a method of producing a zinc oxide varistor for high voltage which is a voltage - current non - linear type of resistor including zinc oxide as the major ingredient and bismuth oxide or manganese dioxide as additives for use with a circuit protector and gapless arrester . the method of the invention includes sintering a zinc oxide composition lacking bismuth oxide at 1200 °- 1350 ° c . under conditions and for a period of time effective to form a primary sintered body , heat - treating a primary sintered body coated with an amount of metal oxide paste containing mostly bismuth oxide at a temperature of 1000 °- 1200 ° c . for a period of time effective to diffuse the metal oxide paste to the grain boundaries of the zinc oxide grains .