Bisque vs Glaze Firing: Temperatures, Timelines, and When to Do Each
Two firings, one pot. That's the rhythm most studio potters settle into: bisque first, glaze second. But if you've ever pulled a lid off the kiln to find pinholes, blistered glaze, or a crack that appeared out of nowhere, you already know the order alone doesn't guarantee results. The temperatures you choose and how you schedule the ramp between them do.
The short version: bisque firing turns fragile greenware into a durable, porous object you can handle, sand, and glaze without it crumbling. A glaze firing then melts that glaze into glass and tightens the clay body underneath it. The longer version involves cones, ramp rates, and a handful of decisions worth making deliberately.
A bisque firing is a preparation step. As the kiln climbs, three things happen in sequence. First, remaining mechanical water leaves the clay. Next, chemically bound water is driven off and organic material — carbon from the body, paper, sawdust, whatever's in your clay or slip — burns out. Finally, the clay particles begin to sinter, fusing just enough to give the pot strength without closing the pores.
That porosity is the point. A bisqued pot drinks water, which means glaze brushed or dipped onto it grips the surface instead of sliding off. A glaze firing does almost the opposite work. The glaze melts, flows, and fuses to the clay, while the body itself moves toward vitrification — denser, harder, sometimes fully non-porous depending on the clay.
Bisque temperatures sit below the glaze temperature for the same clay body. That gap is not a suggestion; it's the thing that keeps the whole process working.
In low-fire earthenware work, bisque usually lands around cone 06 to cone 04 — roughly 1830°F to 1945°F (1000°C to 1060°C) — with glaze firings in the same neighborhood, often cone 05 to 04. Mid-range potters typically bisque near cone 04 and glaze at cone 5 or 6, around 2170°F to 2230°F (1185°C to 1220°C). High-fire work commonly bisques at cone 04 or 06 and glazes at cone 9 or 10, roughly 2300°F to 2345°F (1260°C to 1285°C).
Cones measure heat work, not just peak temperature, so a slow climb reaches maturity at a lower reading than a fast one. Manufacturers publish slightly different numbers, and your kiln's thermocouple and ramps add their own variation. Treat any chart as a starting point and confirm with witness cones in every firing.
A practical rule: bisque at least one cone below your glaze temperature, and don't be afraid of two or three. Bisquing too high is a more common mistake than bisquing too low. Over-fired bisque becomes glassy and stops absorbing glaze, which leads to crawling, poor adhesion, and glaze that flakes off in your hand.
Single firing exists and works, but there are good reasons most people don't start there:
Bisque firings are forgiving about top temperature and strict about the beginning. A typical schedule runs eight to twelve hours to the top, plus a long cool.
Glaze firings can often climb faster than bisque because the water is already gone, but the cooling matters more. A controlled descent gives glaze time to smooth out and heal, and it reduces crazing and shivering in the days after the kiln opens.
Most glaze firings run ten to fourteen hours including the cool. Some potters program a drop-and-hold near the end: come down 100°F or so from the top, then hold for twenty to thirty minutes to let pinholes fill and matte surfaces even out. Others slow the cool from the top cone down to around 1400°F, which affects crystal development and color in ways worth testing.
Loading changes too. Bisque ware can touch and stack. Glazed ware needs a quarter to half inch between pieces, kiln-washed shelves, and stilts or waster slabs wherever glaze meets the shelf. Wipe glaze off the foot before it goes in — a glaze drip that welds a pot to a shelf costs you both.
Once-firing, also called single firing, means glazing raw greenware and firing it one time. It saves hours, electricity, and shelf space, and it suits certain low-fire and mid-range bodies with glazes formulated to bond to raw clay. The tradeoffs are real: glazing greenware is fussy, glazes can flake or crack during handling, and any trapped moisture has nowhere to go but out through the glaze. Once-firing rewards simple forms, reliable glazes, and slow early ramps. Save it for work you'd be willing to lose.
Pick a bisque cone and a glaze cone for each clay body you use, write them down, and don't change both variables at once. If something goes wrong, the firing log tells you whether it was heat work, glaze thickness, or the way you loaded the kiln.
Two habits pay off more than any chart. First, put witness cones in every firing, in more than one spot if your kiln runs uneven from top to bottom. Second, make test tiles from the same clay you're firing and glaze them alongside real work.
If you're new to this, a good default is plain and repeatable: bisque at cone 04, glaze at cone 6, one soak at the top of the glaze firing, cool overnight. Get three consistent loads out of that before you start adjusting ramps. Your kiln, your clay, and your glaze all have opinions, and the fastest way to learn them is to fire the same thing twice and pay attention. Your kiln's manual and your clay and glaze suppliers' published firing recommendations are the right reference points for the specific materials in your studio, especially when you're working with ventilation or kiln safety questions.
Photo: kummod / Pixabay
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