18 KT Gold Expansion Properties and memory
18 KT Gold Expansion Properties and memory
(OP)
Is it possible for 18kt jewelry shipments to damage stone settings as a result of being potentially shipped at -18.0 degrees C in the aircraft cargo hold. The theory is that diaomond mountings contract - gold may have no memory - and on arrival are loose. Can anyone comment if the theory is in a realistic way a potential cause of loose diaomond settings?





RE: 18 KT Gold Expansion Properties and memory
But for -18.0 oC, probably not.
The linear CTE of pure gold is 14 ppm/oC, while that of diamond is only 1 ppm/oC. If the 18-kt gold alloy has been heavily cold worked (work hardened) during fabrication, and if its CTE is as high as that of gold, its elastic limit may be exceeded as its cooling shrinkage is prevented by the diamond. You need the physical properties of the particular 18-kt gold alloy & some knowledge of the setting's fabrication, plus the geometry of the setting, to figure this out.
Argonne National Lab. did a study (Y. Jaski & D. Cookson, 2006) in which a thin (0.2 mm), circular diamond window was brazed between 2 copper washers. Due to difference in CTE, cooling from the 250oC brazing temperature crushed the thin diamond wafer. To avoid this, the copper washers were reduced in thickness to that of the diamond. Thereupon, the copper deformed w/o fracturing the diamond. The study doesn't mention whether the copper yielded plastically as well as elastically.
In your case, the diamond is a solid stone that won't easily fracture, especially in compression. So, your premise is plausible although w/o more data I'd guess the ΔT is too small.
RE: 18 KT Gold Expansion Properties and memory
RE: 18 KT Gold Expansion Properties and memory
However, the CTE of tourmaline (http://prola.aps.org/abstract/PRB/v38/i14/p10072_1) is midway between diamond & gold,
so the thermal stress would be less. An average CTE of tourmaline is about 6 x10-6/oC, so for a 1 cm length and ΔT = -53oC,
Δlength (tourmaline) = 3.18 μm,
Δlength (gold) = 7.42μm
If the stone is a solid chunk & the gold alloy is thin, the ΔCTE x ΔT gives a strain of 0.04% in the gold alloy. Don't know the elastic limit of your alloy, but it's maybe 0.2%. In this case, only get the plastic deformation necessary for the loosening effect if the gold alloy has been heavily work hardened or is already under considerable tensile stress. Some pre-existing tensile stress is likely; I suspect jewelers use it keep the stone tight.
One needs the stress-strain curves for tourmaline & 18-kt gold, and any initial cold work & pre-existing stress in the 18-kt gold, in order to do the calculations.
RE: 18 KT Gold Expansion Properties and memory
You could test this. Take a few rings that you know to be tight. Then put them into a freezer. My deep freezer at home runs at 0F (-18C). Leave them overnight and see what happens.
Are you only seeing this with some styles? It may be related to the way that they are manufactured and mounted.
I think that an unheated cargo hold can get a lot colder than -18C. I recall that in the range of 36,000-40,000 ft the air temp is more like -70F (-55C). It has been a long time since my last international trip so I might be wrong.
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RE: 18 KT Gold Expansion Properties and memory
RE: 18 KT Gold Expansion Properties and memory