Review 1

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Gold Jewellery Assay Methods

22-Nov-11

GoldAnalysis_Review_1.doc

Review of Methods for Measuring Gold Content

 

1. Fire Assay (Cupellation)

The most accurate method, with an accuracy of 2 - 3 parts per ten thousand (0.02%), is the Fire Assay (Cupellation) method.

This involves taking a small scraping from the article, typically about 250 mg, weighing it accurately, wrapping it in lead foil with some added silver (known as 'inquart'), cupelling it in a furnace at about 1100°C to remove all base metals.

Then placing the resulting gold-silver alloy bead in nitric acid to dissolve out the silver (known as 'parting') and re-weighing the resulting pure gold.

This is the standard reference technique used by the national Assay laboratories worldwide for Hallmarking and is covered in the international Standard, ISO 11426:1997

2. Inductively Coupled Plasma – Mass Spectrometry (ICP–MS)

Fire Assay is closely followed for accuracy by Inductively Coupled Plasma – Mass Spectrometry (ICP-MS) which involves taking a smaller sample of about 20 mg, dissolving in acid and subjecting a sample to analysis in an ICP Mass Spectrometer - an expensive instrument.

This technique has an accuracy of 1 ‰ but requires use of comparative standard reference alloy samples of known composition.

This technique is accepted for Hallmarking purposes and has the advantage in that it also measures the other alloying constituents.

It is a quick technique, an assay taking about 3 min., and the results can be automatically printed out by the computer. It also measures the content of the other alloying metals present

3. X-Ray Fluorescence – (XRF)

X-Ray Fluorescence (XRF) is a non-destructive technique suitable for normal assaying requirements such as in-house quality control in manufacturing or for 'certifying' gold content in retail outlets.

It has an accuracy of, typically, 2 – 5 ‰ under good conditions, i.e. where the surface of the jewellery being measured is relatively flat and sufficiently large.

On curved surfaces, the gold X-rays generated and measured are scattered and accuracy is reduced significantly.

However, it only measures the gold content of a thin surface layer, so accuracy is severely compromised where the jewellery article has had a chemical surface treatment (to enhance colour) or has been electroplated with a layer of pure gold.

The more accurate XRF instruments measure the intensity of the generated gold X-rays by wavelength dispersion analysis. The use of energy dispersive analysers results in cheaper instruments but reduced accuracy. Reference alloy standards, of known composition close to that of the test piece, are needed if accuracy is required in XRF testing.

There are several instruments appearing on the market developed specifically for gold jewellery assaying, such as the X-tester, and these are more reasonably priced.

A major jewellery retailer in India has equipped each of their stores with such instruments. The gold content of each piece is measured as it is sold, printing off a Certificate, guaranteeing caratage conformance and providing consumer confidence in a country where national Hallmarking regulations do not exist.

 

Fire Assay is the Standard Reference Technique against which the other methods are compared

Jewellery Assaying Techniques:Review of Methods (1)  Review of Methods (2)  Comparisons: pros / cons (1)   Quantified (2)

Terms: Cupellation  Fire Assay  Reagents  Other Methods  Metals   Sheets: Cupels  Crucibles    Index: Programme