Hi!
Thanks for your tips and advice!
Meanwhile, I think it was the combination ground white (name on the jar Juan, not gesso, surely a kind of acrylic) and aluminum. I started with the same configuration but on linen glued on wood and it is normal.
Thanks again!
PS: About lead: I read overall that essentially, no other "modern" white can replace it. Why? Strong film or rapid drying time? More opaque than titanium white? But sure, it is toxic and even has cumulative effects, although Cadmium and Chromium have too. Comic world....