Diamond tempshift plate slow temperature transfer at around 1000 C

Hello,

I run on  build 4788137

First thank you for the amazing game!

Game save is joinedFortress Cycle 1416.sav

I am doing a rather simple oil boiler to get petroleum using magma from the volcano. 

When magma, or igneous rock is ~ 1700ºC until ~1100ºC the heat exchange between the diamond glass walls and the boiling chamber works great.However I noticed that heat exchange significantly drops when temperature of the the formed igneous rock tiles are inside [1000ºC, 1100ºC] 

Then heat exchange just works great after the temperature drops under 1000ºC

I will try to reproduce in sandbox mode if possible and try to add more information.

Regards

Fortress Cycle 1416.sav