I just found out something that might give me a clue (apart from burning the thread):
The spool on the right, with the dark green thread, is from another company, 'Wallaert Frères' in Lille, France. And I found on Etsy a cardboard box, used for the packaging of this 'Cable Louis d'or 6 fils' that is on the spool, and it says it is cotton!
http://www.etsy.com/listing/16223581...t-freres-lille