Spotting a crack on your pottery piece is never a fun find. You worked so hard on sculpting something and to find a crack feels like the whole thing could be headed for the trash. But that doesn’t always have to be the case.
In this article, we’ll introduce you to some of the techniques you can use to repair cracks in your pottery pieces. Before we even get to those techniques, we’ll show you what could be causing the cracking issues in the first place. Identifying these issues and working to alleviate them is the first critical step in preventing more cracks from happening in the future.
Are you ready to tackle those pottery cracks once and for all? Let’s first discuss a few reasons why cracking occurs.
Why Does My Clay Crack When Drying
Cracks can often occur in common places on a pottery project. When identifying why or how a crack occurred, it’s important to know that the widest point is typically where the crack occurred.
Beyond that, here are a few other reasons why your pottery pieces may contain cracks.
Improper Wedging
It’s very important to wedge even air dry clay before you work with it. This is true for hand-building as well as throwing.
Think of wedging as a way to prepare the clay for working. It’s a way to get the clay to a point where you can manipulate it with greater ease so that it’s got a higher plasticity component. In some ways, it’s a bit like stretching before you work out to get your muscles firing.
In most cases, it’s a good idea to wedge air dry clay 30 times, or for about 2-3 minutes. As you continue to work with clay, you’ll get to know the point at which it feels wedged enough.
While it’s hard to wedge too much in most cases, it’s all about feeling the air dry clay and how it responds to your touch.
If you didn’t wedge your clay long enough, all those clay particles didn’t get enough time to achieve a more malleable state. Like a muscle that hasn’t been stretched, un-wedged clay can become a stress point that forms into a crack later on.
Uneven Clay Thickness
Cracks can occur most often when the air dry clay is thicker in one part than it is overall. For example, many bowls and plates have thicker bottoms, which stands in contrast to edges and lips that are much thinner.
Air-dry clay shrinks as it dries, and when it shrinks at different rates, stress can occur. This stress typically happens when clay changes thickness since thicker parts tend to dry out slower than thin parts.
If your air dry clay is even too thin, to begin with, this alone could cause cracking. Typically, a rule of thumb is that you don’t work in pieces that are smaller than a quarter of an inch.
Too High of Water Content
Clays high in water content will tend to crack more often. Porcelain is a type of clay that contains a high water content, which makes it more prone to cracking.
However, this type of cracking can also occur if the entire piece just has too much water in it altogether. This could be from throwing or a point in the sculpting process where you needed to add water and perhaps added a bit too much.
Water is not an issue just on the surface. It’s something that works within the pores of the clay.
However, when you fire clay that’s not completely dry, the water within the pores will evaporate, which leaves air pockets within the clay piece. These pockets can then fill with steam, which leads to explosions in the kiln.
Type of Clay
The type of air-dry clay you choose to work with could dictate how much cracking you’ll run into. As we mentioned, porcelain clay contains high water content, which leads to more cracking potential.
Plasticity is another factor in the clay’s tendency to crack. Higher plasticity means the piece can crack more easily.
It’s a trade-off between a clay that’s easier to work with but is prone to cracking versus one that’s harder to work but resists cracking a bit better.
Weak Joints
If you had to create a joint at any point in your sculpting, that joint could be weak enough to crack and potentially cause failure in certain components such as handles or rims. Joints that were created with air dry clay that was at different drying points will also be prone to failure.
Improperly Air Drying Clay
Air drying clay may take a long time in some cases. It can take up to a week for clay to dry enough to fire.
For those impatient enough to inconsistently turn their pieces, or even resort to a hairdryer, cracks will be prevalent all over your work. Uneven drying will cause air dry clay cracking. Wet clay will dry slowly compared to the rest of the clay body.
You may be able to cut corners off your air-dry clay projects, but you can’t do the same when it comes to letting your clay piece dry as it should.
Overly Complex Designs
If you’ve bitten a bit more off than you could chew, cracks may be your new best friend. Many potters who create designs that are too complex can shoot themselves in the foot.
Depending on the design you made, your piece could have sharp edges and/or thin spots. Each of these features alone could be the root of a cracking issue.
In the same vein, sculpting over an armament can also cause cracks. As you work the air-dry clay over the armament, it may stretch and flex too much.
Project Size
Choosing to throw or build a large piece is an undertaking all its own, but when it comes to cracks, you should be on the lookout for sure.
Larger pieces tend to have more cracks since there is more surface area to dry, and this is especially true if you didn’t let the air dry clay dry naturally or at a fixed rate considering the various thicknesses of the piece.
Clay Particle Alignment
When you throw it on the wheel, clay particles typically align within the sides. However, bases and lips do not share this same feature. In these cases, the alignment of the clay particles may be off.
Related to this, the grog content of your clay could also be below. The less grog in your clay, the more densely packed your clay particles tend to be.
Environment
The environment of your pottery studio as well as the surrounding weather can all affect your pottery pieces. Low humidity can cause pieces to dry out faster, which is also the case when it comes to drafts.
Improper Firing Cycle
Having an issue with your kiln’s firing process can be the cause of cracks as well. If your kiln is not heating evenly or even maintaining the proper temperature, the clay within will suffer.
Too hot of a firing temperature will also cause cracking to occur.
How to Prevent Clay from Cracking
You can easily repair cracks if you catch them early enough. Here are a few ways you can troubleshoot cracks in your air dry clay and repair them.
Even Thickness
Creating a clay project with an even thickness might seem hard, but with the right tools, it’s easier than you think. Plus, having a consistent thickness in your work will allow the piece to dry more evenly.
Create a Single Piece
The structure of your pottery piece will be tighter and stronger if you form it from a single ball of clay. If you attach various pieces to the main structure, those each contain the possibility of becoming a stress point and/or forming a crack.
Cover As it Dries to Keep Clay Moist
As the clay dries, the clay body begins to lose moisture at varying levels compared to the surrounding clay. Part of controlling the drying process means you might have to cover certain components (or even the whole piece) as the clay sculpture dries. For instance, you can cover lips and edges with a plastic bag or wet towel to keep them moist as the main body of the piece dries.
The main thing is that you should protect areas that dry out quickly to better match the drying schedule of the thickest pieces. You can even cover your project with newspaper and then wrap it in plastic if there’s a lot of humidity present.
If your pottery studio is drafty, cover the piece with a bucket as well as it dries.
Moisten During Drying
If you don’t want to use a plastic bag for fear it might leave an impression on your work, you can also use a spray bottle to moisten certain parts of your clay piece. This will hydrate those thin edge pieces again to help the piece dry evenly.
Most air dry clays won’t need to be moistened during the drying process. Polymer clay dries when it’s fired, so you shouldn’t have to worry about moistening polymer clay.
Let the clay dry slowly so that small cracks don’t form. An air-dry clay crack can be fixed in the early stages but it’s better to prevent cracking altogether if possible.
Place in a Well-Ventilated Room
It’s important to keep airflow unrestricted around your clay project. You can easily achieve this by placing it on a shelf or rack that receives lots of ventilation.
Use Wax Resist
You’ve heard of using wax resist for glazes, but you can also use it for protecting the edges of your air dry clay projects as well. If you do plan on using wax resist, however, you should leave a small gap between the resist layer to let the moisture from the piece itself escape.
Turn Consistently
When air dry clay is hard enough to pick up without damaging it, you can turn it to allow air to get to other spots and dry them out. Lips that may be drying out too fast can be placed down on the shelf to not only allow the bottom to dry out but to protect the lip from drying out any further as well.
Create and Use a Damp Box
A damp box is an easy addition to your pottery studio.
Consisting of a plastic box and some plaster, this damp box can allow you to control the rate at which your piece dries without having to constantly attend to it.
For example, place a 1-2 inch layer of plaster in the bottom of a plastic container. Once the plaster is hard, you can simply make it more or less moist by adding water. Cover your air-dry clay piece within the damp box and periodically check it to ensure the piece is drying evenly.
Join Properly
Joints are properly made using the slip and score method. To score the components, simply create a crosshatch pattern on both surfaces with a needle tool. Then, apply a small amount of slip to fill these scratches and join the components together.
You should also only join clay that’s of similar water content. If the air dry clay is too dry on one component, the project will not dry evenly and cracking can occur.
Work with Pottery Clay That Contains Grog
Grog reduces cracking tendency and shrinks less since clay particles are less densely packed.
Most pottery clays contain some element of grog, but if you’re particularly worried about cracking, it’s something you can add to your clay to improve your chances of avoiding cracks.
How to Fix Cracked Clay Pieces
There are many ways you can repair cracked clay pieces. Here are just a few ideas:
- Identify cracks early on in the process. The greenware stage is the best place in which to find (and fix) a crack.
- Though it might be counterintuitive, widen cracks just a bit to fill them with slip or fresh clay. You’ll need to overcome the clay’s memory of a crack to prevent it from occurring again.
- If you run into larger cracks, place fresh pottery clay over the crack and smooth it out to fit over the crack.
- Cracks found in the bone dry stage are not as easy to fix. You can create paper clay using toilet paper mixed with pottery clay to pack the crack with paper clay slip.
- Bisque ware cracks can be filled in with glaze in some cases, but this can often result in the glaze expanding the crack instead of filling it.
- Sometimes buying a crack-fixing product is the last resort. These can be applied during the bisque ware stage.
One of the most important pieces of fixing a crack is understanding how it happened in the first place. If you keep seeing them, keep track of when they occur and where. You may be able to diagnose an issue in your technique that you can then correct to prevent any further cracking.
How to Stop Air Dry Clay Projects From Cracking
Air-dry clays are water-based and are therefore more prone to cracking the longer they stay out of the packaging. It’s best to store unused air dry clays in an airtight container to stop air dry clay from drying out.
If you need to dry out your air dry clay in a controlled manner, you can heat your oven to 200-250 degrees Fahrenheit. Place the air dry clay in the cold oven and then set a timer for 15 minutes.
You should then turn the oven off and let the air-dry clay cool down naturally. If the clay is not dry enough, then you can do this same technique a few times.
Air-dry clay cracks form for some of the same reasons pottery clay cracks do. That being said, dry clay from cracking can be repaired in similar ways as well.
Wrap Up
We hope you’ve found this article on the reasons why clay cracks are useful. It can be incredibly frustrating to see a crack form on the piece you worked so hard on, but with some effort and a little bit of know-how, your piece could go from cracked to fixed in a short amount of time. After reading this article, you shouldn’t be afraid of cracks, especially since you know how to deal with them now.