Testing Gold the Archimedes Way
February 13, 2011 10 Comments
Just to refresh memories,here is the story:
Archimedes was a Greek scientist who lived in ancient Syracuse . The King of Syracuse wanted a gold crown made. So he gave some gold to a goldsmith to have one made. After few days, the goldsmith brought the finished crown to the King. The King had the crown weighed. He found weight of the crown to be equal to the weight of the gold he had given the goldsmith . However the color of the crown made the King suspicious. He believed that the goldsmith had pocketed some of the gold he had been given for his own personal use. The King wanted to find out the truth. So he asked his court scientist Archimedes to find out how pure the gold in the crown was.Obviously Archimedes could not melt down the new crown to determine its purity.So to find a solution Archimedes thought about the problem day and night. One day,while he was preoccupied thinking about this problem, he proceeded to take a bath. He was so preoccupied that he failed to notice that the water in his bathtub was already full to the brim.So as he got into into the bathtub ,a large quantity of water flowed over the rim . Archimedes noticed this and had a brainwave . He was so excited to have at last found the solution to the King’s problem that he jumped out of the bathtub,and ran naked down the street, shouting, “Eureka! Eureka!” Eureka in Greek means ‘I have found it.’
What Archimedes had remembered was this:Different metals of the same weight have different volumes. This is because different metals have different densities.Also 0bjects, put in water, will displace water. The displaced water will be equal to their volumes. A metal or alloy that is less dense will displace more water than a denser one.
Using these basic facts, Archimedes worked out a plan to find out the purity of the gold in the crown. He took two bowls and filled them with water to the brim. He then he placed each bowl separately in the middle of 2 other larger vessels. He then placed the crown in one bowl and as the water overflowed he collected the water at the bottom of the outer vessel. Then he took a piece of pure gold,equal in weight to the crown. He put this gold piece in the middle of the second bowl. Here too the water overflowed and got collected at the bottom of the outer bowl. Archimedes then measured the quantity of water in the two vessels. He found out the difference in the water that overflew in each case. The crown had displaced more water than the pure gold piece.So he concluded that, the gold in the crown had some other metals mixed in it. Archimedes reported his finding to the King who was then able to wring out a confession from the goldsmith.
This same principle can be used to determine the purity of gold ,approximately, at home.First weigh the unknown sample. Then determine the water displaced by a known quantity of gold of given purity.Then use ratios to determine how much water a piece of gold of the given purity and the same weight as the unknown sample would displace.If necessary use ratios again to adjust for a different caratage.Here is a worked example on how to do it http://www.chemicalforums.com/index.php?topic=8490.0 .Then compare the amount of water displaced by the unknown sample with what it should theoretically displace and check the purity of the unknown sample of gold.
This method comes in handy to detect the modern day gold -plated titanium fakes.Titanium is denser than gold and so would displace less water.For more details check this post :http://strats4ever.wordpress.com/2011/02/12/rumours-of-gold-plated-tungsten-how-to-easily-test-yours/ .This is also why it’s safer buying 22k gold over 24k for investment purposes.