It was also used as a metaphor that captured a characteristic of iron, 'easily heated but cools down just as easily,' because high thermal conductivity is one of the characteristics of metals.
また、金属の特徴の一つでもある熱伝導率が高いことが、いわゆる「熱しやすく冷めやすい」という鉄の特徴を、捉えた比喩としても用いられた。