by Tim Harry

Created on: July 19, 2009   Last Updated: February 06, 2010

Carat is one of those words that most people know, but are unable to actually describe its specific meaning. Specificity though is an issue as it does have two different meanings, one meaning relates to the weight of precious stones, and the second meaning relates to the level of purity of gold and gold alloys.

Known as the karat in Canada and the United States, the carat is simply calculated as one carat being 1/24th purity of mass. Purity being measured by dividing the mass of gold in an item by the total mass of the said item. Thus an item made from all gold would be 24-carat and one made from only 50% gold would be 12-carat gold. The same basic calculations also ensure that a fineness scale can be achieved by dividing the mass of gold by the total mass of the item.

The millesimal fairness scale works as 999 (rather than the achieved 0.999 or above) as the equivalent of 24-carat gold, with 750 being 18-carat gold and so, the three digit number being a representation of the percentage of gold in an item. Going back into English history though a further measure of the carat was also devised based each carat being made up of four grains, and each grain being four quarts.

Knowing though what carat a piece of gold is, is different from being told what carat it is. If an item is displayed as 24-carat or 18-carat most people take it on trust that this is correct, after all testing is out of the question for most people.

Pure gold can be tested by undertaking a density test. Pure gold, or 24-carat gold has a density of 19.32gm/cm^3, something that can be easily determined by weighing an item and see how much water it displaces in a graduated cylinder of water. Density is calculated by dividing the mass of the item by its volume.

This test though can not be used on anything less than 24-carat gold though as the density is altered by other metals involved in the alloy, and it would be necessary to know the exact makeup to derive any meaningful results. For example yellow gold is made up of gold, silver and copper; white gold is an alloy of gold, nickel, zinc, copper, tin and manganese; and grey gold is made from gold and iron.

There are various testing kits that can determine the make up of a gold item, and can thus determine the purity and carat of that item. These testing kits though make use of acid and destroy a sample of the gold to get results.

Most purchasers of gold items therefore look to the assay or hallmark present on the gold item, although it should

• 1
• 2
• Next Page >>