A particle having a magnesium aluminate core and a fluoride salt coating on the core. The particle has been heated in an oxidizing atmosphere to a temperature in the range of about 400.degree. C. to about 750.degree. C. A method of making a particle by mixing a magnesium aluminate core with a solution of a fluoride salt in a solvent to form a slurry and spraying the slurry into a drying column. The slurry enters the column as an aerosol under thermal conditions that avoid boiling the solvent. The thermal conditions in the column evaporate the solvent as the aerosol moves through the column to form a coating of the fluoride salt on the core while substantially avoiding spalling.