I am really delighted at the progress that is being experienced throughout the class these days. As your instructor I can say that my real reward for being at good dirt is to see how well my students are coming along. Even with the occasional frustrations of a failed pot there are valuable lessons being learned. It is all a part of the growing experience.
One point I should have made to Sandrine but which could have been made to the class as a whole is something that Jayne learned from a conversation she had had with Alya a few days ago. The issue relates to the different consistencies of the clay that you use. I always try to make the point that if your clay is too stiff to center you are often better off setting it aside with some water in the bag until it softens up some. If you can't wait or can't switch to a new bag of clay you may be able to center smaller lumps and confine your aspirations to more modest sized pots. One other possibility is to cone up and down as long as it takes to get some moisture worked into the clay. May take a while, but the point is that you can't just assume that you can set out to make what ever you want without first analyzing the limitations inherent in the materials and the possible avenues for correcting/dealing with them.
The specific issue that Sandrine had was that she was working with her reclaim and that it was especially wet, so it was sort of the opposite scenario. While being wetter may make it easier to center, and therefor center larger masses of clay, this does not mean that your ambitions should be for larger pots as well. The wetter the clay the less able it is to stand up to and resist gravity. The clay will want to sag if it is put in a vulnurable shape or if there is too much weight being supported by too thin a wall. This may mean that you have to leave extra clay in the walls for support, you restrict your shapes to low walls and undramatic curves, or that you choose a smaller scale that is not subject to the same threats of gravity.
What I should also have told you, Sandrine, was that instead of trying to throw the form upside down as we had originally strategized, it may just as well have been thrown right side up and then just trimmed down to the correct thickness and shape. Try it both ways to see what the difference will be. If you get to your trimming while the clay is still moist enough you can still do the slip decoration you want to investigate. Good luck!