In the Northern Qi dynasty, in the 6th century AD, the urine and fat of draft animals were used for quenching different parts of sword blades. Urine contains salt, and cools the metal faster than pure water, making for a hard, sharp edge; but fat cools more slowly, giving the blade a resilient back which absorbs shocks and does not easily break.
牲尿含有鹽分，淬火時比水冷卻得快，淬火後刀刃鋼質堅硬銳利；用牲脂淬火冷卻慢，因此刀背鋼質柔韌，可以抵銷刃口受到的衝擊，以免折斷。