"Discoloration" in high purity Al2O3 1

[josephn16]

I am having some troubles with white spots showing up on high purity Al2O3 that is normally an ivory color. The assumption has been that the atmosphere in the kiln is causing the "discoloration", but I have been unable to prove any significant atmosphere differences. Other theories include lower than normal MgO content (since the MgO gives the ivory color), and possible Si contamination. Has anyone had any similar experiences?

Most of the time, the ivory color can be returned to the parts by using UV light.

 

[JimMetalsCeramics]

Are you sure it's alumina? Mullite is ivory in color.

 

[josephn16]

Positive, we make both.

 

[oleman]

I experienced this 40 years ago. We rationalize it as slight difference in density,i,e difference in light reflectivity (?). One other cause in density is the formation of spinel structure, an alumina - alumina spinel formed in the kiln due to slight ly reducing conditions. This can be proven by X-Ray diffraction and unit cell size measurements. You should resolve if it is chemical or not by Scan Election microscope. Good luck