 "Indican" was quite unstable and resisted further purification.Later he also investigated Polygonum tinctorium and believed that it contained the same "indican".Only in the early years of the 20th century was it shown that Schunck's "indican" from woad was not the same as the indican from tropical indigo plants and Polygonum tinctorium, and it was renamed isatan, which was even later shown to be a mixture of isatan A, B and C.

The presence of indigo in urine has long been the subject of much curiosity and Schunck tried in 1857 to show that "indican" was the origin.