Platinum percentages - Weight versus Volume

[Steve in Arizona]

Am I correct in my assumption that the platinum alloy in rings marked “950” indicates that the ring’s total weight is a combination of 95% platinum and 5% copper or other metal. Does anyone know what the percentage of the volume of a platinum ring is actually platinum? I am wondering if we are all buying copper rings with a little platinum added for a shiny gray color.

Thanks, Steve

 

[JohnQuixote]

Date: 5/19/2006 1:36:25 PM
Author:Steve in Arizona
Am I correct in my assumption that the platinum alloy in rings marked “950” indicates that the ring’s total weight is a combination of 95% platinum and 5% copper or other metal. Does anyone know what the percentage of the volume of a platinum ring is actually platinum? I am wondering if we are all buying copper rings with a little platinum added for a shiny gray color.

Thanks, Steve

Platinum 950 indicates 95% by weight. The different metals that are alloyed with platinum have differing weights, so the actual volume of platinum in a platinum-ruthenium alloy, as opposed to platinum-iridium is variable. For fine jewelry platinum is most commonly alloyed with those 2 metals.

Cobalt is sometimes used because it melts faster and flows better (for example, this alloy might lend itself to a basket-type head). In South Africa, the copper alloy you mentioned is used for similar reasons to cobalt. However, the drawbacks of Pt-5%Cu is that is has a brownish-grey look and is more susceptible to scratching.

Ruthenium and iridium platinum alloys are by far the preference for hand-working and machining - they are also both in the platinum family of metals. The other metals in that family are palladium (used mostly for gold alloys to produce white gold), rhodium (used as a plating finish for jewelry and silverware) and osmium, which is as dense as iridium but not associated with fine jewelry as it oxidizes easily.

A search of PS will turn up prior threads with good supplemental info.

 

[Steve in Arizona]

Thanks for the reply. Does anyone have an idea of the approximate percentage by volume of platinum in a platinum ring.

 

[denverappraiser]

It will depend on your alloy and what the other metals are made of. If you really are interested in being precise you will find it varies with temperature as well. As John points out, the alloying is done by weight, not volume. If you know the recipe of the metals you can use the specific gravity of the various components to work out a conversion formula. With your copper example, platinum is a little less than 3 times the density of copper.

Neil Beaty
GG(GIA) ISA NAJA
Professional Appraisals in Denver