Monday, February 24, 2014

The terms of caratage and weight-fraction fineness for gold concentrations of alloys

Gold (Au) is a precious metal. It is so precious that it has a special unit of purity: karat, also spelled carat. Frequently used unit symbols are kt or k.  Uppercase K is also in use, but may conflict with the symbol for the temperature unit Kelvin.

The term caratage refers to the purity of gold, i.e. the fraction of pure gold when this metallic element is alloyed with other chemical elements. Karat purity—using the kt unit symbol—is defined as follows:

Karat purity = (24 · MAu/Mtot) kt 

MAu is the mass of pure gold in a material sample and Mtot is the total mass of that sample. Thus, pure gold corresponds to 24 kt.

C. J. Raub reviews the composition of various gold alloys in relation to color and other properties [1].  He provides his readers with the following understanding of carat alloys:

The gold concentration of alloys can be expressed either in terms of caratage, 24 carat representing the pure element gold, or in finess which is the weight fraction expressed in 1000ths ( ‰). In European countries, 8, 14 and 18 carat jewellery is most common. (333, 585 and 750 ‰). 
                                                                                                                  C. J. Raub, 1999.

Dissolution reactions of gold alloys and reaction rates depend on the karat value. Nitric acid, aqua regia and other testing solutions are employed to determine the karat of a gold sample [2]. Testing materials and reagents are typically obtained from a jewelers' supply store.

Gold is known to be malleable. For many applications, a gold material with highest hardness at highest caratage is wanted—such that the material keeps its shape and guarantees wear resistance under ambient conditions. The 990 gold-titanium alloy with a fineness of 23.75 kt (24 · 990/1000)  is such a material, which has been developed as a fine gold alloy with desirable color, durability, and mechanical properties [3].

Keywords: gold alloys, purity calculation, concentration units, unit symbols, unit conversion.

References and more to explore
[1]  C. J. Raub: Gold Metal and Gold Alloys in Jewellery. In Hubert Schmidbaur, editor: Gold - Progress in Chemistry, Biochemistry and Technology. John Wiley & Sons, Chichester, England, 1999; page 110.
[2] Philadelphia Museum of Art: Finishing Techniques in Metalwork. Determination of Gold Karat [].
[3] G. Gafner: The development of 990 gold-titanium, and its production, use, and properties. J. S. Afr. Inst. Min. Metall. 1989, 89 (6), pp. 173-181 [].

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