seancisse

Thanks again for your comments, I'll see another shape or something else.
Keep you informed in case of success 

Hi Callie, distributed by a french supplier CERADEL. The clay is GT5700B (to be fired between 1250°C and 1280°C). The glaze is "vert pré" EG225 (to be fired between 1180°C and 1250°C).
I fired à 1220°C.

Thanks for your advices, I'll try something like a kind of fusible pillar in the center

Thanks for your advices, I'll try something like a kind of fusible pillar in the center

Your collapsed piece is quite interesting as it is and demonstrates the nature of clay as it interacts with fire.
Romance!
Physics!
As you have discovered, you will need a different design to stand tall in high fire. Re-think your construction and make something that is structurally sound to show off your glaze. 

Good question!
My guess is the clay softens just enough to overcome the form, where a cylinder or even an open shape - bowl - has enough structure to resist collapse, your four twisted ribbons of clay cannot, so.
Perhaps if the four arms had some arc, and no twists, they would stay upright.
You might try using a prop - a narrow cylinder - to hold the form upright, where the interface isn't glazed and has a bit of kiln wash or somewhat to prevent sticking. Even so, the twisted ribbons may still sag.
Good luck! I'm curious to know how your project comes out.