MadSci Network: Science History

Re: Where did the numbers designating gold purity originate?

Date: Tue May 11 20:51:48 1999
Posted By: Greta Hardin, Secondary School Teacher, Chemistry, George Washington University
Area of science: Science History
ID: 925879731.Sh

Whee - This is a jewel of a question (so to speak).

As to Who came up with the karat (or carat) system or exactly When, I fear 
this is lost in the murky past.  However, as to Where, I can point you in 
the right direction. I fear my ancient history is lacking so I can only tell 
you this was already going on MORE than 3000 years ago.  (There are 
spectacular ruined cities in the Indian jungles that are older than that... 
but I digress)  But anyway, when I say the When and Who are lost in the 
murky past I mean the Ancient Past.

While Western europeans were still drawing on animal skins with burnt sticks 
the Persians and Chinese and other parts of the Asian, Indo-European and 
Near Eastern world had already adopted standardized measurment. When it came 
to precious metals and gems, the standard had to be small and reliable.  
Carob seeds became the standard, and one standard unit of gold was 24 of the 
carob seeds. The word karat (and carat, the unit of weight used to measure 
gems) derives from the; Italian "carato", the Arab "qirat", or the Greek 
"keration", all meaning the fruit of the carob tree.

Now, we all agree that 24 karat is pure gold, or 24 parts gold.  So 18K is 
18/24 gold and 14K is 14/24 parts gold.  Incidentally 10K is the lowest that 
can be sold as "gold" in the US, though things can also be "gold plated" or 
even "electroplated"  (more chem... ).  Some countries set the threshold 
higher (14K or 18K) and a few as low as 8K or 9K.

So what are the other parts?

Well it depends on what one wants.  You can make different color alloys;

Yellow gold      - usually copper and zinc alloy
Red or Rose gold - more copper and less zinc
White gold       - this one has many ingredients and can therefore vary 
	widely in price, quality and durability.  First, please nothe that
	sometimes Pt (platinum) is called "while gold."  However gold in a
	white alloy is mixed with avariety of things including: zinc, copper
	(far less than in yellow or red gold), nickel and/or platinum.
Green gold       - copper and silver
Blue gold        - iron

The metric craze has also hit the world of fine jewelry, so many european 
markets prefer to work using a "fineness" scale based on a) a decimal 
version of the 
karat component and b) parts of 1000.

24K gold or 100% gold has a fineness of 999
18K gold or  75% gold has a fineness of 750 and
14K gold or 58.3% gold has a fineness of 585
and so on..

As to where can you find more?

General advice.  Look under headings such as Gemology, Precious Metals, and 
of course Gold in a library index.

More specifically:  here are a bunch of jewelry and prospecting sites I used 
to research this.  Many of them have links I didn't need to use, but they 
may be of interest to you.  Beyond these sites, you may want to find a 
gregarious independant jeweler who does his/her own work, or find a 
university with a Gemology department and start nosing around.
Happy Hunting!

The sites:

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