excess of silicon carbide in said ends over the intermediate zone from the following examples the utility and versatility of the present igniters will be readily apparent a composite billet 10 such as that shown in fig1 measuring approximately 5 inches in length 16 0 9 inch in height 18 and 0 45 inch in thickness 17 was hot pressed in a graphite mold assembly with the top plunger of mold removed the cavity therein was about 3 4 filled with a finer than 10 micron mixture made up of 60 by weight of silicon nitride and 40 by weight of silicon carbide and the powder spread level on top of the foregoing was spread a finer than 10 micron mixture of 70 by weight of silicon nitride and 30 by weight of silicon carbide the top plunger was put in place and the powders were hot pressed at 1775 u00b0 c for 60 minutes referring to fig1 the 70 30 layer is indicated by 12 and the 60 40 layer by 14 the layers are integral with each other forming a monolithic structure the billet of fig1 was then sliced into blanks 20 shown in fig2 measuring 0 9 inch on the side 28 0 45 on the side shown as 27 and 0 125 on the side 26 the 70 30 compositional stratum is shown as 22 and the 60 40 section as 24 the blanks 20 of fig2.