Single Firing Clay: Why I Only Fire My Sculptures Once

Most ceramic pieces go into the kiln twice.

Mine only go in once.

I didn’t start single firing to save time, electricity or money – although those are welcome benefits. I do it because it’s the only way I can make my sculptures as I want to.

My sculptures often have highly decorated interiors, partly enclosed by the outer form. Those interiors need to be finished before the sculpture is closed. Once the form is complete, I can no longer get a brush – or sometimes even a finger – where it needs to go.

So I apply the glaze while the clay is still unfired.

Then I cross my fingers and put the whole thing into the kiln.

What is single firing?

The usual ceramic process involves two firings.

The first is the bisque firing. The word bisque is generally used in the UK too, although it is sometimes explained as a shortened form of biscuit – supposedly because someone in the Middle Ages thought fired clay looked like biscuits. The clay goes into the kiln dry and unglazed, and is fired hot enough to turn it from fragile, water-soluble clay into ceramic while remaining porous enough to absorb glaze.

The piece is then glazed and returned to the kiln for a second, usually hotter, glaze firing.

Single firing – sometimes called once firing – skips that middle stage. Glaze is applied directly to unfired clay, or greenware – often called raw glazing. The clay and glaze make their journey through the kiln together.

Simple.

Well… in theory.

What is actually happening in the kiln?

Clay going into a kiln isn’t simply getting hot and becoming hard. Quite a lot is happening.

First, remaining physical water is driven away. As the temperature rises, chemically bound water is released from the clay minerals and organic material burns away. Eventually, the clay’s structure changes irreversibly.

It becomes ceramic.

At higher temperatures, particles begin to bond and the body becomes denser and stronger through sintering, with varying degrees of vitrification depending on the clay and firing temperature. At the same time, glaze ingredients melt, flow and form a glassy surface.

In a conventional two-firing process, some of these changes have already happened before the glaze is introduced.

With single firing, everything happens in one journey.

That’s partly what makes it fascinating – and partly what makes it less forgiving.

So why single fire?

There are some good reasons to do it.

The obvious one is that you fire the kiln only once. Kilns use considerable electricity, so eliminating an entire firing can reduce the energy needed to make a piece. For me, that matters. Ceramics will never be a zero-energy process, so I’m interested in making thoughtful choices about the energy and materials I use.

There’s an economic benefit too. One firing means less electricity, less kiln time and potentially less wear from running an additional cycle.

It can also shorten the making process. There’s no need to wait for a bisque firing, unload the kiln, glaze everything and reload it.

But those are bonuses rather than the reason.

Single firing allows me to make work I couldn’t otherwise make.

My sculptures are about interiors and exteriors, hiding and revealing, and the tension between what we show and what we keep protected. Openings offer glimpses of another surface, colour or texture within the form.

Technically, that creates a problem.

I need to complete those hidden surfaces while I can still reach them. Construction and decoration therefore have to become part of the same process rather than two separate stages.

What’s the catch?

If single firing saves energy, time and money, why doesn’t everybody do it?

Because there are more opportunities for things to go wrong.

Greenware is much more fragile than bisque-fired clay. You’re handling and glazing something that can still be damaged remarkably easily.

Then there’s the relationship between wet glaze and the unfired clay beneath it.

When you apply glaze to bisque, the porous ceramic draws water out of the glaze quickly. The glaze settles onto a solid surface that has already undergone many of the physical and chemical changes of firing.

Greenware behaves differently.

Adding water to clay creates slip, so applying glaze to greenware can unintentionally mix clay into the glaze. The surface can become disturbed, and once that happens, it isn’t always easy to wash or smooth it without adding even more water. Anything involving a lot of wet handling therefore needs particular care.

The glaze also needs to remain attached while the clay beneath it shrinks during firing. Clay bodies can shrink considerably as they fire, so the glaze layer has to accommodate those changes.

Meanwhile, gases and water vapour need to escape before the glaze becomes molten enough to trap them. Get that sequence wrong and you can end up with bubbling, pinholing, crawling or other glaze faults.

Single firing isn’t simply a matter of taking your usual glaze routine and skipping the first firing.

Glazing greenware

I’ve found brush-on glazes particularly useful because they give me more control over where the water goes.

I also prefer to glaze before the work is completely bone dry. At the right stage, the clay can cope better with the moisture than very dry greenware suddenly receiving several coats of wet glaze.

There’s a balance, though. Too wet and the clay is still moving and shrinking significantly as it dries. Too dry and you’re asking thirsty, fragile clay to absorb a lot of water quickly.

Because adding water can create slip, I try to avoid rubbing or repeatedly reworking the surface once glaze has been applied. I also avoid washing surfaces back or using techniques that introduce too much water. Clay can become suspended in the glaze, altering the surface before it reaches the kiln.

Glaze application may also need to be thicker than you expect. On bisqueware, water is absorbed quickly, leaving glaze materials on the surface. Greenware doesn’t necessarily build a glaze layer in the same way, particularly when brush glazing, so getting the application right takes testing.

And that word – testing – is probably the most important in this entire blog.

Before you try single firing…

Know your materials.

Not every clay and glaze combination will behave well when single fired, even if both work perfectly in a conventional two-firing process.

Make test pieces. Keep notes. Test different glaze thicknesses. Observe how your clay behaves. Consider the piece’s thickness and how evenly it has dried. Think about your firing schedule too – a slower early stage can give moisture and gases time to escape before the kiln reaches higher temperatures.

And don’t rush the drying.

Although I glaze my work before it is completely dry, it must be properly dry before firing. Any remaining physical water needs to leave the clay before it reaches the point where it turns rapidly to steam. Trapped moisture and a rapidly heating kiln are not a happy combination.

Single firing isn’t a shortcut around good making and firing practice.

If anything, it asks you to understand your materials more closely.

Sculpture can be more forgiving

There’s another reason I’m comfortable using single firing: I make sculpture.

A glaze pinhole on a mug may become a hygiene issue. A warped rim might mean a lid no longer fits. A crack can make a bowl leak. Functional ceramics have to do a job, and some firing faults mean they can’t do it safely or successfully.

Sculpture gives me a little more freedom.

That’s not to say I don’t care whether my work cracks or whether the glaze behaves – I most definitely do! But an unexpected surface, a pinhole or a small movement in the form doesn’t necessarily destroy the piece.

Sometimes it can even add something.

There is a point in every ceramic process when you have to give up control. You can make carefully, understand your materials, programme the kiln and test endlessly, but ultimately fire and heat have the final say.

Single firing perhaps makes that relationship more visible.

Is single firing better?

Not necessarily.

It’s different.

It can use less energy, cost less and remove an entire firing from the process. For some ways of working, those are significant advantages.

But it also introduces different risks and requires careful thought about clay, glaze, moisture, drying and firing schedules. For functional ware in particular, the consequences of a fault may be much greater than they are in sculptural work.

For me, though, single firing isn’t really a choice between two methods.

It’s part of how the work comes into being.

The outside protects and conceals the inside. Openings offer glimpses rather than revealing everything. To create those hidden surfaces, I need to work on the interior while the clay is still being formed, decorating it while everything is raw and vulnerable.

Then the whole sculpture goes into the kiln together – clay, glaze, inside and outside – and somewhere in the heat, all those separate materials become one finished piece.

There is risk in that.

But then, perhaps that’s part of the magic of clay.

Want help to try single firing yourself? Check out my mentoring programme where this is one aspect we can chat about ☺️