The reason why the character 柴 (brushwood) is used for saitogoma (柴燈護摩) of the traditional Shingonshu sect Tozan school is that they made a ceremonial mound with brushwood and firewood because they could not decorate even esoteric tools due to the ascetic practice in the mountain.
伝統的な真言宗系当山派の柴燈護摩に柴の字が当てられているのは、山中修行で正式な密具の荘厳もままならず、柴や薪で檀を築いたことによる。