Martensite crystals are the hardest form of steel that is formed when the steel undergoes heat-treatment/quenching. When polished, its colour looks frosty white, not unlike snow.
The Chinese in the Han Dynasty call them "baijian" or "white hardness"; in the Tang Dynasty, it was commonly known as "shuangxue" or "snow flakes".
According to textual evidence, differential heat-treatment using clay was still practiced in the late Ming and throughout the entire Qing period, though surviving examples of such heat-treated sabers are few...
Click on the pic to see more pics of swords that had undergone differential heat-treatment without using clay.