On the J.S. Bach it is no discolouration: Montblanc leads this colour as 'Sepia' and the material is by design dark-brown. Real discolourations or deposit formations on Montblanc writing instruments are, according to collector experience, very rare and can be attributable to unfavourable storage (case materials, humidity, neighbouring materials like furniture outgassing) as well as to inks used. If deposits can be removed with a silver-cleaning cloth or careful wiping, mostly no substance damage is given. For a well-founded judgement, the writing instruments together with the case must be sent in to Montblanc with detailed information on storage location, climate, ink and cleaning.