Handmade ceramic plate cracking after putting hot food on it
It is the glaze and the lack of annealing the piece. In lampworking, glass  needs to be annealed to decrease any stress in the glass so it will not crack.  Ceramics that are glazed are glass covering the ceramics, so it also  needs to be annealed, with a slow cooling of the piece when fired.
It would also help if you find out the COE of the glaze if you can so you can hold the temperature of the kiln at the best temperature to reduce the stress of the glaze.
I personally do not know what that would  be.  Possibly, just slowing down the firing process; reducing the temperature very slowly to say twice what  you did before.