Fire assay to determine Au/Pt content in Au-Pt alloy

[Sy-Spark]

How we can determine gold content or platinum content in alloy contains both metals using fire assay method, in another word, how old jewelers dealt with Au-Pt alloys before XRF released specially when alloy has low Pt content so can't be recognized by color.

 

[Yggdrasil]

How we can determine gold content or platinum content in alloy contains both metals using fire assay method, in another word, how old jewelers dealt with Au-Pt alloys before XRF released specially when alloy has low Pt content so can't be recognized by color.

I'd guess they used other spectrographic methods.
Or combinations of different refining techniques along their good scales/balances.

 

[4metals]

A dore bead resulting from cupellation was classically re-digested in aqua regia, the excess nitric is removed, the gold is dropped, preferably with ferrous sulfate, and the Platinum is dropped with saturated ammonium chloride to come down as ammonium chloroplatinate. The salt has to be rinsed in a dilute ammonium chloride rinse and dried in a tared funnel in a drying oven, cooled in a dessicator to avoid re-absorbing atmospheric moisture, and weighed to determine the Platinum content.

 

[Sy-Spark]

A dore bead resulting from cupellation was classically re-digested in aqua regia, the excess nitric is removed, the gold is dropped, preferably with ferrous sulfate, and the Platinum is dropped with saturated ammonium chloride to come down as ammonium chloroplatinate. The salt has to be rinsed in a dilute ammonium chloride rinse and dried in a tared funnel in a drying oven, cooled in a dessicator to avoid re-absorbing atmospheric moisture, and weighed to determine the Platinum content.

But standard fire assay doesn't use aqua regia only nitric acid, am asking cause here in my local (jewelry main market) they only use fire assay without any kind of xrf analysis, in case there is few percents of platinum in the alloy they will not recognize by color or density, so I think anyone can cheat them since gold price more than 3 times platinum price.

 

[4metals]

But standard fire assay doesn't use aqua regia only nitric acid

True but the presence of Platinum requires additional steps if you want to quantify it. The cupellation to produce a doré bead is standard but to separate the Gold from the Platinum aqua regia is needed.

 

[4metals]

You should note that in a fire assay if the Silver added exceeds 10 times the Platinum, the Platinum will follow the Silver into the parting and not be weighed as gold. If the Platinum is high enough to remain with the Gold it will be visible when the coronet is annealed.

 

[Sy-Spark]

You should note that in a fire assay if the Silver added exceeds 10 times the Platinum, the Platinum will follow the Silver into the parting and not be weighed as gold. If the Platinum is high enough to remain with the Gold it will be visible when the coronet is annealed.

I think this is the trick, really thanks for your knowledge