24 Karat gold is the purest gold you can get in gold bullion, coins, bars or gold jewelry.
Coins should originate from a Mint. Smaller items such as rings and other gold jewelry should have an assay mark, or hallmark to show the purity of the gold.
Bars and ingots should originate from a foundry. Since 1994 all SMALL bars coming out of modern foundries are100% pure and at the exact weight they are prescribed to be at, due to various recent advancements in production and accurate weighing. New bars, especially ones that are polished and are 1kg or less, and are considered jewelry quality, are weighed and formed while the gold is in a semi-liquid state. Gold at a certain temperature, will flow down a special ramp at a prescribed speed and can then be cut into exact sizes and weights.
Bars that were made before mid 1994, however, or are above 1 kilo weight (above 5 kilos after 1997) or are graded pure raw gold are almost never 100% pure gold nor are they the exact weight.
Hence the Asian foundries always tend to make their bars slightly heavier, adding up to one gram after the traditional weighing, to make up for any discrepancy in purity. This means that a kilo bar of 999.9 percent will have more than 1000grams of 100 percent pure gold in it..
But as a result of the .999 purity standard adopted in the 80s, to replace the 24 ct norm, non-Asian bars had to be made to meet the stencil marks as closely at possible. Prior to that, and even continued by smaller foundries until the early 90s, in Asia a kilo bar always weighed more and was stenciled as 24ct pure.
The carat (abbrev ct or K) is a measure of the purity of gold. In the US and Canada the spelling karat has been adopted solely for the measure of purity and carat referring to the mass weight. These are two different things. As a measure of weight,
1 carat is one 24th purity by weight. 10 carats are ten 24ths. 24 carat gold is pure gold or 999.999% gold with some possible impurities. 18-carat gold is 75% gold, 12-carat gold is 50% gold, and so forth. There is actually no such thing as absolute pure gold. In the refining process it is possible and even likely that a tiny amount of copper will be alloyed as part of the refining process.
However, moves are being made to change the current karat system to the millesimal fineness system by which the purity of precious metals is noted more accurately by parts per thousand of pure metal in the alloy. The most common carats used for gold in bullion, jewellery making and by goldsmiths, for example, are:
24 karat (millesimal fineness 999)
22 karat (millesimal fineness 916)
20 karat (millesimal fineness 833)
18 karat (millesimal fineness 750)
15 karat (millesimal fineness 625)
14 karat (millesimal fineness 585)
10 karat (millesimal fineness 417)
9 karat (millesimal fineness 375)
But this system of calculation gives only the weight of pure gold contained in an alloy.
18-karat gold means that the alloy's weight consists of 75% of gold and 25% of alloy(s). The quantity of gold by volume in a less than 24-karat gold alloy will be different according to the alloys used. We know that standard 10 karat yellow gold that standard 18-karat yellow gold consists of 75% gold, 12.5% silver and the remaining 12.5% of copper by weight. The volume of gold in this alloy will be 60 percent. As gold is more dense than the silver or copper.
But for most people it is a matter of, how much gold is in the ring or gold jewelry or even gold bar. And how do you know that the figure given to you is correct?
In addition the various colors of gold also have different mixes of other metals.
50% silver and 50% copper
Nickel, zinc, copper, tin and manganese
Pink (rose) Gold
90% copper and 10% silver
High proportion of silver or cadmium
You can test the purity of gold yourself with the right test equipment. Mostly this consists of acids which you can purchase for that purpose although electronic testing is now becoming popular and electronic testers can be found with a little investigation and research and purchased. The cost of the test equipment means that you would want to test a lot of gold to warrant the cost of the equipment and along with the other factors such as safety measures, time involved etc, you would likely be better off just getting an independent jeweler or laboratory or even a Mint to test the gold for you.
Bear in mind however that any testing of gold with chemicals will mean a loss of some small amount of gold. For a kilo or more gold bar this may not amount to much but for a ring, it could be significant.
Many a person has been surprised to find that their bar, coin or ring has somewhat less gold than they were given to understand. By the same token it is quite possible to buy a ring and find it has a lot more gold than originally thought.
Gold purity is an important factor when it comes to owning gold!