Can the length of a firing (the number of hours of a firing) cause crazing or shivering of a glaze?

Yes. It is possible that firing too fast can cause a glaze to craze or shiver. This is because the fast firing does not allow enough time for the glaze to properly meld with the clay. Try firing sample pieces at a slower rate. What happens if you leave the peephole plugs out of the peepholes while firing a kiln that has a downdraft vent? The kiln will fire very unevenly--perhaps the difference of several cones from top to bottom. The peephole plugs must be inserted throughout the firing on a kiln that is equipped with a downdraft vent such as the Orton Vent Master.