the concentration of clf 3 is so high the cleaning operation is liable to be accompanied by considerable reaction of clf 3 with carbon to result in the formation of an interlayer compound in carbon or graphite body and it is difficult to prevent the unwanted reaction by suitably setting cleaning conditions besides use of an unnecessarily large quantity of clf 3 gas is unfavorable for the economy of the cleaning operation the cleaning operation is carried out at a temperature in the range from about 200 u00b0 c to about 300 u00b0 c if consideration is given only to the cleaning efficiency it is favorable to raise the cleaning temperature however as the cleaning temperature exceeds 300 u00b0 c the reaction of clf 3 gas with carbon is gradually promoted at relatively high temperatures the reaction of clf 3 with carbon forms poly carbon fluorodie called graphite fluoride which is a powdery substance so that a change occurs in the surface state of the carbon member subjected to cleaning also when the cleaning temperature is lower than 200 u00b0 c the reaction of clf 3 with carbon becomes significant in this case the reaction product is an interlayer compound which is a powdery substance and hence changes the surface state of the carbon member.