Hi there, I am new to Sourdough baking. Did an afternoon sourdough baking course, watched sourdough baking on TV and then kicked off. Leaven rises well, first prove all good (four hours approx in sealed plastic container) knock air out and shape and place in bread tin, cover with tea towels - two hours. Sometime leave bread in tin and sometimes bake without tins. Bake at 250, prior to baking spray with fine mist of water and add semolina. Been enjoying very nice bread with a good crust but can't seem to produce bread with those lovely open crumb/holes that everyone seems to do. Is there any advice for a novice like me....? Thank you Greg D.