Both the bisque and the final cone 10 firing are done in a gas kiln that I built. The kiln has about 50 cubic feet to be filled with the pieces as well as the shelves that they sit on. Just as there are many factors to the glazing process that determine the final appearance of the piece, there are also many factors at play during the second firing process.
Everything from the weather during the firing and cooling period, to where pieces are positioned within the kiln, to how much heat is given and when that heat is given, to the length of the firing all play a factor in the final outcome of the appearance of the pieces that are created.