indigo

'The vinegar sets the indigo and keeps it from fading,' he says.

他说，醋可以使靛蓝变稳定，防止牛仔裤褪色。

The indigo plant was highly valued until the discovery of chemical dyes in the 19th century.

植物靛蓝在当时的欧洲非常昂贵，直到19世纪化学染料的出现。