18 KT Gold Expansion Properties and memory

[NPHX]

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?

 

[kenvlach]

If it were taken to -180.0 ^oC, I'd say yes.
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 250^oC 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 [Δ]T is too small.

 

[NPHX]

Thanks for the reply. I talked it over with the end client and they say microns is the measure that make the stone loose - unsurr how PPM would equate to a 53 degree celcius change in shipment. They also said the stones are tourmaline which too seem to have some piezioelectric and unique expansion qualities that are temperature related. (

http://www.minsocam.org/msa/ammin/TOC/Articles_Free/1995/Hawkins_p491-501_95.pdf

)

 

[kenvlach]

ppm is short for 10^-6 unit/unit, so a 1-meter long diamond with a CTE of 1 ppm/^oC would expand 1 micron for each 1^oC increase in T.

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 [Δ]T = -53^oC,
[Δ]length (tourmaline) = 3.18 [μ]m,
[Δ]length (gold) = 7.42[μ]m
If the stone is a solid chunk & the gold alloy is thin, the [Δ]CTE x [Δ]T 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.

 

[EdStainless]

I am with Ken, it sounds like too little of change in temperature to get the effect you are seeing.
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.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[NPHX]

Thanks for all your help. There has been quite some effort in ensuring quality and in this last shipment there was an additional final inspection. With tourmeline having a piezioelectric quality maybe its package xrays that excite it to vibrate at some level.

 

[ornerynorsk]

It doesn't sound plausible for thermal to be affecting it in this way. You're sure everything's tight when shipped? Sorry, just stating the obvious. Is there anything funny going on with treatments of the stones to "enhance" carat weight or coloration, that might be affected by atmospheric change? Just a thought.