Thanks for your help! The thread seems to be cotton.
Why I don't use mixed fiber or polyester?
Most polyester thread is made for machine sewing, and I do everything by hand, the piecing and the quilting. Machine-sewing thread is often not well suited for hand sewing.
Furthermore, my mother always said that for something that I meant to last long, I should never use a thread with a fiber that was stronger than the fabric's fiber.
In her book about handquilting 'That Perfect Stitch', Roxanne McElroy says something alike:
'My teacher told me that if we were to make a garment of polyester, we must use polyester thread. (...same for silk, cotton....) If you were to make a garment out of polyester and sew it with cotton thread, you would constantly have to re-sew your seams, because the friction of movement would rub one fiber againt the other. The polyester would win every time because it is stronger. If you made a garment of cotton and sewed it with polyester thread, the polyester thread would cut the cotton fiber.
In looking at older quilts, I have seen evidence that this "balance of power" between fabric and thread is very imporetant. Quilts made as recently as the 1950s, when polyester first became popular, are already beginning to come bac to quilt shops for repairs. "
She continues/
"think of the reasons you are making your quilt before you choose your thread. If it is a quilt to fit in with a room decor that might change in a few years anyway, or if it is for a child who will love it to death, matching th thread to the fabric may not be important to you. But if you are making a heirloop quilt or want you quilt to have maximum longevity, take care with thread selection."
Jinny Beyer too, in 'Quilt making by Hand', suggests to use 100% cotton.
And I admit, I'm not such a good quilter that I can hope my quilts will become heirloom, especcially since I'm in Europe, where quilting isn't such a rich tradition as in the States. But because all my quilts are 100% made by hand, I do want to give them maximum longivety, although that may sound a bit vain...