Gold - The Relationship between Purity, Carats and Fineness

Introduction

Gold is a soft metal with a characteristic yellow color, and is also the most malleable and ductile of all elements. Except for aqua regia", HNO₃/HCl, gold is not affected by to water, air, acids, and alkalis. The fact that this metal is chemically unreactive means that it is usually found in its natural state.

It is a good electrical and thermal conductor and has excellent reflective properties to both infrared and light. It is found in the earth's crust with an abundance of 0.0011 ppm.

Most of the gold is retained for use as bullion reserves, but it is also used within the jewelry and electronics industries, where it is often alloyed with other elements such as silver and copper to improve the metal’s mechanical properties.

Gold is also used as a decorative medium as well as a heat reflecting coating for glass.

Background

The purity of gold is expressed in carats, often abbreviated to ‘ct’ of ‘K’ in the USA and some other countries. Pure gold has a purity of 24 carats. Various other purities exist that are measured relative to 24 carats. These are summarised in the table below.

Table 1. The relationship between gold content (by weight), its carat rating and hallmark rating for standard gold alloys.

An alternative method of expressing purity is ‘fineness’. This expresses the purity of gold in parts per 1000.

Gold may be alloyed with other metals such as silver, copper, zinc or silicon to produce purities less than 24 carat. Silver and copper and most commonly used alloying elements for gold. Pure gold is too soft to be able to be used for jewellery.

The metal chosen as an alloying element my influence properties such as workability and colour of the resultant gold alloy.

The purity of the gold is ‘marked’ onto jewellery by stamping or laser engraving. This acts a quality control stamp. Often this is done after the alloy is tested by a qualified testing laboratory or facility. Depending on which country the gold is produced, the caratage or fineness may be stamped into the item of jewellery.

According to the International Hallmarking Convention, there is a “no negative tolerance” policy. This means that a gold alloy determined to consist of 749 parts of gold per 1000 would not qualify for a 750 quality mark.

Methods used to assay or test the purity of gold include:

•         Inductively Couple Plasma Spectrometry (ICP)

•         X-Ray Fluorescence

•         Fire Assay

•         Touchstone

•         Electronic Pen

•         Density

Source: AZoM.com