No matter what kind of kiln you are loading, you do not want to load ware directly on the kiln's floor. Doing so can damage the floor, especially in glaze firings, and can seriously disrupt the proper heating of the kiln by restricting air flow. Check the kiln's floor for cracks, debris, glaze drips, or other problems. If the floor is solid, make sure that it has a good coating of kiln wash. Do not kiln wash floors with electrical elements embedded in them. If the floor has a vent system port or burner port, make certain no kiln wash gets into those areas.
Begin the stack by setting posts for a bottom layer of shelving. You can use one-inch posts if the kiln does not have heat coming from below it. If your kiln does have a floor element, ventilation port, or floor-burner, I suggest using two-inch posts.
Before loading the kiln, make certain that all shelves are clean and free of cracks. Especially for glaze and test firings, also make sure they have been well kiln washed. Read Kiln Shelf Basics for more about these essential pieces of kiln equipment.
Stack (or Load) the Kiln
Although we load pottery into the kiln, potters often refer to stacking the kiln. This is because of the importance of keeping all weight distributed straight downward. Unsupported clay that has any pressure on it is highly likely to crack.
Weight Distribution in the Kiln Load
Stacking to keep weight supported in straight downward lines is also extremely important as you load the kiln furniture. Look at the posts on the left side of the photograph. See how they are perfectly aligned with each other? This completely straight vertical arrangement reduces the stress on the kiln shelves. Each shelf's weight is supported by the posts, which in turn is directly supported by the posts below them.
Take this weight distribution into account. It is absolutely essential if you are stacking ware without kiln shelves, as can be done in bisque kilns or with unglazed ware. As you can see in the photo, there are two bowls nested inside of each other for this bisque stack. Note, however, that one bowl is slightly tilted. If left this way, the lower bowl may crack due to the weight that is resting on its wall, rather than being directly supported by its foot.
Tight Stacks Are Better
For both bisque and glaze firings, pack the kiln as tightly as possible, given the circumstances. Full kilns heat more efficiently then loosely stacked kilns do.
As you may surmise, you can stack pottery to be bisqued very tightly. As long as there is no unsupported weight resting on the clay, pot walls can be as close as an eighth of an inch between them. This is only true of pottery that is being bisqued or fired unglazed. Pots that has been glazed need to have at least a quarter of an inch of space around them so as to avoid pots becoming welded to each other when the glaze melts. Remember, pottery expands in the kiln as it heats.
As much as possible, you should also stack the kiln so there is not over-much empty space above the top of each layer of pots and the bottom of the next shelf, and between the top of the last layer and the ceiling (or lid, for a top-loader). In practical terms, this is much easier to accomplish if you deliberately load each shelf layer with pots that are roughly the same height. The pots in the lowest level in the photograph show this clearly.
Do remember that you need to take expansion into account. The top of the tallest pot does need to be at least a quarter of an inch headroom between its rim and the bottom of the shelf above it.