Kilns & Firing field guide
How to Load a Kiln: Shelves, Spacing and Witness Cones
Bisque versus glaze loading, shelf and post rules, where to place glazes in an uneven kiln, witness cone packs on each shelf, and safe unloading.
Loading a kiln is a puzzle with consequences: a glazed pot touching its neighbour is fused to it for good, and a badly placed shelf can leave the top of the kiln a cone cooler than the bottom. A few rules, and a plan made before you start, solve most of it.
Bisque loads and glaze loads are different
| Bisque load | Glaze load | |
|---|---|---|
| Pieces touching | Allowed; nest bowls and stack similar forms | Never — glaze fuses whatever it touches |
| Feet and bases | No preparation needed | Wiped clean of glaze, usually a few millimetres up the side |
| Density | As full as possible | Gaps for air and heat to move |
| Main risk | Ware that is not bone dry | Glaze running onto the shelf |
Before you load
- Check the ware. For a bisque, every piece must be bone dry (how to tell). For a glaze firing, feet wiped, no glaze drips on the base, and no glaze thicker than the glaze is meant to be applied.
- Check the shelves. Kiln wash on the top surface only, never on the underside where flakes can fall onto the ware below. Scrape off old glaze drips.
- Group by height. Sort the load into tall, medium and short pieces before you start; it decides the shelf heights.
- Make a plan. Decide which pieces go on each shelf before lifting anything. It is much easier to rearrange a list than a half-loaded kiln.
Shelves and posts
- Three posts per shelf. Three points always sit level on an uneven surface; four can rock. Stack each post directly above the one below.
- A short first level. Raising the bottom shelf on short posts lets heat circulate under it.
- Stagger half shelves. With half shelves, offsetting their heights helps heat move through the load.
- Mind the clearances. Keep pieces away from the elements and the thermocouple, and leave room under the lid. Your kiln manual gives the distances for your model.
- Protect the shelf from runny glazes. Fire glazes that may run on a cookie — a disc of bisque or a shelf protector — under the foot.
Where to put what
Most kilns are not perfectly even. A kiln log with witness cones on each shelf shows you where yours runs hot or cool after a few firings. Use it: put glazes that need full heatwork where the cones bend furthest, and glazes that run easily where it is a little cooler.
The kiln loading calculator estimates how many pieces of a given size fit on each shelf, and the kiln volume calculator gives the usable volume of your kiln.
Place witness cones
Put a cone pack — one cone below your target, the target and one above — on each shelf, where you can see it through a spy hole or read it after unloading. A cone that has bent until its tip is level with its base tells you the shelf received the right heatwork. For cone 6, Orton's chart gives 1222 °C (2232 °F) at a 60 °C/hour final ramp; see the full cone chart.
Unloading
- Wait for the kiln to cool. Opening a hot kiln exposes pots to cold air, which can crack them (dunting) or craze glazes. Your firing log shows how long your kiln usually takes.
- Read the cones first. Before moving anything, note how far each cone bent on each shelf.
- Check each piece against its notes. Record the result per piece and per shelf — good, crazed, pinholed, under-fired — while you still remember where it sat.
Sources and further reading
Frequently asked questions
Can pottery touch in a bisque firing?
Yes. Bone-dry ware can touch, nest and be stacked in a bisque firing because there is no glaze to fuse it.
How far apart should glazed pots be in the kiln?
Far enough that no glazed surface touches another piece, a shelf or a post, with the feet wiped clean of glaze. Keep the clearance from elements and the thermocouple that your kiln manual specifies.
Why use three kiln posts instead of four?
Three points always sit level on an uneven shelf; four can rock. Stack each post directly above the one below.
Use these calculations with the procedure
Editorial note
Found a source or calculation that needs correction?
See how formulas are selected, how changes are recorded, and how to send a reproducible correction in the ClayCalc methodology.
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