Thank you very much all,
Rodney:
I put a sun light at the top of the object with these options:
Width : 800 cm
Shadows:
Rays cast : 50
Darkness : 60
And for the rendering, I made 36 passes.
Xtaz:
No I don't use porcelain, here the wireframe:
http://perso.wanadoo.fr/ffave/img/Tire_bouchon_wire.jpg
There isn't a lot 5 points, only in connections:
http://perso.wanadoo.fr/ffave/img/Tire_bouchon_wire2.jpg
Fabrice.