Shattering CZ

[Orange Pirate]

I'm having a bit of trouble with some CZ shattering. It was bought as lavender but I am fairly sure it's a color change of sorts - Lavender to wishy washy blue.
It's happened twice in a row. Both were large cuts, one 10mm the other about 12mm square.
Both times polishing the largest facets on the pav. I'm polishing using 100k on a matrix, prepolish 8K on copper.


The first time I thought it was because of heat generation from applying too much pressure. But the second time I was being fairly conservative with pressure, less than I'd prefer. I don't think the lap is running to dry. I'm using the drip (normally with the matrix I use the spray bottle), admittedly a slow drip. The stone doesn't feel hot.
I did notice it would sometime scratch for no apparent reason. Indeed, on the facet that exploded on the second stone, deep scratches (One large one evident in the image.) kept appearing on that facet yet did not effect a following facet





Any thoughts. I am thinking low quality cheap CZ? Anybody else have any thoughts.


Cheers
OP

[MakkyBrown]

Wow, is your lap ok? That big cz heart I cut I could feel the heat in my fingers when polishing. I thought cz was fairly heat resistant.

[MakkyBrown]

It must of been the design, you know if you were cutting that square design I did for you it wouldn't have exploded 

[Orange Pirate]

Wow, is your lap ok? That big cz heart I cut I could feel the heat in my fingers when polishing. I thought cz was fairly heat resistant.

That's what I thought. I even checked weather it was heat resistant after the first attempt, which it is supposed to be.
I seem to remember something about a quality difference between Yttrium stabilized and Calcium stabilized.
Also annealing can be a problem and is common with cheap CZ.


But I don't know if any of that is true.

I must of been the design, you know if you were cutting that square design I did for you it wouldn't have exploded 

If I had some CZ left I'd test that theory. Guarantee and cast iron are coming to mind. 


It was supposed to be another go at the 'Robin' cut by John Wren but in lavender because it looks awesome in that color. I want one.

[MakkyBrown]

You look to have a good bit of thickness left on the crown you could almost recut or do the cracks run deep. Might be ok now the stress? has gone out of it. Were you annealing it? are you meant to anneal cz I never have on the few bits I've cut.

[FlashGP]

I haven't heard of having to anneal rough before cutting it.  Do you have to do tgis with much rough?

CZ can withstand casting temperatures for silver
  One of our club members experimented witb embedding CXs in her mould before cssting.  We had to preheat the mould a little to reduce the thermal shock.  Likewise they don't  like being quickly cooled from 5 or 6 hundred degrees.

Regards
Gordon

[Orange Pirate]

The annealing occurs during manufacturing. -

It can be brittle, especially if the annealing process has been hurried or gone wrong which is often the case when ‘bargain material’ is offered. This brittleness can cause chipping on sharp edges such as keels and acute girdle angles as can be found for instance on trillions and marquise shapes.

http://facetingacademy.com/UKFCG/CubicZirconia.pdf


And it seems to cause only brittleness. The same goes for the stabilizer - Calcium or Ytrrium.


I can only put it down to 'thermal shock' as Flash has suggested. Perhaps because I was using a drip where the flow was intermittent. Maybe the facet being so large just got heated a lot quicker, became momentarily dry and the sudden drip cooled it too quick. I felt the 'pop' between my fingers.
But I'm still surprised as after the first one cracked I was taking it a lot easier.


Third time lucky. Just need some more lavender CZ. 


Cheers
OP

[Orange Pirate]

You look to have a good bit of thickness left on the crown you could almost recut or do the cracks run deep.

It's a mighty deep crack, right through the guts of the stone. There is no second chance for this sucker.