TABLE 2 shows the relationship between the purity of gold and international standards for that purity:
There are several ways in which to test a material of its gold content. Some are destructive (i.e. after the test the material has been damaged in some way, therefore; losing its original value) and some are not. One of the most accurate and well-known ways is by fire assaying. Fire assaying is destructive, but its accuracy is within 2-3 parts per 10,000. The first step in this process is to take a scraping of the material (this is why this method is ‘destructive’). The scraping is very small, around 250 mg (milligrams). It is then precisely weighed, wrapped in lead foil with some silver. The silver is added so in the cupelling process it alloys with gold. It is then cupelled (a metallurgical process in which ores or alloys are heated to a very high temperature in order to remove noble metals, like gold and silver; from base metals like lead, copper) in a hot furnace around the temperature of 1100°C. After this procedure all that is left of the ore/alloy is gold and silver as an alloy. Then, nitric acid is used to remove the unwanted silver so what remains is pure gold. It is then reweighed and compared with the weight of the scraping which was taken at the start of the process. If the weight is the same as before, then the ore or alloy in subject is 24k gold. If the final weight is half that of the original, then the original scraping is 12k gold. This is the formula used to find out the karatage of the ore/alloy, provided you know the starting and end weights of the scraping.
Where
Mg = mass of the pure gold after the process
Mm = mass of the original scraping at the start of the process
One of the main advantages of this process is that it is quantitative (as opposed to qualitative) which means that the results from the process have numerical values, and are just observations. This increases accuracy greatly as numbers are more meaningful in this case than words, since the exact purity is very important in today’s world and there is no room for error.
A disadvantage is that the scraping taken may not be representative, i.e. it may not represent the actual content of the whole alloy. So it may contain a higher percentage of gold than another part of the material. Another disadvantage is that this is a time-consuming process, compared to some of the other way of testing gold for its purity.
GOLD ALLOYS
If it weren’t for alloys, gold wouldn’t be used as jewellery as it would be too heavy to be worn every day. Alloys are two or more metals chemically bonded, which share properties of its different metals. This is a bit like selective breeding, where plants or animals which share desirable characteristics are bred to produce offspring with the best characteristics. For example, metal A could be heavy and strong, and metal B could be light and weak. Hopefully, by alloying these two metals together, the resulting alloy would be light and strong. If unsuccessful, the resulting metal would be heavy and weak. Alloying involves trial and error. In gold jewellery, the metals alloyed with gold not only increase its strength, but also change its colour. Gold is most often alloyed with copper, zinc or silver. Alloying gold with copper gives give the final material the red colour (more commonly known as ‘red gold’). TABLE 3 shows the composition of popular different coloured (and different karatage) gold alloys. As you can see from the table, there are quite a few different alloys for gold which yield different colours. 9k gold is often said to be a harder metal than 18k gold. Well, since gold is a soft metal, 9k gold allows more of the content to be composed of other, harder metals; unlike 18k gold where there’s less space (percentage) for other, harder metals. But this is not true. The hardness of the gold alloy depends on other factors such as how the material was treated during manufacture.
Note: the shaded areas indicate 0% usage of that metal in the specific alloy
OTHER USES OF GOLD
Today gold is used everywhere around you. Gold is an excellent conductor of electricity and doesn’t corrode or tarnish. It can also work at very high temperatures. This makes it ideal for use in electronics. It is the best metal to use in for small electronic contacts, switches and other components. These electronic parts can be found in computers, TVs and telephones. Because of gold’s high price, only a very thin film of gold is used for gold-plated contacts. Also, gold has often been used for many years as the reward for highest achievement at the Olympics and other major sporting events like the FIFA World Cup. In reality, the ‘gold medal’ in the Olympics is actually a medal made out of silver, but plated in gold. However, the FIFA World Cup trophy is made out of 18k gold. Gold has also been invaluable to space travel, as NASA used 40.8 kilograms of gold for the construction of Space Shuttle Columbia in places like radiation shields, where it is necessary to deflect the Sun’s otherwise-damaging heat. Gold has also been used in food and has the E Number of 175. Because the price of gold doesn’t change much and stays around the same price, it is used by people as investment. When the value of currency goes down, gold can be used instead as its value is mostly stable. Alloys of gold are also used in dentistry for tooth restoration (e.g. crowns and permanent bridges). The isotope (gold-198) is used for treating some diseases and is also used in some cancer treatments.
QUESTIONNAIRE - WHAT DOES THE PUBLIC KNOW AND THINK ABOUT GOLD?
For this CDA, I conducted a survey with the intention that its results will tell me how much the public know about gold. I decided to ask 20 people in my street and write down their answers on a data collection sheet. Luckily, every house I went to there was an adult to answer and no one was away. I chose to ask at around 7 p.m. so that there will be less chance of people not being available as they would’ve come back from work by then. I think my results could have been more reliable if I’d asked more people, and also if I’d asked people from other streets, as these results may not have applied to everyone. These are the questions I used:
- Do you own gold jewellery?
- Do you know the karatage of your gold jewellery?
- Were you aware of this before you purchased the jewellery?
- Do you feel as though the staff in jewellery stores don’t reveal the karatage of a product easily, and that they should be more honest?
- Do you know the karatage of pure gold?
- Did you know that most gold jewellery in this country isn’t half made out of gold and instead they are mostly made out of other, cheaper materials such as copper and silver?
- Do you like gold jewellery? Why/why not?
ANALYSIS
* People aren’t actually allergic to gold; it’s the nickel in the gold (in certain alloys of gold where nickel is present). Contact dermatitis is the name used for irritation on the skin caused by allergens, such as nickel. People claim to be allergic to gold, but actually chances are that they’re allergic to the nickel in their gold jewellery.
I think that my results were quite valid, since everyone I asked were adults, and not just teenagers who would mess around and give me silly answers which would ruin my results. My results could have been more valid if I’d gone to other streets and asked them the same questions, since the people I asked are from around the same wealth group (I assume this because all of the houses in the street are around the same size) and results may have varied minutely or greatly if I tried other people. Also, in order to increase reliability, I should have asked more people about their opinion and knowledge of gold. This would’ve given me more accuracy in my results. This would be because as I would be asking more and more people, my results would be forming more and more into results which represent the population of UK. Of course, I couldn’t ask all of the country and also no matter how many people I asked (up until the point where I’d asked everyone in the country), my results weren’t going to represent the entire population.
In conclusion I have learned a lot about gold during my research. I now understand its importance to this world both aesthetically and chemically. I can also conclude, by looking at my results, that people do not know much about gold. When I asked question 6 for example, people were bewildered by that fact and seemed somewhat shocked. They were quite surprised to know that most ‘gold’ jewellery in the UK consists of less than 50% of gold itself.
GLOSSARY
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Alloy – when two or more metals are combined to make an alloy which shares some of the characteristics of its constituent metals.
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Metallurgy – the study of metals.
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Ductility – the ability to be made into wire without fracture.
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Malleability – the ability to flatten a metal down into a thin sheet.
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Tarnish – layer of corrosion that forms over metals such as copper and zinc, but not gold, since gold doesn’t easily oxidise.
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Corrosion – deterioration of metals caused by oxidation or other chemical reactions.
REFERENCES
- Gold - Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Gold
- Gold - Simple English Wikipedia, the free encyclopedia
http://simple.wikipedia.org/wiki/Gold
- Composition of Gold Alloys - Gold Alloys in Colored Gold Jewelry
http://chemistry.about.com/od/jewelrychemistry/a/goldalloys.htm
- Gold - The Relationship between Purity, Carats and Fineness
http://www.azom.com/Details.asp?ArticleID=2430
- Homepage World Gold Council, the information resource for gold, investment, jewellery, science and technology, historical and culture
http://www.gold.org/ (webpages within the website used as well)
- Cupellation - Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Cupellation
http://www.jewellerycatalogue.co.uk/gold/gold_alloys.php
http://www.jewellerycatalogue.co.uk/gold_jewellery/gold_hardness.php
- Ductility - Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Ductility
- Tarnish - Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Tarnish
- Contact dermatitis - Wikipedia, the free encyclopedia
http://en.wikipedia.org/wiki/Nickel_allergy
Images:
Many images were taken from Wikimedia Commons.
- Olympics+Day+13+Athletics+VZ0NfF6Ep2al.jpg 594×396 pixels (Usain Bolt)
http://www4.pictures.gi.zimbio.com/Olympics+Day+13+Athletics+VZ0NfF6Ep2al.jpg