When Yellow Gold Is Not Yellow

 
A recent problem came up in production with 18kt yellow gold jump rings. The wire department supplies coils of 18kt yellow gold for jump ring production, but the jump rings produced from a recent wire delivery were a different color than expected. The jump rings should be a nice deep yellow color, but after using the recent coil, the jump ring machine operators were complaining that the jump rings produced were “greener” than usual – a completely different color. Comparing stock to the newest jump rings confirmed this. Was there a problem with the wire supply and had the second coil been made from a different alloy by mistake? Had the melt make-up when prepping for casting the rod been incorrect giving the wrong composition? I needed to investigate this further to determine the root cause.
 
As with all problem solving, a logical approach is best. Tick the most obvious off the list then go from there. I started off by doing an XRF analysis on both “good” and “bad” jump rings to see if there was anything obvious with differences in composition. The stock jump rings of the correct color measured spot on at ~ 75% gold, 10% silver and 15% copper. On to the latest jump rings and something was obviously amiss. The XRF gave results of ~ 85% gold, 14% silver and 1% copper! What was going on here and how did we miss this in QC?
 
First of all, we don’t make an alloy with 85% gold in it. Secondly, this should have been picked up in assay control if a mistake had been made when mixing the alloy. All material produced must pass assay before being released to the production floor. So again, what was going on? What we must remember is that XRF is a surface technique. The X-rays only penetrate the surface by microns, so was it possible that the latest batch of wire, when being processed into jump rings, was in some way heavily oxidized during processing. This would mean the jump rings produced would need pickling. If this was a “heavy” pickle to clear the oxide and residual pink surface, there is a risk that the copper from the surface would be removed to a point that a gold + silver-rich surface layer remained….which is green gold!  To quickly check this hypothesis, I abraded some of the surface from the jump ring and tested this area. Bingo! Back to ~ 75% gold, 10% silver and 15% copper. My guess was correct. It turns out that the jump rings had indeed been oxidized in processing – a cover gas issue with a belt furnace. This meant they needed to be heavily pickled to remove the oxide and pinkish copper-rich surface. The pickling process had removed the surface copper, increasing the relative gold and silver content significantly. To all intents and purposes they had depletion guilded the jump rings.
 
Problem solving is not about jumping to conclusions (pun intended). Just start simple, let it lead you down a logical path and the cause and resolution should eventually become evident. In the words of one of the world’s greatest – if fictional – detectives…..Elementary, my dear Watson! (Pun again intended!)
 ~Stewart Grice.

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