There is disclosed a highly densified, sintered aluminum nitride body having a thermal conductivity in excess of 200 W/m.degree.K. and a strong resistance to attack by hot HC1. There is also disclosed a method of sintering aluminum nitride bodies which employs a variable thermal cycle whereby the body is either heated slowly through a pre-sintering temperature range of 1300.degree. to 1750.degree. C., or held at a temperature within that range for at least a half hour. Optionally, a green aluminum nitride body may be provided with up to 10% by weight of at least one metal fluoride dopant, and may be provided with a sufficient weight to maintain flatness during firing.