AAAAAHHHHH!
Luckily I had one more night, one more firing to fix it.
So I glued it with special "ceramic glue" and fire it again. It worked but you can still see the crack. The up side is that it was an artistic fracture in a nice symetric swirl.
I realised afterwards that there had been a flaw in the porcelain plate. It was a miracle that it didn't break in the first firing. Next time I will pay more attention to that