へー、天然黒鉛は不純物として遷移金属元素を含んでてそれが酸化促進させるので、純度を上げ2800℃以上で加熱処理する事で不燃性を持たせているのね

> The oxidation of graphite by air was enhanced by the impurities in the graphite such as Co, Ni and V.

Air Oxidation Behavior of Carbon and Graphite Materials for HTGR, Haruo Kawakami,
https://doi.org/10.7209/tanso.1986.26