You can protect the bottom of your kiln by placing a full shelf there and coating it with kiln wash.
Apply kiln wash to the bottom of your kiln and one side (top) of your shelves before your first glaze firing.
Kiln wash is normally used to protect the bottom of a kiln and one side of shelves from glaze drippings. If you prefer, however, you can use asbestos paper. After paper becomes brittle or spotted, you can change to a new piece.
Never apply kiln wash to the sidewalls, lid or bottom of shelves, as it will fall off during firing and ruin your glazed ware.
Kiln wash is a non-hardening compound that consists of a combination of dry powder and water, and is applied by painting a fairly thick coating to one side of shelves and the floor of the kiln.
When you are giving your kiln bottom a new coat of kiln wash, be sure to put cardboard against the sides to protect the elements.