These all items are made out ofSteatite C-221.STEATITE - C221Basically magnesium silicateImpervious material structureVery good Mechanical StrengthHigh volume resistivity upto 1000 degree C.Low dissipation factorGood dielectric strengthVery popularCartridge Heaters and End Seals are used at the end portion of Thermocouple or Heaters as Insulator.