is the plug 10 for purposes of comparison but not in accordance with the present invention additional spark plugs identical with the plug 10 except that in one instance talc was substituted for the seal according to the present invention and in another instance a mixture of 41 percent talc 40 percent aluminum and 19 percent silicon was substituted for the sealing material according to the invention spark plugs with the talc seal and spark plugs with the talc aluminum silicon seal were subjected to the gas leakage test described above with the results plotted in fig3 it will be noted that after having been subjected to temperatures above 1300 u00b0 f the plugs with the talc seal and also the plugs with the talc aluminum silicon seal had undergone catastrophic failure insofar as gas leakage is concerned spark plugs 10 were also subjected to a different test to determine holding power of the seal 15 this test involved heating plugs to various temperatures holding them at temperature for a period of twenty four hours allowing them to cool removing the terminal 20 and then ascertaining the force in pounds exerted in the direction of an arrow designated 28 in fig1 required to move the electrode 14 relative to the insulator 3 the maximum force applied was 500 pounds the results of this test are plotted in fig2 from which it will be observed that a force of at least substantially 300 pounds was required to cause such movement of the electrode 14 in spark plugs 10 which had the seal according to the present invention after heating for twenty four hours at temperatures ranging from 900 u00b0 f to 1400 u00b0 f for purposes of comparison but not.