Test Methods to Find Gold Content (Fineness)

 Test Methods To Find Gold Content (Fineness) 

There are a number of methods for measuring the gold content - or ‘fineness’ - of carat gold jewellery. 

Measuring the gold content is known as assaying and many of the most commonly used methods are described in a recent World Gold Council technical publication ‘The Assaying and Refining of Gold - a guide for the gold jewellery producer’. 

Which method of measurement is selected depends on the accuracy of measurement needed and the speed and ease of measurement. The cost of the equipment (instrument) will also influence the decision.

01) Fire Assay   (ISO 11426:1993)

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 milligrammes, weighing it accurately, wrapping it in lead foil with some added silver, cupelling it in in a furnace at about 1100°C to remove all base metals and then placing the resulting gold-silver alloy button 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 11426:1993. A good description of the process is given in an article in Gold Technology, No 3, January 1991.

A simplification of this technique involves omitting the initial cupellation stage and just melting the sample with added silver and/or copper, rolling to a thin sheet and then dissolving out the silver and base metals with nitric acid. This is satisfactory only when there are no other impurities present, but will be less accurate.

(02) X-ray fluorescence

X-ray fluorescence (XRF) is a non-destructive technique that is 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 parts per thousand 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. It is a quick technique, an assay taking about 3 minutes, and the results can be automatically printed out by the computer. It also measures the content of the other alloying metals present. 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 retailer in India has equipped each of their stores with such instruments. The gold content of each piece of jewellery is measured as it is sold, printing off a Certificate, thereby guaranteeing caratage conformance and providing consumer confidence in a country where national Hallmarking regulations do not exist.

(03) Touchstone testing

Touchstone testing is an ancient method for measuring gold content whereby a rubbing of the jewellery is made on a special touchstone alongside rubbings of known reference samples and treated with acids. The colour of the reacted area is compared to that of the reference sample. It is not sufficiently accurate (about 15 parts per thousand at best) and is only useful as a sorting test to differentiate between different caratages. It is less accurate at high caratages and with white golds. A more detailed description of the technique can also be found in Gold Technology, No 3, January 1991.

(04) Gold Testing Pen 

The Electronic Gold Tester (or the so-called Gold Pen) is a cheap, although portable technique based on the capacitance decay principle. Accuracy is poor, being correct to only 1-2 carats (4-8%) and is compromised if the surface is gold-plated, for example. It is useful only as a sorting test.

(05) Density Measurement

The density of carat golds reduces as caratage is lowered and this gives rise to density measurement as a possible method of measuring gold content, using Archimedes principle. However, density is also influenced by the other alloying constituents and so the accuracy of the method is poor. Jewellery containing defects such as porosity would further reduce the accuracy of density measurement. It is not recommended by world Gold Council.

Comparison of Assaying Techniques

Technique	Versatility	Sample size	Accuracy	Limitations	Equipment Cost
Fire Assay	Only gold	~ 250 mg	0.02%	Modifications for Ni and Pd	Moderate $50,000
ICP	Complete analysis	~20 mg	0.1%	-	High $150,000
XRF	Complete analysis	Non-destructive	0.1 - 0.5%	Surface layer, flat samples	Moderate $25,000+
Touchstone	Only gold	Almost non-destructive	1-2%	Unsuitable for high carat and white golds	Low $100
Electronic Pen	Only gold	Non- destructive	4-8%	Not consistent	Low $200
Density	Only gold	Non-destructive	Poor	Only for binary alloys	Low $500

For high accuracy, consistent with marking/Hallmarking regulations, only Fire Assay is sufficient. This technique involve taking a physical sample (a scraping) from the jewellery item.

For good accuracy, X-ray Fluorescence (XRF) analysis is suitable. Accuracy depends on the shape (geometry) of the item; it is best on flat surfaces. This technique is suitable for quality control in production and for certifying caratage conformance in a retail environment. It is a quick technique (3-4 mins) and does not require technical expertise to operate. Results are automatically displayed/printed out by the computer control. Suitable bench-top instruments developed specifically for jewellery use are available on the market. Reference standards are necessary.

For sorting jewellery into different caratages, then the touchstone and electronic gold pens are suitable cheap, quick techniques.

Density measurement not recommended to certifying gold caratage..

Source : WORLD GOLD COUNCIL