A process for treating a luminescent halogen-containing material includes contacting the luminescent halogen-containing material with an atmosphere comprising a halogen-containing oxidizing agent for a period of at least about two hours. The luminescent halogen-containing material has a composition other than (i) Ax[MFy]:Mn4+, where A is Li, Na, K, Rb, Cs, or a combination thereof; M is Si, Ge, Sn, Ti, Zr, Al, Ga, In, Sc, Y, La, Nb, Ta, Bi, Gd, or a combination thereof; x is the absolute value of the charge of the [MFy] ion; and y is 5, 6 or 7; (ii) Zn2[MF7]:Mn4+, where M is selected from Al, Ga, In, and combinations thereof; (iii) E[MF6]:Mn4+, where E is selected from Mg, Ca, Sr, Ba, Zn, and combinations thereof; and where M is selected from Ge, Si, Sn, Ti, Zr, and combinations thereof; or (iv) Ba0.65Zr0.35F2.70:Mn4+.