A correspondent wrote to me (not an exact quote): a quick look a the literature scrub shows transmissivity of alumina quickly drops to zero past ~7 micron wavelength. Much of the data seems to be in relation to the crystalline variant (e.g sapphire) vs non-crystalline.
http://www.janis.com/Libraries/Window_Transmissions/Sapphire_Al2O3_TransmissionCurveDataSheet.sflb.ashx Notice the Ecat report clearly states on page 4 “The spectral range for both cameras is from 7.5 to 13 μm.” That seems to indicate that any transmitted IR through the alumina wouldn’t be detected by the IR cameras.
