What is "Kama-yaki", the technique of rust prevention for Nambu cast iron kettles (Tetsubin)?

In the year 1884, a significant fire engulfed the city of Morioka, during which iron kettles were buried. Strikingly, these kettles underwent transformation and exhibited increased resistance to rusting. It is recounted that artisans of that era recognized this unique attribute and subsequently adopted it as a rust prevention technique. This marked the genesis of "kama-yaki," a distinctive trait of Nambu iron kettles.

Upon removal from the mold, subjecting an iron kettle to high temperatures of 800 - 1,000 degrees Celsius leads to the formation of a protective layer known as the "oxide layer." This layer is colloquially referred to as "Kuro-sabi" or "black rust" in Japan. Despite its name, this layer isn't intensely black; rather, it constitutes a diverse form of rust. Upon the commencement of kettle usage, the likelihood of red rust developing on the surface diminishes significantly.