Beecken, H.; E-M. Gottschalk; U. v Gizycki; H. Krämer; D. Maassen; H-G. Matthies; H. Musso; C. Rathjen; Ul. Zdhorszky (2003). "Orcein and Litmus". Biotechnic & Histochemistry. 78 (6): 289–302. doi:10.1080/10520290410001671362.
H. Musso, C. Rathjen (1959). "Orcein dyes. X. Light absorption and chromophore of litmus". Chem. Ber. 92 (3): 751–3. doi:10.1002/cber.19590920331.