I have heard of this, but, I haven’t seen it in any burner factories.
The story goes that they heat at a low temperature for a long time which doesn’t melt the rutiles in the sapphires, but still gives them more colour, so, they still certify as unheated.
FOLLOWING IS OPINION ONLY
I believe that this story was started outside of Sri Lanka to account for the seemingly meteoric rise in GIA certified unheated sapphires.
I believe the truth is, there are just as many unheated sapphires as there always were. Some sapphires can’t be heated because gas bubbles will expand and crack the stone. (although the new high pressure, high heat seems to have solved this problem)
Because of the massive change in the industry, everyone is certifying everything these days, I certify some stones that I sell as cheap as $250-$300.
Since there are so many more stones being certified, there are many more stones being certified as unheated.
Funny enough, my favourite broker in Ratnapura always tells me EVERYTHING is heat treated. (because he knows it will come back if it is said to be unheated and is heated)
ALL THIS SAID: I can be 100% wrong and people have systems like this running outside of burner factories.