Sometimes you want to put a larger than usual piece in the kiln. But your standard shelf is just too small. There are some things you can do to allow the firing of the larger piece.
You can put a two part or multiple part shelf into the kiln. However, joining two or more small shelves to make a larger one will continue to show lines where they join.
You can put in a single larger shelf. If this is a mullite shelf, you need to be sure you can get your fingers into the gap to get enough purchase to lift the shelf back out.
You can put a large fibre or vermiculite board over the existing shelf and so extend the area. You need to prepare these boards by firing and preparing them for firing with glass on top.
But, there may be problems with adding more shelf area.
If you have a side firing kiln, the glass will be much closer to the elements, which will require baffles and certainly slower firing schedules.
The reason for the gap between the sides and the shelf are to allow air circulation underneath the shelf to get even heating and even cooling. Restricting this air flow requires slower schedules. Some experts suggest cooling from one side – which this now will exhibit – requires annealing at half the rate of cooling from both sides, i.e., with air space along the sides of the shelf. This means doubling the anneal soak, and a reduction by half of the annealing cool rate.