Platinum Purity 

I use "Classic" 90% platinum / 10% iridium in my work (PT 900)

 

I get a lot of requests for "950" platinum.  I've been asking those who request "950" platinum - which one? - there are several -  and what benefit do you desire from your choice of alloy?

The answers that I've received include vague mentions of "purity" as well as hardness and durability. I'd like to address some of these issues at this time.

Precious metals are generally mixed with other metals to establish their rating, karat, or numerical designation by weight. In "950" platinum, 95%  of the weight of the net  alloy will be platinum mixed with 5 % of another metal - by WEIGHT. Those who are familiar with the "periodic table" remember that each element has it's own specific gravity or mass. This means that some elements are "heavier" than others. A 1" cube of one material will weigh more or less than another of equal volume.  As an example of how the various weights per unit of different materials can influence an alloy or mix of different elements, please try to picture the following example in your mind's eye. 

Picture a shopping bag full of feathers and imagine the weight of this bag full of feathers. You can pack the bag any way you choose  - it's your mind's eye, after all. Just think about this for a second.

Now, please try to picture an identical shopping bag - but this time it's full of rocks - granite rocks, about fist size or smaller will do for this example.

Both bags are the same size. Which bag do you feel will be heavier? 

If you think that the bag of rocks will be heavier than the bag full of feathers, then I believe that I'll be able to clear up some misconceptions regarding the "purity" of "950" platinum for you.

As I mentioned above, different elements have different atomic weights. There are four commonly used platinum alloys and many more that see less usage. Let's discuss the top four platinum alloys because these are the ones that you are most likely to encounter as you research your choices.  For the record, platinum has an atomic weight of 195.078.


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.


4) 90% platinum (900) - alloyed with 10% iridium   "Classic Platinum" - Vickers hardness = 110

Again: Iridium has an atomic weight of 192.217 - pretty close to that of platinum.

10%  iridium, by volume, is a touch over the volume of ruthenium required to equal 5% of our platinum. 

By this measure (volume), 10% iridium platinum is only a bit less "pure" than 5% ruthenium platinum. 5% cobalt is miles away from being as pure as either.   

This alloy is of upper-medium hardness - resists dents and dings well - is reasonably malleable - and requires less pressure to be imposed on a stone in the setting process than the harder alloys - is about as resistant to scratching as the 950 ruthenium blend and features all of the longevity that platinum is renowned for. This alloy works extremely well for both casting and hand fabrication. This is my platinum alloy of choice. 

 


I believe that the desire for "950" platinum is primarily driven by the fact that 950 is a bigger number than 900 - it  looks better in "print" - so it must be "better". As you have read above, the "950" designation doesn't really mean anything with regard to the "purity" of the metal. The most "pure" of the 950 alloys (5% ir) is too soft to serve the  needs of my my clients properly, in my opinion.  

 Many folks request "950" platinum but none that I have spoken with about this have associated any tangible benefit with their request.  "950" just sounds better.

Folks, my reputation, my livelihood and the welfare of those who depend on my financial support are all contingent on my making good choices in my work.

My platinum work is done, specifically, by means of casting and hand fabrication. I believe I have made the best choice of platinum alloy for my method of manufacture. Other methods of manufacture as well as manufacture on a much larger scale than mine will benefit from the use of some of the other available alloys. If my work does not perform as expected, my lifetime commitment to building an honest product is nullified. Do you really think I would make a decision as important as which platinum alloy to use in my work without having given serious thought to my options? In an effort to satisfy my client's requests, I have worked with all of the above alloys as well as a couple of lesser known "950" platinum blends. I have even gone as far as creating my own blend of "950" platinum by mixing iridium and ruthenium in various ratios with a 95% platinum base. I did this to satisfy the "950" content while maintaining the working characteristics of my favored  90/10 iridium mix. In the final analysis, I couldn't justify the added process of my private mix. At the end of the day, the tangible benefits of my private blend - to you, the consumer - were the same as 90/10 IR (900) platinum.  

Respectfully,

Mark Morrell

Thanks to my friend - Bob Lynn - for his input on some of the technical issued addressed above - and for encouraging me to stop working long enough to write this document. 

References:

Information on the periodic table found at  http://www.webelements.com/

Information on the hardness and wear resistance of  platinum alloys found through my personal experience and reinforced in the February, 2004 issue AJM Magazine - in an article written by Jurgen J. Maerz. - Director of technical education for Platinum Guild International, USA.