Platinum

Platinum is a popular precious metal for fine jewellery, as it is the perfect setting for a large, good quality diamond. Dippal Manchanda, technical director at Assay Office Birmingham, outlines its main characteristics.

Platinum was officially recognised as a precious metal for fine jewellery in 1975 when the new Hallmarking Act introduced the requirement that all articles over 0.5 grams be hallmarked. After a slow start with less than 150 articles hallmarked in the first 10 years, the advent of the marketing initiative Platinum Guild International (PGI) finally pushed volumes into the thousands in 1985.

Steady growth has continued, and PGI has been able to exploit the opportunities presented by the trend for white metal and higher consumer disposable income and establish platinum as a much sought-after status symbol.

Platinum’s peak year by volume was 2004, when 364,000 articles were hallmarked in the UK. Platinum prices have continued to rise since the 1980s, driven not by the jewellery trade but by other industrial sectors where platinum is used – most particularly the automotive industry, where platinum is a crucial element in catalytic convertors. Platinum prices have been extremely volatile since 2008, and have been hovering around £1,000 per ounce this year, hitting a high of £1,173 in May 2010. The rise in the price of gold means that the price differential between the two metals is narrowing, but platinum retains its superior status, partly due to the worldwide marketing efforts of the Platinum Guild, which is still very active in the UK, whereas both the World Gold Council and the Diamond Trading Company have abandoned promotion within the UK market as it is deemed too small to be cost effective. Platinum in its pure state is very soft and cannot be used for jewellery. So, as with all other precious metals, it is alloyed to make it harder. Recognised international standard finenesses for hallmarking are 850, 900,950 and 999 parts per thousand, but almost everything hallmarked is 950.

The specific composition of the alloy will vary according to the requirements of the manufacturing method. Standard 950 platinum means that 95 per cent of the weight is made up of fine platinum, but other platinum group metals or cobalt may make up the remaining five per cent for good technical reasons. Iridium and rhodium raise the melting point and arguably make the alloy whiter still. Cobalt lowers the alloy melting point, improves fluidity in casting and makes the alloy very slightly greyer. Palladium also lowers the melting point and makes the average density lower. Density is significant as platinum itself is exceptionally dense, at 21.45 grams per cubic centimetre. The five per cent ‘non-platinum’ weight in the alloy may therefore represent more than 10 per cent of the volume and have a significant impact on the behaviour of the metal under manufacturing conditions. Platinum (21.45 gms/cm3) is nearly double the density of palladium (11.9 gms/cm3) and more than double that of cobalt (8.71 gms/cm3) meaning the ‘other’ metals have a disproportionate effect on the alloy.

Manufacturers therefore need to select their platinum alloy carefully. A common choice for universal needs, such as casting, fabricating and die striking, is 950 parts platinum alloyed with 50 parts ruthenium, another platinum group metal. This hard, malleable and ductile alloy has a good clean colour and is hard at 130HV, making it durable and capable of taking on a high lustre when polished. It is used by many high-end manufacturers, and most of the machined wedding bands sold in the UK and US are made from this alloy.
For hand fabrication, 950 parts platinum and iridium is a better choice, as the metal is workhardened by hammering and rolling it. Pt900/ iridium is good for casting, as Pt950/iridium may be too soft and the piece of jewellery will scratch, bend and dent easily, but as the 950 standard is the general expectation, this is not widely used in the UK.

When manufacturing, jewellers need to be aware that carbon is the enemy of platinum and the two elements should be kept firmly apart to avoid creating porosity in the alloy. Carbon is present in oxy-acetylene gas and torch melting of platinum should always be carried out using hydrogen/oxygen to avoid this problem.

The Laboratory is currently melting and assaying large volumes of platinum for customers, and is careful to use carbon-free white crucibles to avoid any contamination of the platinum. Melting is carried out in an induction furnace, which can hit temperatures of 2,200 degrees C, although most platinum alloys will melt at 1,700 degrees C. As with gold and silver, the Laboratory will take a ‘dip sample’ from the molten metal to assay to ensure that the sample tested is homogeneous and representative of the whole bar.

The popularity of white metal continues, and Assay Office Birmingham expects to see a continuing steady volume of platinum for hallmarking and, subsequently, melting.