Another quality of line that's more difficult to achieve is variation in the softness and hardness.
线的另一性质更加难以企及，那便是软硬的变化。
Cast iron manhole cover of good quality is to see its toughness and hardness.
质量好的铸铁井盖是要看其韧度和硬度的。
Generally speaking, the higher the hardness, the better the quality.
一般来讲的话，硬度越高，质量越好。
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