the present method uses a methane - containing nitrogen gas sintering atmosphere to sinter aluminum nitride to a high transmittance . the methane gas replaces the solid carbon charge material used in prior art sintering methods as the source of gaseous carbon . the amount of carbon in the methane - containing nitrogen gas is easily controlled by varying the partial pressure of methane in the nitrogen gas . in addition , the methane flow is stopped prior to the end of the sintering cycle to prevent darkening of the sintered part .