Many years ago, I created a set of dinner plates and planter saucers to be used underneath potted plants. They came out beautifully. However, after they dried, several of them developed cracks. So naturally, I asked what caused the cracked clay and what I might do to prevent them in the future? So, as a result, I would like to share with you my tips on how to prevent cracks in pottery.
Cracks in pottery are caused by stress on the clay. Cracks form when parts of the clay pot dry out faster than other parts of the pottery. Clay dries at different rates because of the various thicknesses of the clay pot. The bottom is thicker and dries last. Clay has a grain or alignment of particles so clay pots need to be turned and flipped during drying so that the clay pot dries evenly.
The base of a piece of pottery is where cracks appear. They are quite rare in hand-built pots, although they occur often in wheel-thrown pots.
A crack (or maybe even a hairline crack) might appear as the pot dries out before being fired. It can also show up during a bisque or glaze firing. A crack starts while greenware clay is drying out before being fired. Instead of breaking, the clay rips, causing a crack. While clay is in the greenware stage, it’s very fragile. A simple bump or light knock and break the clay body easily.
You might try this product. It’s basically gluing your pottery back together. I have done this before on keepsakes in which I can’t replace. Sometimes I get lucky and the pieces fit back together so well so I can’t see the crack. Click here: Clay Repair Kit
Pottery is fragile and prone to cracking by its very nature. Uneven drying of the piece causes cracking, which means that certain portions of the pottery shrink more than others. The key to avoiding these hazards is to dry the item in a constant atmosphere and to do the required prep work to guarantee that the entire piece dries at the same rate.
The size and depth of cracks vary. Some of them go to the pot’s base. I can only see others from the outside or inside of the pot and do not go all the way through.
Smaller objects, such as mugs and bowls, might develop cracks at the base. Larger, broader items, such as plates and platters, are more likely to have them.
1. Uneven Drying Is A Cause Of Cracks In Pottery
Uneven drying is the most common cause of cracks in pottery. Clay shrinks when it dries. The clay is stressed if one component of your pottery dries and shrinks faster than another. A drying gradient is what this is called.
Exposed areas of pottery will dry first, and they will dry faster than the rest of the piece. Do all you can to ensure that the drying process is as equal as possible. No matter how you arrange it, certain areas of intricate forms will dry before others.
For example, an early unshielded draft or pot left out too long after forming might cause this. Clay, in its pliable condition, rapidly absorbs pressures caused by a difference in water content between the piece’s driest and wettest portions.
However, once introduced, this spread tends to persist throughout the drying process, and weaker and thinner parts can only find relief by cracking in later stages.
The walls of a pot, plate, or cup will usually dry out faster than the base. This is because the pot’s side is more exposed to air. This is especially true of pottery that is allowed to dry flat on a firm surface. When a pot’s base rests on a solid surface, the whole underside of the pot is sealed off from ventilation.
The sides, particularly the rim, are, on the other hand, exposed to the surrounding environment. As a result, moisture on the edge and rim of the container will evaporate more quickly. As a result, the exposed sections of your pottery will dry faster.
In a climate of steadily dropping humidity, surface water is removed from ware by a high-velocity circulation of damp air. The cool, damp breeze passes across all surfaces, sucking up the same quantity of moisture. Controlling how much outside dry air and moist recirculating air is allowed in and out can prevent cracking.
A great book on the subject of crazing is “Crazing: A Potter’s Guide to Understanding and Preventing It” by Ann Marie Gillet. Here is what I like about the book: It covers crazing, causes, how to prevent it and how to repair it. The benefit of this book is, it teaches you the causes of crazing so you can avoid them and a bit about troubleshooting crazing problems in you own pottery. Available in paperback too. Check it out now.
2. Too Much Water Can Cause Cracks In Pottery
When throwing on the wheel, a pool of water will frequently form at the base of your pot. As you throw your pot, the small puddle will sit there, making the clay mushy as it lies in the base.
As a result, the clay in the base of your pot will have a larger water content after you’re finished. Even if you wipe up the extra water with a sponge, the clay will have sat in water for a time.
Do not underestimate the escalation of issues associated with drying pots that is larger than what you have previously made. For example, a small plate or pot may have been trouble free, but switching to a larger size might skew your drying and cause cracks.
Some geometries with a wide range of thicknesses (thick layer) and angular curves are especially challenging. Large plates, especially those made of porcelain, might be difficult to dry. Keep in mind that shape and contour have an impact on drying cracks.
3. Clay Thickness Can Cause Cracks In Pottery
Thicker clay takes longer to dry than thinner clay. It will dry out more slowly if the base of your pot is thicker than the sides of your pottery. One side of the pot is thicker.
Shrinkage is uniform in all directions when they are randomly arranged in the plastic stage or greenware stage, and there is no stress during uniform drying.
But when particles align, however, shrinkage along their length is smaller than shrinking along their breadth. As a result, even if drying is uniform, cracking can develop. Clay can be particularly thick around the pot’s bottom edge, where the side meets the base.
As a result, pots with thicker bottoms are more prone to crack. For example, particle direction across the bottoms of wide thrown pieces is more random than up the walls. To avoid tension during drying, compress the bottom to try to line them up parallel to the surface if you’re hand throwing pottery.
Because there is more distance between the rim and the center of the base, plates are more prone to cracks. As a result, the plate’s base is subjected to increased tension and stress.
4. Type Of Clay Used Can Cause Cracks
The kind of clay you use can also influence whether your pottery breaks or cracks when it dries. Clay (organic matter in clay) with a high level of plasticity cracks more easily as it dries.
Consider using ‘grog’ clay, (clay with less water content) which has been fired and pulverized before being added to the clay. The grog allows the clay particles to move about, allowing moisture to reach the surface and evaporate more quickly.
One possibility is to swap out some of the ball clay for less plastic ball clay or some of the kaolin for less plastic kaolin. It will not form as well due to the lesser plasticity, but it may be easier to adapt to the lack of plasticity than to cope with drying cracks.
You can do drying and shrinkage tests yourself to confirm what is happening. Cracks are prone to appear in porcelain clay. Additionally, stoneware clay with a high ball clay concentration is more prone to cracking. Ball clay has a high water content and shrinks a lot when it dries, therefore this is why. As a result, if one part of your pot dries faster than the other, your piece will be put under a lot of stress.
The Number One Solution To Prevent Cracks In Pottery
Here is what I do, and it works every time to prevent cracks in pottery.
The sides and rim shrink as they dry. When the sides and rim of the pot shrink, they pull on the pot’s base, causing the base to tear.
You may avoid this pulling motion on the base if your base and sides are drying at the same time. The clay particles on the foundation are pushed inwards and together when they dry at the same rate.
I can create a humidity chamber in a particular area of my studio. I use plastic to create a plastic-enclosed area. Please keep in mind that humidity is an unavoidable result of confining wet ware or wet clay pottery.
When pottery is enclosed, it dries much more slowly, but more uniformly. That is the trick and secret to success. Pottery can be removed for final air-drying once complete shrinkage has occurred. Potters and hobbyists can expand this technique by putting ware on batts, covering it with fabric, then covering it with plastic. The drying process will be slowed and thus evened out because of this. Some potters will use what’s called a wax resist. Wax Resist is a waxy compound that is used to keep slips, engobes, and glazes from adhering to the clay body or a previous slip covering, for example. To produce designs, the resist can be applied beneath or over underglazes, glazes, and slips.
Different orientations, such as turning bowls over as soon as possible, placing, putting mugs in circles with handles in the center, or air-flow adjustment or shielding, might help speed up sluggish drying areas in complex or odd designs.
Use Your Kiln As A Drying Chamber
Because electric kilns lack the airflow needed to remove surface water, a kiln-venting system should be considered not just for safety, but also for appropriate drying. Whether there is a vent or not, a gradual heat above boiling point is required to allow all pore water to leave before firing process can begin. If required, set the temperature to 240°F (above room temperature) for several hours or even days before continuing the firing process.
How To Repair A Pottery Crack
Word of caution: Ceramic restoration materials are not food safe, liquid proof, or heat resistant above 190 degrees F, so repaired items should not be used for cooking or serving food.
When your beloved piece of pottery is cracked or chipped, don’t throw it away. Rather, try to repair it first. I use use a 2-part epoxy glue adhesive to repair broken pieces of pottery and fill chips with epoxy filler to make them look virtually new again. Here is how to fill cracks:
1. Bucket of Sand to Hold The Pot
This isn’t required for all repairs, but it can be quite useful when working on items that must be held at an angle. Place the broken ceramic piece on the sand with the cracked side facing up. This allows you to do repairs with both hands. If you don’t have any sand on hand, cat litter will suffice. I always have plenty of cat littler. As a cat owner you might too. You might also hold the sand in a large pot, a shallow serving dish, or something similar as long as it’s deep enough to support the pottery completely.
2. Prepare The Area Of The Pottery Where The Crack Is Located
I sand the edges of the broken-off piece and the main piece of pottery with fine sandpaper. But sometimes this is not necessary because the larger pieces fit back together and you can’t tell where the crack is. And if you only have two pieces then you are in luck because your work just got a lot easier. So test fit the pieces before sanding, you may not need to. Apply gentle pressure and a back-and-forth motion until the edges are smooth. If you’re repairing a crack rather than a complete break, you don’t need to sand the piece right now.
3. Clean The Cracked Area
Wipe down both the main piece of pottery and the broken-off piece’s edges. After that, let them air dry completely before moving on to the next stage.
4. Mix Up 2-Part Epoxy
Squeeze the epoxy onto a non-porous disposable surface, such as a thin sheet of plastic, and quickly mix the two components together using a wooden or plastic stick. After 3 to 4 minutes, it will begin to harden, therefore you must work quickly.
5. Apply Epoxy To The Cracked Pot
Apply epoxy to the edges of the piece of pottery as well as the cracked piece with the wooden or plastic stick. I usually start by applying a thin layer then build up as needed. How much epoxy you use will depend on the wall thickness of your pottery. I do not let the epoxy run over the edges because ones dry its hard to clean up.
6. Let Dry
If the epoxy squeezes out around the edges, that’s all fine for some folks but again, like I mentioned above I am careful this does not happen. It’s just a real mess to have to clean up later. I am not too worried about the inside of the pot so if some epoxy get squeezed out on the inside that’s OK because no one will see that. Get it as straight as I keep it in place for 30 seconds or as long as the epoxy instructions say.
7. Clean Up
If you allowed the epoxy to drip on the outside of the pot you will now need to do a clean up. I find the easiest way to do this is to wait till the epoxy is almost dry but still in a pliable state. I take a razor blade and carefully scrape off any excess epoxy. At this point you will have to look at the pot and determine if sanding and painting is necessary.
Other Methods Of Fixing Hairline Pottery Cracks
1. Paper Clay To Fix Pottery Cracks
Sometimes you will find you have a very small hairline crack in your pottery. And if you have some paper clay on hand you are in luck. Many times you can fill in the hairline crack using paper clay. Paper clay, also known as fiberclay, is a type of clay that contains processed cellulose fiber or paper. Left over throwing slip, after wheel throwing can be used to make paper clay. It’s commonly used by potters to patch cracks in dry, traditional clay and dry paper clay.
How to make paper clay using toilet paper
Make your own paper clay. You choose the correct toilet paper with no shiny coatings, soak it in water, and then combine the combination into pulp with a paint mixer. The paper pulp is then wedged into your clay. You could start by adding around 5% pulp by volume. Be careful on adding water as you want the right consistency.
2. Use a Kintsugi Kit To Repair Pottery Cracks
Kintsugi is a Japanese technique for repairing shattered artifacts by applying urushi lacquer to the fractured places and then dusting with powdered gold. Kits usually include detailed instructions, and allows you to practice the Japanese Art of Repair from the convenience of your home me.
While the origins of Kintsugi are unknown, scholars assume it dated from the late 15th century. According to mythology, the technique began when Japanese shogun Ashikaga Yoshimasa returned to China to have a cracked chawan or tea bowl repaired.
3. Fix Hairline Cracks In Porcelain Using Milk (I can’t believe this trick)
Fill a pot halfway with milk and place your cracked porcelain piece with a hairline inside. After that, heat for an hour on low heat. Allow to cool in the milk before removing and rinsing. If the split wasn’t too deep, your porcelain piece should have resealed itself by now.
Pottery Cracks And Causes
The kind of clay you use can also influence whether your pottery cracks when it dries. Clay with a high level of plasticity cracks more easily as it dries. Consider using ‘grog’ clay, which has been fired and pulverized before being added to the clay. The grog allows the clay particles to move about, allowing moisture to reach the surface and evaporate.
Stress on the clay causes cracks in pottery. When parts of a clay pot dry up faster than other parts of the pottery, cracks emerge. Because of the varying thicknesses of the clay pot, the clay dries at different rates. The bottom is thicker and takes longer to dry. Because clay has a grain or particle alignment, clay pots must be twisted and flipped during drying to ensure that they dry evenly.
The bottom line is as a potter, you should know your clay. Therefore, you know the limits of that can be accomplished with the type of clay you are using. The only way that I know of how to gain this knowledge is to work with the clay and experiment using different techniques to find out what works and what does not. It took me years to develop techniques working with different clays to find solutions on how to stop or minimize cracks. My advice is to get out there and just do it. You will gain so much knowledge by doing.
References
Bronitsky, G. (1986). The use of materials science techniques in the study of pottery construction and use. In Advances in archaeological method and theory (pp. 209-276). Academic Press. sciencedirect.com/science/article/pii/B9780120031092500088
Rawson, P. (1984). Ceramics (Vol. 6). University of Pennsylvania Press. https://books.google.com/books?hl=en&lr=&id=1e79xqbRqEEC&oi=fnd&pg=PA1&dq=Is+Pottery+Art&ots=JYVyRyIXYR&sig=AQZbSLk4xQyCjy9e1dLnol83yQA#v=onepage&q=Is%20Pottery%20Art&f=false
Peterson, S., & Peterson, J. (2002). Working with clay. Laurence King Publishing. https://books.google.com/books?hl=en&lr=&id=hJ4BOhm-XicC&oi=fnd&pg=PA8&dq=How+To+Wedge+Clay&ots=sqI8oBwnGm&sig=NnNyDhF8yak-UJJ5I-QHkp99uJU#v=onepage&q=How%20To%20Wedge%20Clay&f=false
