Possible reasons for discolorations on the zirconia restauration and how to prevent them 

Due to various reasons, during and prior to the sintering process white spots or discolorations on the zirconia structures can occur. However, certain measures can be taken to prevent these unwanted discolorations.  
Problem: A possible reason for white spots on your work might be small quartz particles, which can chip off the heating elements from time to time and remain lying on the zirconia works.
Solution: Vacuum the combustion chamber to eliminate eventual residues of quartz particles. To protect your works, use either a ceramic hub or a sintering shelve, on which you can place a firing tray or additional sintering shelve as a cover.
In this way, quartz splinters can no longer fall onto the structures.
Problem: Touching the restorations with sweating or greasy hands during or prior to coloration can cause sealing of the pores of zirconia works and prevent the Colour Liquid from penetrating into the zirconia. This can lead to brightening of the zirconia structure.
Solution: Use gloves for handling unsintered zirconia crowns and bridges.