning is a process that uses electrolysis on a liquified ore to extact a pure metal.13
Gold cyanidation is one of the most common methods of extraction for gold in ores. In this process, the ore is ground into fine particles and sodium cyanide, or cyanide salt, is added along with water. The cyanide then binds and reacts with the gold particles in the ground ore, and they react, changing the gold to make it soluble in water. The gold dissolves in the water and is therefore separated from the other ore particles. The following equation can be used to show this:

(1)
$$4 Au + 8(NaCN) +O2 + 2 H2O → 4 NaAu(CN)2 + 4 NaOH$$

After this, in order to recover the gold from the solution, a very fine zinc powder is added. This zinc reacts with the cyanide and the following equation occurs:

(2)
$$2Au(CN) + Zn → 2Au + Zn(CN)4$$

Producing, separated gold.14

Silver:

• Symbol: Ag (comes from the Latin word, 'argentium')
• Atomic number: 47
• Belongs to group 11 in the periodic table
• Atomic mass: 107.8682 amu
• Transition metal

Silver atom electronic configuration:
15
To gather the silver needed in jewellery, it must be mined and extracted. Traces of silver can be found in many naturally-occurring minerals. Silver is often mined from lead ores, copper ores, and cobalt arsenide ores and can be the result or by-product of copper and lead mining. Pure silver can be extracted from these minerals in processes such as smelting, electro-refining and froth flotation.16
Smelting is a process which involves applying heat and chemical agents to an ore in order to melt the metal to be extracted and driving away the other substances as gasses or 'slag'. Electro-refining is used to separate the impurities of a metal by use of electrodes placed in fluids. Flotation involves using the hydrophobic or hydrophillic (hydrophobic meaning repellent of water, hydrophillic meaning attracted to water) properties of substances to separate them.17 Similarly to gold, silver can also be placed with cyanide to extract it from it's ore. For example in argentite, a silver containing ore, the equation is as follows:

(3)
$$Ag2S + 4NaCN ⇌ 2Na[Ag(CN)2] + Na2S$$

Zinc powder is added to the solution and the equation is as so:

(4)
$$Zn + 2Na[Ag(CN)2] → Na2[Zn(CN)4] + 2Ag$$

Again, producing silver which has been separated.

Platinum:

• Symbol: Pt
• Atomic number 78
• Belongs to group 10 in the periodic table
• Atomic mass: 195.078 amu
• Transition metal

Platinum atom electronic configuration:
18
In order to extract platinum ready for use in jewellery, platinum can be found in various alloys and minerals. Polyxene, is a very rare alloy containing about 80 - 90 % platinum, 3 - 11 % iron, and small amounts of other platinum group metals, gold copper and nickel. Platinum can also be found in Platiniridium, an even rarer alloy composed mainly of platinum and iridium.19
However, platinum is most often collected as the by-product from mining other metals such as copper and nickel, or can be extracted from Sperrylite sulfide and nickel ores, using methods similar to that of silver, such as smelting, gravity separation and froth flotation.20 (Gravity separation is the separation of two components with different weights, using gravity).
When trying to separate pure platinum from other platinum group metals, aqua regia or nitro-hydrochloric acid is used. Pure platinum will dissolve in aqua regia, whereas the other platinum group metals do not. The reaction that occurs between pure platinum and aqua regia can be shown:

(5)
$$Pt + 4NO3 + 8H → Pt + 4NO2 + 4 H2O$$
21

Once dissolved, the pure platinum is then removed from the aqua regia in the form of a 'platinum sponge', heated, then melted to it's pure metal form.2223

Gold, silver and platinum are all types of metals. Metals are elements in which the compounds are composed of positive ions suspended in a sea of delocalised electrons. This structure is what give metals properties suitable for jewellery such as being malleable, ductile, dense and have the ability to reflect light (lustre).

After extraction from ores and minerals, gold, silver and platinum can be applied to jewellery. They are often made into alloys (see evolution) and then undergo heat and pressure to shape them in rings, necklaces, bracelets and earrings for people to wear.

## Environmental Impact

When extracting gold, silver and platinum, the processes needed to collect the metal often result in impact to the environment. When mining the ores, the machines needed to drill and carry the rocks require a substantial amount of fuel and energy. After excavating minerals which have been sitting inside the earth many thousands of years, the air and moisture can set off chemical reactions, producing acids and toxic substances. Mining companies often have to mine a lot of earth to produce small amount of pure gold. The waste and leftover material which arises is full of harmful contaminents such as cyanide, mercury and arsenic. These products must be managed affectly in order not to hurt surrounding wildlife or contaminate water and food sources.24

In the process of creating the jewellery itself, jewellery makers can also leave a large carbon footprint in what they use. For one thing, there are many harmful chemicals such as cyanide, ammonia and petroleum-based-coolant used in the refining process that are uncarefully disposed of, causing pollution to the environment. Even simple things like the packaging used to ship jewellery. Nondegradable styrofoam peanuts are quickly thrown away and build up in landfills.25

## Properties

These are the various properties of gold, silver and platinum which make them suitable for use in jewellery:

Property Definition Use in Jewellery
Malleability The ability of a material to be moulded or ‘hammered’ into different shapes. Malleability is a prominent property required in the materials which make jewellery. Different jewellery and pendants are often designed with beautiful shapes and patterns so that they are aesthetically pleasing and will attract people. In order for the jewellery to be made into these designs they must be malleable.
Ductility The property that allows a material to be drawn into thin wires. This property is similar and just as important as malleability in the creation of jewellery. Various types of jewellery such as necklaces, bracelets, earrings, and rings are all thin wire-like shapes, needed in order to curve around body parts like our necks and wrists. The ductility in this case is not only attractive but also suitable to create a light weight object that fits to our bodies.
Durability The capability of a material to withstand wear, pressure or decay without damaging Gold, silver and platinum are all extremely durable materials which have been well-known for their ability to withstand wear and tear. Jewellery made from metals such as these are treated as precious and valuable treasures which can be passed down from generation to generation.
Non-corrosive A substance which does not have the tendency to be corroded/eaten away at by another substance, through a chemical reaction This property is similar to durability. Not only are gold, silver and platinum not be susceptible to everyday wear and tear, but they are also not easily influenced by other chemicals. People buy jewellery made using these metals with the confidence that they will last.
Lustre The shininess of a material, the ability of a material to reflect light. The lustre of jewellery is important because it is aesthetically appealing to people. It is the lustre of the jewellery which attracts people to buy it.
Hardness The quality of being rigid and resistant to pressure Hardness is important in jewellery so that the shape which it is bent into will maintain it's shape and will not easily be bent when hit. In the case of gold and silver, which are quite soft metals, other metals are often added in order to create alloys which are harder. People want jewellery to be hard so that the shape and design will stay the same.