Firing on the kiln bottom, is sometimes necessary in smaller kilns. You might find the need to put something inside the kiln, but there is not enough room with the shelf inside. This is a common thing with the smaller kilns.
Newer kilns usually have an improved uniformity of heat distribution throughout the kiln. This guide is only meant as a starting point. You will need to rely on your own experience and that of others who have tried this process.Inside the kiln the hot air needs to move and circulate. As the kiln is heating up, warm air circulates through the kiln. The heated elements radiate direct heat onto the glass. As the hot air moves through the kiln it transfers heat around and under the glass. There might be some advantage to heat circulating beneath your mold, so be sure to place it on prepared kiln posts.
You will need to protect the bottom of the kiln. Use Kiln wash, fire blanket, fiber board or fiber paper to protect kiln bottom. You should always coat the firebrick bottom of your kiln with kiln wash, even if you are firing the glass on a shelf.
If using a shelf out of ½” fiber board, place it on the bottom of the kiln. Be sure to take it out when not being used to keep it from getting smashed or dented. Always check to see that your foundation is flat and level, and use a conservative firing schedule.
When and if you feel the need to fire a piece on the kiln bottom, I hope this information will come in handy and assist you in this process. As always take notes and refer to them when doing future firings.