Throughout history, gold has been treasured for its natural beauty and radiance. For this reason, many cultures have imagined gold to represent the sun.
Yellow gold jewelery is still the most popular color, but today gold is available in a diverse palette. The process of alloying—mixing other metals with pure 24 carat gold—gives malleable gold more durability, but can also be used to change its color.
White gold is created through alloying pure gold with white metals such as palladium or silver. In addition it is usually plated with rhodium to create a whiter surface that matches other plated, white gold jewelry. Some will say this is because the rhodium will give the item a harder surface with a brighter shine, but that is not true. It is to make all of the items in a sale case look the same color. Depending on the ally, white gold still has varying shades of yellow when compared to other alloys. This is to facilitate sales and that is all.
The addition of copper adds strength to the alloy and increases resistance to wear. If you add significant amounts of copper, the result will be a soft pink complexion of rose gold.
The weight of gold is measured in troy ounces (1 troy ounce = 31.1034768 grams), however its purity is measured in ‘carats’.
‘Caratage’ is the measurement of purity of gold alloyed with other metals. 24 carat is pure gold with no other metals. Lower caratages contain less gold; 18 carat gold contains 75 per cent gold and 25 per cent other metals, often copper or silver.
The minimum caratage for an item to be called gold varies by country. In the US, 10 carat is the legal minimum accepted standard of gold caratage, 14 carat being the most popular. In France, the UK, Austria, Portugal and Ireland, 9 carat is the lowest caratage permitted to be called gold. In Denmark and Greece, 8 carat is the legal minimum standard.
- .375 = 9 carat (England and Canada)
- .417 = 10 carat
- .583 (.585) = 14 carat
- .750 = 18 carat
- .833 = 20 carat (Asia)
- .999 (1000) = 24 carat pure gold
Strictly speaking, 14 carat should be 583 (14/24 = .583333), but most manufacturers have adopted the European practice of making 14 carat gold slightly over 14 carat. Thus, the fineness mark is 585 in most 14 carat gold jewelery.
Similarly, 24 carat should be 1.0 (24/24 = 1.00). However, in practice, there is likely to be a very slight impurity in any gold, and it can only be refined to a fineness level of 999.9 parts per thousand. This is stated as 999.9.
Accepted tolerances on purity vary from market to market. In China, Chuk Kam (which is Cantonese for ‘pure gold’ or literally ‘full gold’) still comprises the majority of sales and is defined as 99.0 per cent minimum gold, with a 1.0 per cent negative tolerance allowed.
Fineness is another way of expressing the precious metal content of gold jewelery, and represents the purity in parts per thousand. When stamped on jewelery, usually this is stated without the decimal point.
This table shows some examples of the composition of various caratages of gold.
So, what are the metals that are alloyed with gold? Gold will form alloys with most metals, but for jewelry, the most common alloying metals are silver, copper, and zinc. However, other metals may be added, especially to make colored gold. Here's a table of the compositions of some common gold alloys:
White gold compositions listed here are nickel free. Nickel-containing white gold alloys form a small/very small percentage of white gold alloys and generally contain other base metals such as copper and zinc.
The following are the common standards of fineness that are used:
|Caratage||Gold(Au)||Silver (Ag)||Copper (Cu)||Zinc (Zn)||Palladium (Pd)|
|White Gold||18k||75%||25% (or Pt)|
The following are unusual gold alloys. You will not see these very often.
|Color of Gold||Alloy Composition|
|Gray-White Gold (18K)||Gold 75%
|Soft Green Gold (18K)||Gold 75%
|Light Green Gold (18K)||Gold 75%
|Green Gold (18K)||Gold 75%
|Deep Green Gold (18K)||Gold 75%
|Blue-White or Blue Gold (18K)||Gold 75%
|Purple Gold||Gold 80%
The alloying metal compositions above are typical of those used by the jewelery industry to arrive at the color/caratage combinations shown, but these are not the only ways to arrive at these combinations.The alloys listed in the table do not include the more unusual colors. That is because they are made from the addition of patinas or oxides on the alloy surface. Black gold for example derives its color from cobalt oxide.
Mixing an alloy
- Weigh metals carefully in the proportions listing in gold alloying tables (the larger the amount prepared, the more accurate the karat the resulting gold alloy will be).
- Pickle all metals to make sure they are all thoroughly cleaned.
- Mix the metals, except the gold, in a crucible fluxed with borax.
- Melt the metals with a reducing flame (starting with the highest melting point metal). Do not bring the metal to a boil. Stir constantly with a carbon stirring rod.
- Keep the metal molten and add the fine gold.
- Pour the molten gold alloy into an ingot mold or depression formed in a charcoal block. If the resulting gold alloy is brittle when later worked, this will indicate impurities. Streaks of color will indicate that the alloy was not mixed properly and may have to be remelting and remixed with a carbon stirring rod.
Formula for Raising to a Karat
( (K1 - K2) x W) / (24 - K1) = Amount of pure gold to add
- Weigh the gold you want to raise ( W ).
- Subtract the starting karat ( K2 ) from the desired karat ( K1 ).
- Multiply the result of step 2 by the starting weight, W.
- Subtract the desired karat ( K1 ) from 24 (for 24 karat).
- Divide the result of step 3 by the result of step 4.
- You now have the amount of 24K gold you need to add to your original karat gold to raise it.Ex. How much 24K gold do you have to add to 7 grams of 10K gold to raise it to 14K?( (14 - 10) x 7) / (24 - 14) = 2.8 grams of 24K gold.
Formula for Lowering to a Karat
( (W x K1) - (W x K2)) / K2 = amount of alloy you need to add to lower the karat of gold.
- Weigh the gold to be lowered ( W ).
- Multiply this weight, W by its karat ( K1 ).
- Multiply the starting weight, W by the desired karat ( K2 ).
- Subtract the result of step 3 from the result of step 2.
- Divide the result of step 4 by the desired karat ( K2 ).
- You now know how much alloy to add to your gold to lower its karat.Ex. How much alloy do you need to add to 6 grams of 18K to make it 10K?( (6 x 18) - (6 x 10)) / 10 = 4.8 grams of alloy.
Helmenstine, Anne Marie, Ph.D. (2017, March 24). Composition of Gold Alloys in Colored Gold Jewelry. Retrieved from https://www.thoughtco.com/composition-of-gold-alloys-608016
most of this information has been pulled from the World Gold Council web page. https://www.gold.org/about-gold/gold-jewellery 2018-01-25 15:42:22. Other various sources have been used as well to create this page.