Kiln furniture is the shelves, props, stilts and wash that hold work in the chamber, and it is a consumable rather than a one-off purchase. Shelves warp and crack, wash flakes and is renewed, and stilts are lost to glaze runs, all of which is normal.
Shelves and props
Shelves carry the work and props set the height between levels. Both are refractory and both have limits: a shelf loaded unevenly or supported at three points that are not aligned across levels will warp over time. Stack props directly above each other from level to level so the load runs straight down through the stack rather than bending the shelves.
Kiln wash and why it matters
Wash is painted on the upper surface of every shelf so that a glaze run can be chipped off rather than welding a pot to the shelf permanently. It flakes and is renewed periodically. A shelf without wash is one glaze run away from being scrap, which makes this the cheapest insurance in the studio and the least glamorous job in it.
Stilts and where they belong
Stilts hold a piece off the shelf where the glaze runs to the foot, which is normal on some earthenware and unusual on stoneware, where fluid glazes make it risky. They leave small marks that are ground off afterwards. On stoneware most potters leave the foot bare instead, because a stilt under a running glaze fails spectacularly.
Buying it
Furniture is often not included with a kiln and a full set is a significant purchase. Buy shelves rated for your kiln's temperature, enough props to fill the chamber at the spacings you use, and a tub of wash. Suppliers stocking kilns stock the furniture beside them, which is the section to look in rather than among the clays.
Questions people ask about stilts
Do I need kiln wash?
Yes. It lets a glaze run be chipped off rather than welding a pot to the shelf, and it is renewed as it flakes.
Why did my kiln shelf warp?
Usually uneven loading or props that are not aligned from level to level, so the load bends the shelf rather than passing straight down.
When should I use stilts?
Where the glaze runs to the foot, which is more common at earthenware temperature. On fluid stoneware glazes a bare foot is safer.