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Platinum Metals Rev., 1975, 19, (4), 155

The Melting of Iridium

A Historical Note

  • J. C. C.
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Recent developments in the application of iridium as an electrode in high-duty sparking plugs lend interest to a little-known paper published over ninety years ago entitled “The Fusion, Casting, Dephosphorising, and Plating of Iridium, together with a Bibliography of the Subject”. This was written by Nelson W. Pevry, and was published in three parts in issues of Chemical News, commencing on January 2nd, 1885.

The paper records efforts made by a succession of inventors in America over a period of more than 40 years to develop melted iridium as a substitute for the natural-occurring mineral iridosmine (or osmiridium) for the hard-wearing tips of gold nibs for fountain pens. These were known as McKinnon nibs.

The earliest experimenter in this field is said to have been G. W. Sheppard, who died in 1862, when his business was taken over by John Holland. Six years later Holland accepted a contract to supply tips for a new type of nib which required a centre-drilled “iridium” tip. Being unable to secure sufficient grains of iridosmine of the size needed, and under a threat of being sued for breach of contract, Holland offered 1000 dollars in 1868 to anyone who would fuse for him a mass of 1 ounce of iridium. With no solution forthcoming, and observing the fluidity imparted to iron by phosphorus, he tried the effect of phosphorus additions to iridium and, according to the paper, “made half an ounce at the first go”.

On May 10th, 1881, Holland patented in America the fusion of iridium with phosphorus and about this time formed the American Iridium Company of Cincinnati, with Professor W. D. D. Dudley as general manager. The method used for making pen points was to fuse iridium with phosphorus and pour the alloy between two hinged iron plates which were closed to give a slab inch thick. This was broken up and the pieces soldered to a brass strip for grinding to shape with corundum or diamond, countersinking with a diamond drill, and piercing. Later, Professor Dudley suggested that the phosphorus be removed by heating the alloy on a bed of lime.

This account of activities in the New World brought a curt letter from Johnson Matthey & Co Ltd of London in reply, commenting that some of their staff carried a record of preparing iridium-phosphorus as far back as 1837. And the editor, Sir William Crookes, added a footnote that in April 1882 Mr Sellon, the Chairman of Johnson Matthey, had presented him with a beautiful specimen of iridium sheet.

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