Gold is omnipresent in the world of investment-grade art. Serious connoisseurs of art and antiques frequently encounter items made either partially or entirely from gold. This isn't surprising considering gold is a premier luxury material. The yellow precious metal has been lavishly used in everything from high-end jewelry to luxury watches to vintage fountain pens to breathtaking objets d'art.
Gold purity in jewelry and other antiques is usually measured in karats, with 24 karats representing pure gold. Gold alloys commonly encountered are 18 karat (75%) gold, 14 karat (58.3%) gold and 10 karat (41.67%) gold. In addition to the aforementioned alloys, British antiques and jewelry can sometimes be 22 karat (91.67%) gold, 15 karat (62.5%) gold or 9 karat (37.5%) gold.
Now a question that we might naturally ask ourselves as investors in antiques is which of these alloys is acceptable and which aren't? At what point is a gold alloy too diluted to qualify as "investment-grade" anymore? The short answer is "the point where the gold alloy stops acting like gold." In other words, how low can one descend the karat ladder before your gold alloy becomes vulnerable to corrosion and tarnish? Another important factor is that low purity gold alloys have difficulty achieving an intense, true gold color.
In order to determine a reasonable lower bound for acceptable gold purity, we need to understand a basic, fundamental law of gold alloys that is not widely known. A gold alloy's corrosion resistance is closely related to the percentage of gold atoms it contains. The higher that percentage, the more the alloy will behave like pure gold.
Any other precious metal atoms in the alloy (usually silver, although palladium is sometimes an alloying agent in white gold) also help enhance corrosion resistance, although not to the extent that gold does. Precious metals are sometimes referred to as the noble metals because of their chemical inertness. Therefore, we want a gold alloy to not only have a high percentage of gold atoms, but also a high percentage of noble metal atoms - aka noble nuclei.
So, even though we know the percentage of our alloy that is gold by weight, this isn't the information we need. We must convert this proportion by weight to proportion by number of atoms. Luckily, we can derive this information with a bit of math and science, assuming we know all the constituents of the alloy in question. The formula we're going to use for this is below:
Number of Atoms = (Percentage of Element in Alloy x Avogadro's Constant)/Atomic Weight of Element
Avogadro's number (6.022141527 × 10^23) allows us to estimate the number of atoms, molecules or ions in any substance provided we know its mass and molecular or atomic weight. To make this calculation work we also need to know the atomic weights of metals commonly found in yellow gold alloys for our calculations. However, in this case, we can ignore mass because we only care about the ratio of gold to its other alloying constituents.
Now if we plug the various numbers for a typical 18 karat yellow gold alloy (75% gold, 12.5% silver and 12.5% copper by weight) into the formula above we get the number of atoms each element contributes to the alloy. Then, using a little more simple math we find the relative proportion of each element in the alloy in terms of number of atoms present, instead of by weight, which is how alloys are usually represented.
In this case, our 18 karat yellow gold alloy is composed of 54.92% gold atoms, 16.71% silver atoms and 28.37% copper atoms. The noble nuclei (gold and silver) together are fully 71.63%. The very high percentage of noble nuclei - and gold nuclei in particular - give this alloy excellent corrosion resistance and a rich, deep gold color.
You may have noticed that the proportion of any element's atoms is inversely correlated to its atomic weight. In other words, very large atoms - like gold - tend to contribute fewer atoms to an alloy than their percentage by weight in the alloy would suggest. Conversely, smaller atoms - like copper - will contribute more atoms to the alloy than their percentage by weight would imply.
Now let's make the same calculations using a typical 14 karat yellow gold alloy consisting of 58.3% gold, 12% silver, 19.7% copper and 10% zinc by weight. The percentage of gold atoms contained in this 14 karat yellow gold allow has now declined to 34.01% while the noble nuclei have dropped to 46.80%. This 14 karat gold alloy, although possessing slightly less desirable properties than its 18 karat gold counterpart, will still have good corrosion resistance and a pleasing yellow-gold color.
Let's try this one more time, this time using a typical 9 karat yellow gold alloy composed of 37.5% gold, 10% silver, 45% copper and 7.5% zinc by weight. The percentage of gold atoms has declined precipitously to 17.22% in this 9 karat yellow gold alloy. Even the level of noble nuclei has dramatically fallen to 25.60%. This gold alloy will be inferior, with poor corrosion resistance and a pale, washed out yellow color.
Once a gold alloy declines below 30% to 35% gold atoms or 35% to 40% noble nuclei, it starts to look and behave much more like its constituent base metals than gold. This transition occurs right around the 14 karat mark.
Once a gold alloy dips below this level, even to just 12 karats, its physical properties rapidly deteriorate. Lower karat gold alloys like 9 or 10 karat are simply lost causes. No matter how you manipulate their component elements, it's impossible to get an alloy that has good color and good corrosion resistance.
The conclusion is clear: only buy 14 karat gold or better jewelry for investment purposes; avoid anything lower. This sage advice is echoed by the quality distribution apparent in fine jewelry as well.
18 karat gold alloys and above have traditionally been exclusively restricted for use in very high quality jewelry. 14 and 15 karat gold is the middle ground, where workmanship can vary tremendously, ranging from mediocre to excellent. Lower karat gold alloys - like 9 and 10 karat gold - have traditionally been reserved for mass-produced items with little regard for quality.
