Platinum Iridium Alloy
Platinum-iridium alloys are alloys of the platinum group precious metals platinum and iridium. Typical alloy proportions are 90:10 or 70:30 (Pt:Ir). These have the chemical stability of platinum, but increased hardness. The Vickers hardness of pure platinum is 56 HV while platinum with 50% of iridium can reach over 500 HV. This improved hardness has also been considered as beneficial for use in platinum jewellery, particularly watch cases.
1) 95% platinum (950) - alloyed with 5% iridium - Vickers hardness = 80
Iridium has an atomic weight of 192.217 - pretty close to that of platinum.
We'll need just over 5% of the VOLUME of our platinum - in iridium - to make our "950" blend.
This is a very soft alloy - is extremely malleable - Setting stones is the least risky in this alloy. Dents and dings occur easily. Is resistant to wear and abrasion but not resistant to scratching, bending and being deformed and dented by impacts. This alloy works extremely well for both casting and hand fabrication. It just doesn't age very gracefully.
2) 95% platinum (950) - alloyed with 5% ruthenium - Vickers hardness = 135
Ruthenium has an atomic weight of 101.07 - a bit over HALF the weight of iridium
By VOLUME, we'll need almost twice as much ruthenium to equal the weight of the iridium in the above "950" iridium example.
This alloy is fairly hard - resists dents and dings well - is less malleable - is resistant to wear and abrasion but prone to cracking and requires more pressure to be imposed on a stone during the setting process than the softer alloys. When it doesn't crack, it features all of the longevity that platinum is renowned for. This alloy works reasonably well for both casting and hand fabrication - when it doesn't crack. This is a very good alloy for machine cut parts.
3) 95% platinum (950) - alloyed with 5% cobalt - Vickers hardness = 135
Cobalt has an atomic weight of 58.933200 = roughly only 30% of the atomic weight of platinum and iridium.
When we consider the VOLUME of this element required to equal 5% of the weight of our platinum, we need almost 3 1/3 times the volume of our iridium to generate 5% of our platinum by WEIGHT.
This alloy is fairly hard - resists dents and dings well - is not reasonably malleable or workable at the bench - is much less resistant to wear and abrasion than the iridium and ruthenium alloys - lacks longevity because of it's LACK of resistance to wear and abrasion and will require that more pressure be imposed on a stone during the setting process. This alloy works extremely well for casting. This metal polishes quickly because polishing is "abrasion" and the alloy doesn't resist abrasion as we'd otherwise expect platinum to do. This alloy features a natural "bluish" color cast due to the high volume of cobalt. Rhodium plating is often employed to hide it's native color. This metal it is magnetic. You can actually pick up a piece made of this metal with a strong magnet. Welding and hand fabrication have proven to be problematic for bench jewelers / repair technicians.
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