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Meet the Experts cadmium

 

cadmium testing

Changes to legislation surrounding the use of cadmium are on the horizon, and will affect jewellers making, importing and selling both fine and costume jewellery. Dippal Manchanda, technical director of Assay Office Birmingham, explains the situation and how the Laboratory will test that products comply.

Cadmium is widely used in the jewellery industry, despite having long been recognised as a toxin and a known carcinogen, harmful when it is ingested or inhaled. Increasing concern with regard to consumer and employee health and safety has pushed it into the spotlight, and this has been accelerated by a recent trend for Far East manufacturers to substitute cadmium for lead, which is now tightly restricted in many countries.

The EU has extended the restrictions on cadmium in the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) Directive. A new regulation, EU 494/2011, was issued on 2 May, enforceable from 10 December 2011. The regulation restricts cadmium content in jewellery to 0.01 per cent (100 mg/kg) by weight of metal, and this applies to “metal beads and other metal jewellery components, metal parts of jewellery and imitation jewellery articles and hair accessories (ie bracelets, necklaces, rings, piercing jewellery, wristwatches, wrist-wear, hair accessories, brooches, cufflinks). Jewellers at all stages of the supply chain clearly need to respond by ensuring their products are compliant.

Cadmium: where and why is it present in jewellery?
Cadmium is a heavy metal, which has been used for over a century in both fashion and fine jewellery products. Small amounts of cadmium may be added to alloys used to make jewellery to impart specific technical and functional attributes to the metals. It may be present in jewellery as part of the metal alloy, solder or gold coating for electroforming/electroplating, or as a pigment or stabiliser in non-metal components.

Cadmium is used for various specific reasons:
De-oxidiser in silver alloys When silver is melted it takes up oxygen – about 22 times its own volume – and the copper that is usually alloyed with it also introduces more oxygen in the form of cuprous oxide. This creates the phenomenon known as ‘spitting’ or ‘sprouting’, when silver cools after pouring. The presence of oxygen also interferes with rolling and drawing operations. Cadmium has proved to be the most effective de-oxidiser to resolve this problem.

Improved malleability
Sterling silver alloys containing cadmium are significantly more malleable and ductile, rendering them easier to spin and to draw. Cadmium may also be alloyed with tin to improve melt and flow. This property has also resulted in cadmium being widely used in solders and such solders melt and flow better at a lower temperature than non-cadmium products. They are also widely used in gold and silver solder-filled wire.

Colour
Cadmium is used in gold alloys in place of silver to obtain different shades of colour such as pale yellow or pink. Certain mixture of cadmium with gold gives green alloy, like the Cu-Cd-Ag-Au mixtures. Greenish 18 carat gold alloys have traditionally been obtained with mixtures containing as much as 12.5 per cent cadmium. Adhesion and corrosion resistance Cadmium is used in some plating processes to promote adhesion and minimise corrosion. Plating baths may contain trace amounts of cadmium from earlier platings that could be incorporated in the final plated item.

Cadmium in paints, colours and glaze Cadmium in various forms is used for colouring glass and porcelain and in the preparation of enamels. Such colours are absolutely stable on exposure to light and air, as none of the modifications of cadmium take up oxygen at ordinary temperatures. In addition to use in metals, solder and solder-filled jewellery, cadmium is used as a stabiliser in certain plastics and may be used as a pigment in crystal, glass, ceramics, enamel or plastics, or in paint and surface coatings.

Scrap
With such widespread use the jewellery supply chain needs to audit and review its practices in respect of cadmium. Its prevalence is apparent when gold and sterling silver scrap is melted. While levels are generally low, below 300 ppm for 95 per cent of the time, cadmium may be present at levels of 1,000 ppm and occasionally very much higher in both gold and silver melts.

How will articles be tested for cadmium compliance?
The Analytical Laboratory has a variety of techniques available for measuring the concentration of base metals in jewellery. Most popular are: X-ray Fluorescence Spectrometry (XRF); Analysis, Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES); and Atomic Absorption Spectroscopy (AAS).

X-ray Fluorescence Testing (XRF)
This technique is reasonably accurate for measuring cadmium content but, as a surface analysis method, the presence of coatings on the tested item can interfere with the detection of metals below. As many costume jewellery articles have some form of coating this could result in an inaccurate analysis. XRF method sensitivity also depends on the composition of the alloy, the shape and size of the object and the calibration models available for use.  Assay Office Birmingham proposes that a proper validation of testing jewellery articles to the new cadmium levels using XRF be undertaken before making a final decision on the suitability of XRF for accurately determining the concentration of cadmium in articles. XRF could certainly be used for initial screening of articles and quick measurements of cadmium content before a more accurate (and also more expensive) method such as ICP-OES could be used. If it is not necessary to determine the concentration of the metal but merely to confirm its presence, XRF is definitely a viable option.

Atomic Absorption Spectroscopy AAS
Atomic Absorption Spectroscopy (AAS) has also been suggested as a possibility for determining cadmium content; a detection limit of 0.005 mg/l for cadmium has been suggested.

ICP-OES
ICP-OES is a destructive technique that is capable of reporting several elements simultaneously. The turnaround times are slightly longer than the XRF method of analysis, however the accuracy levels are far superior.

This method can only be used by (accredited) laboratories. When a sample of the jewellery article is dissolved, the method can find even trace elements; for metals the results can be at the parts per million. An estimate for the detection limit is 0.001mg/kg so this method is currently the recommendation to ensure that products comply with the new regulations.

 

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