What actually happens inside the kiln? From raw clay to finished ceramics
When you hold a finished piece of pottery in your hands, you’re looking at the result of many different steps: throwing, drying, trimming, glazing and much more. What you don’t see are all the hours in between – and, most importantly, what happens inside the kiln.
Every piece goes into the kiln twice. Two firings that transform dried clay first into a solid ceramic body and, eventually, into a finished piece of pottery.
The bisque firing – when clay becomes ceramic
Before a piece can even go into the kiln, it needs to be completely dry. Especially when I first started pottery, I didn’t always get this part right – with some rather spectacular (or disastrous) results. 😉 These days, I know that patience is incredibly important when it comes to drying.
For the bisque firing, I slowly heat the kiln to 950°C. During this process, quite a lot happens inside the clay: the last remaining moisture evaporates, chemically bound water is driven out of the clay minerals, and organic materials burn away. At around 573°C, the quartz in the clay undergoes a small transformation, which is why it’s recommended to move through this temperature range particularly slowly.
After many hours, the dry clay has transformed into a solid but still porous ceramic body. It is now strong enough to be glazed while remaining porous enough to absorb the glaze well.
It’s moments like these when I’m especially glad that I can rely on my kiln. My Nabertherm TOP 190 heats so evenly that I don’t have to worry when it reaches these critical temperatures. That gives me a little peace of mind in the studio while I’m already busy working on something else.
Between two firings
After the bisque firing, I inspect each piece, sand it where necessary and then apply the glaze. The glaze is what will later bring colour, texture and character to the finished piece.
Then comes the challenge of loading the kiln efficiently. Glazed pieces must not touch one another, otherwise their surfaces can fuse together during firing. With my plates in particular, the kiln can fill up very quickly.
This is where I really appreciate the generous interior of my TOP 190. For example, I can fit four dessert plates next to each other – or one large plate and two smaller ones. Any remaining gaps can then be filled perfectly with smaller pieces.
The glaze firing – When glazed pieces become finished ceramics
The second firing takes things to a whole new level, reaching temperatures of around 1230°C to 1250°C.
The glaze melts and, as it cools, forms a smooth, glass-like surface. At the same time, the ceramic body becomes denser. Its tiny pores begin to close – a process known as vitrification.
How dense the finished piece becomes also depends on the type of clay being used. With the clay bodies I work with, the clay becomes vitrified at these temperatures, but this varies from one clay body to another. This process is what ultimately gives the finished ceramic its density and the properties it needs for everyday use.
And then comes the part I still find a little exciting every single time: waiting.
Even once the kiln has reached its maximum temperature, the firing process is far from over. Controlled cooling is an important part of the entire process. That’s why one complete glaze firing cycle can take up to 35 hours in my studio.
1250°C isn’t always just 1250°C
What the kiln displays on its screen doesn’t tell the whole story.
The temperature is measured at a specific point inside the kiln. Depending on where a piece is positioned, the actual amount of heat it receives can vary slightly.
That’s why I also use what are known as pyrometric cones. These small ceramic cones react to a specific combination of temperature and time. After the firing, they show me whether the kiln has actually achieved the intended heatwork.
Many small decisions
A firing programme isn’t simply a matter of setting the kiln to “950°C” or “1250°C”. It consists of several different segments: How quickly should the temperature rise? At what point should the temperature be held? How quickly can the kiln cool down again?
Things such as the air inlet during the bisque firing and the kiln’s insulation also play a role.
You may never see most of these details in the finished piece. But these invisible steps can make all the difference between a successful firing and one that doesn’t quite go to plan.
And perhaps that’s one of the things I find so fascinating about firing ceramics: you spend weeks working with a piece, shaping it with your hands, allowing it to dry and carefully glazing it – and eventually, you have to hand over part of the control to the kiln.
From clay to finished piece
There are often around three weeks between the first touch of clay and a finished piece of pottery. In between are countless individual steps, two firings and many small decisions.
Whenever I open the kiln after a glaze firing, there’s always a little sense of anticipation. Because only then do I finally get to see what has become of something that, just a few weeks earlier, was simply a piece of soft clay.
Perhaps that’s one of the reasons I love pottery so much: you can plan, measure and control so many parts of the process – but in the end, there is always that small moment of wonder.
And every finished piece has passed not only through my hands, but through fire as well.
Yours Anna Maria
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