Why Does Pottery Crack? And Can It Be Prevented?
Cracks happen in pottery. Sometimes they appear while a piece is drying. Sometimes they don't show up until after a firing. And sometimes a piece that seemed perfectly fine suddenly develops a crack that leaves you wondering what went wrong.
Usually, the crack didn't come out of nowhere.
Clay shrinks as it dries and continues to go through major changes as it is fired. When different parts of a piece shrink, dry, heat or cool at different rates, stress can build in the clay. If there's enough stress, something has to give.
Understanding when and where a crack appears can tell you a lot about what caused it and, more importantly, how to help prevent it the next time.
The crack opened dramatically at the neck of the vase. A circular crack developed around the base of the vase. Working with a crack can turn the damaged vase into an experiment.
It Starts with Preparing the Clay
Preventing problems begins before you make the pot.
The goal of wedging is to create a mass of clay that is even and consistent throughout. You don't want one part wetter or softer than another, or hidden pockets of air interrupting the clay body. You want the clay to behave as one uniform material when you throw or build with it.
Why does that matter? Clay shrinks as it dries. If the clay itself is inconsistent, different areas may respond differently as the piece dries and later goes through the kiln. Air pockets can also create weak spots and cause problems during construction and firing.
Wedging also tells you something about the clay before you begin. As you work it, you can feel whether it is too soft, too stiff or uneven and decide whether it's suitable for what you're planning to make. For instance, very soft clay might be fine for a small bowl, but may not have enough strength to support a tall form or a larger piece.
Good wedging won't prevent every crack, but it gives you what you want at the starting line: a consistent piece of clay that can dry, shrink and fire as evenly as possible.
Why Does Clay Crack as It Dries?
Wet clay contains water, and as that water evaporates, the clay shrinks.
That's perfectly normal. The trouble starts when one part of a piece dries and shrinks faster than another.
Think about a mug. The rim and handle are exposed to air and can begin drying fairly quickly, while the thicker bottom may still contain considerably more moisture. Those parts are trying to shrink at different rates, creating stress in the piece.
Uneven thickness can make the problem worse. So can setting a freshly made pot in a warm or drafty spot where the outside dries much faster than the inside.
One of the simplest ways to prevent cracking is also one of the most important: slow down.
For some work, that means loosely covering it with plastic and allowing moisture to leave gradually. Large pieces, thick pieces and forms with handles or other attachments may need particularly careful drying.
Why Do Handles and Other Attachments Crack?
Anywhere two pieces of clay are joined can become a weak point.
Handles are a classic example. If a wet handle is attached to a mug that has already become much drier, the two pieces will continue to shrink at different rates. As the handle shrinks, it can pull away from the mug.
The same thing can happen with coils, feet, sculptural additions and pieces joined together in handbuilding.
Good attachment technique matters. The surfaces need to be prepared, joined securely and compressed well. But moisture matters just as much. Whenever possible, the pieces being joined should be at compatible stages of dryness.
Then they need time to dry together.
What's That S-Shaped Crack on the Bottom?
If you've spent much time around wheel-thrown pottery, you've probably seen one: a crack running across the bottom of a pot, often in the shape of an S.
The appropriately named S-crack is a common problem in wheel-thrown work.
One way potters help prevent it starts while the pot is still on the wheel. After opening the clay, the base is compressed by repeatedly running a finger or tool across the bottom before pulling up the walls.
A bottom that is too thick can also contribute to problems because it dries much more slowly than the walls. And while a pot is sitting on a bat or another solid surface, very little air reaches the underside.
Once again, even drying matters.
How Dry Is Dry Enough to Fire?
A piece may look dry on the surface while still holding moisture inside, especially if it is thick.
Before pottery can go into a bisque firing, it needs to be completely dry. Potters call this stage bone dry.
Why are we so fussy about it?
As the kiln heats, any remaining water in the clay turns to steam. If enough moisture is trapped inside the piece, the rapidly expanding steam can damage the work and, in extreme cases, cause it to break apart in the kiln.
That's why we don't rush pottery into a firing just because the surface looks dry.
Can Pottery Crack During Firing?
Yes. Not every crack is a drying crack.
During firing, pottery expands as it heats and contracts as it cools. If different parts of a piece heat or cool at significantly different rates, the resulting thermal stress can cause cracking.
Potters sometimes call cracking caused by thermal stress "dunting."
Large, flat forms and pieces with dramatic differences in thickness can be particularly challenging. The way a kiln is loaded, heated and cooled can matter too.
This is one reason firing pottery isn't simply a matter of getting the kiln hot enough. The entire heating and cooling cycle matters.
Can You Fix a Crack with Glaze?
It's tempting. There's a crack, you're about to glaze the piece anyway, so why not fill it with glaze and hope for the best?
Unfortunately, glaze isn't glue.
A structural crack in the clay is still a structural crack after glazing, and another firing can sometimes make the crack larger.
For functional pottery, that's particularly important. A cracked mug, bowl or other piece intended to hold food or liquid may no longer be suitable for its original purpose.
Decorative work gives you more freedom.
Remember the cracked vase at the beginning of this post? Rather than throwing it away, John decided to play with what had happened. The cracks became part of the piece.
Sometimes a crack means starting over. Sometimes it leads somewhere you hadn't planned to go.
So How Do You Keep Pottery from Cracking?
There isn't one trick that prevents every crack. It's a combination of good habits throughout the process: prepare the clay well, build with reasonably consistent thickness, compress where needed, make strong joins, pay attention to moisture levels and give your work enough time to dry evenly.
And then start paying attention to your failures.
A handle that repeatedly separates from a mug is telling you something different from an S-crack across the bottom of a wheel-thrown pot. A piece that cracks while drying presents a different problem from one that cracks during firing.
The more pottery you make, the better you become at reading those clues.
Cracks Are Part of Learning Pottery
Every potter loses pieces.
Clay cracks. Pots warp. Handles occasionally come off. Glazes don't always look the way you imagined. And sometimes a piece you were sure was going to be wonderful simply doesn't survive the process.
That's part of working with clay.
Learning pottery isn't about reaching the point where nothing ever goes wrong. It's about understanding the material well enough to recognize what happened, make an adjustment and try again.
At Throw Clay LA, our six-week beginning and intermediate courses give students the time to learn the entire process, while members continue developing those skills through studio practice, courses and regularly scheduled free Member Clinics.
Because sometimes the cracked pot teaches you just as much as the one that comes out perfectly.
About Throw Clay LA
Whether you’re curious to try pottery for the first time or ready to develop your skills, Throw Clay LA offers one-time pottery experiences, six-week Build Skills courses, and six-week Refine & Explore courses in wheel throwing and handbuilding at our studio in Downtown Los Angeles.
We also offer pottery workshops and private pottery events for groups, while studio memberships provide experienced potters with dedicated space and time to pursue their own work, along with access to studio equipment, glazes, firing services and free weekly Member Clinics.
