Why Evenness Matters in a Biscuit Firing

Biscuit firing is the foundation of every successful glazed pot. You are turning raw clay into a porous, durable ceramic that will accept glaze evenly. If the heat is patchy – too hot in one corner, too cool in another – you risk cracked rims, dunted bases, warped tiles, or a surface that refuses glaze. The kiln does not heat like an oven; it radiates, convects, and conducts in complex ways. Your job when loading is to help heat travel freely, so every piece reaches the same temperature at roughly the same time. That means thinking about airflow, shelf spacing, and where you place your pyrometric cones. Get the loading right, and the firing becomes predictable. Get it wrong, and you will spend hours chasing faults that started before you even switched on.

Start with Clean, Level Shelves and Props

Before any pot goes in, check your kiln shelves. They should be flat, free of loose kiln wash, and not cracked. A shelf that rocks on its props will shift as it expands, and that can topple a whole stack. Use three props per shelf for stability – never four, as three points always find a level plane. Arrange the props so they are not directly above the ones below; staggering them slightly encourages radiant heat to move around the shelf rather than channelling straight up. If you have a tall kiln, vary the shelf heights. A gap of 5–8 cm between the top of the tallest pot and the next shelf is a good minimum, but 10 cm is better for even circulation. I always set my bottom shelf at least 5 cm above the kiln floor, using short props, so heat can sweep underneath.

Leave Gaps Between Shelves and Pots

The single most common loading mistake is crowding. You need gaps – vertical and horizontal. Between shelves, that means don’t stack them like a sandwich. Leave a clear chimney of space at the centre or back of the kiln if you can, so hot air can rise from the bottom to the top. Between pots, aim for a finger’s width – roughly 2 cm – of clear space on all sides. That includes the gap between a pot and the kiln wall, and between pots on the same shelf. Arrange pieces in a loose grid rather than tight rows. Tall, thin pieces go near the edges where they can radiate heat; thick, chunky pieces sit closer to the centre, where heat tends to be more consistent. If you are firing flat tiles or plates, stand them on edge in a kiln rack, not flat on the shelf, so both faces dry and fire evenly.

Place Cones Where You Can See Them Through the Peephole

Pyrometric cones are your eyes inside the kiln. You need to see them without opening the door. Place a set of three cones – guide, firing, and guard – on a small cone plaque or as self-supporting cones. Position them on the middle shelf, close to the peephole, but not so close that they block your view of the pots. Make sure no tall pot or shelf edge obscures them. A good trick is to place a witness cone on the top shelf and another on the bottom shelf, but keep the main set at peephole level. That way you can watch the middle of the kiln, which usually represents the overall heat. If the peephole is small, use a mirror or a piece of kiln shelf to angle the cones into view. Never bury cones behind greenware – you will fire blind.

Never Stack Glazed Work – and Be Careful with Biscuit

This is a hard rule: never stack glazed work. Glaze melts and fuses. If two glazed pots touch, they will weld together, and you will lose both. Even in a biscuit firing, where everything is unglazed, stacking is risky. Thin flat pieces like tiles or small plates can sometimes be stacked with kiln wash or fine sand between them, but only if you are experienced and the clay body is forgiving. For most makers, keep every piece separate. If you are loading a mixed kiln – say, a few glazed pieces from a previous firing that need a second go – put those on their own shelf with plenty of space. Do not let any glazed surface touch a shelf, a prop, or another pot. Biscuit ware is more forgiving, but crowding still traps moisture and can cause steam explosions if the clay is not bone dry. When in doubt, leave it out and fire it next time.

Final Checks Before You Close the Door

Once everything is in place, do a slow walk around the kiln. Gently push each shelf to check for wobbles. Look for any pot that overhangs the shelf edge – it might fall when the shelf expands. Make sure nothing is touching the heating elements; keep at least 5 cm clearance. Check that your cones are still visible and upright. If you use a kiln sitter or controller, confirm the sensing rod is not blocked. Finally, close the lid or door and latch it evenly. A kiln that does not seal properly will fire unevenly no matter how well you loaded it. Take ten minutes for these checks. They are the difference between a load of lovely, ready-to-glaze pots and a frustrating pile of shards. With practice, loading becomes second nature – and your biscuit firings become reliably even, every time.