In the Studio: Fixing Cracks Sunshine Cobb

Appears in the Jul/Aug 2020 issue of Pottery Making Illustrated.

Cracks, cracks, and more cracks! Why do cracks happen? It could be that you’re working with a clay body that is prone to cracking. It could be the way you handled your clay during the building process. Was water sitting in your pot for an extended period? Did you form an attachment late in the drying game? Whenever two pieces of clay of different states of workability are joined, there is the potential for cracking

As you can see, there are many reasons for cracks to occur. The good news is that you can learn to identify the cause of a crack as you gain experience. And don’t despair if you get many cracks at first! The more skillful you become, the less cracks you’ll see.

Before I discuss repair, I want to share a warning as well: don’t spend too much time fixing your piece. Once a structural crack appears, you will likely have to deal with that crack repeatedly during the rest of the process, including after the piece is fired. So, when a crack appears, don’t panic. You will have several repair options, but be mindful of the time you’re spending to save a piece. Sometimes it’s best to start over.

Repairing Soft to Leather-Hard Greenware

Greenware is the best stage to catch a crack and stop it in its tracks. I recommend repairing cracks or structural problems as you go because cracking will often start to appear at points of structural weakness. For example, cracks are common where one coil is attached to another or where two walls meet. Cracks can appear right away in the building process, or they may show up once there’s added stress such as the weight of another coil or new elements, like a flange. As a piece dries out, cracks begin to appear in these vulnerable areas.

Leather hard and soft leather hard are perfect times to fix a crack or a weak spot in the clay before it becomes a more serious problem. While it may seem counterintuitive, the best way to remedy a developing crack (1) is to open it up more. Clay has a memory, so in an effort to erase the memory of a crack, score the area in question deeper and larger than the crack itself (2), then place a bit of scored soft clay into the space you’ve made (3), and compress it with a rib (4). This will heal a crack in most cases, depending on how dry the cracked clay is.

After you have repaired a crack, if the clay still seems weak, you can wrap the form in plastic and allow the moisture levels of the form to equalize. Be very wary of adding water to a crack at any time, though. Water can swell the crack and lead to its growth, rather than eliminate it as you might hope.

Here are a few specific situations where you’re likely to encounter cracks, with advice on how to proceed:

• Coil pots tend to develop thin spots when you are first learning to use the rib to create even wall thickness. Those spots should be shored up with extra clay as soon as they’re discovered to help prevent collapses and cracks.
• When you make coiled pots, be mindful of your initial connections (coil to floor and coil to coil). These connections can be tricky when clay is too dry to connect, or you are connecting a new wet coil to a leather-hard, partly built form. Scoring well and making sure the connections are true and thoroughly compressed will help minimize potential cracks.
• Hard-slab forms tend to crack most frequently at the seams and in the corners. Again, scoring the initial connections well and using small amounts of water or slip will help lay the foundation for a solid, well-attached seam.
• The lip or rim of a form can also be a problem area. I’ve seen many pots start to crumble or crack from the top down. Make sure you maintain a decent thickness at the top of your piece, shoring it up if necessary with a small coil or using a chamois (a wrung-out sponge or a thin strip of plastic will also work) to compress the lip. Quickly troubleshoot any cracks that begin to form.

Repairing Bone-Dry Ware

Your choices at the bone-dry stage are limited, so don’t waste a lot of time trying to repair a piece. If you have a structural crack, such as a corner where one wall meets another, it’s only going to become a larger crack once you fire the piece. If something small has popped up during the final drying process, you may have a better shot at fixing it in the bisque stage.

At this point, if you do attempt to repair a crack, adding water, wet slip, or wet clay is a mistake. This will lead to a bigger crack or the new clay falling off and leaving more of a mess. Generally, wet clay doesn’t want to stick to dry clay. Instead, I recommend trying to repair small cracks with paper slip.

To do this, make a small amount of very thick slip of the same clay your piece is made of. In another container cover a wad of toilet paper with water and let it sit until the paper has broken down into pulp. (Tip: You can speed this process by using an immersion blender.) Wring out a small amount of the paper pulp (you don’t need the extra water) and add it to the slip, then mix well. Don’t add too much water, and let the slip dry out so it’s the consistency of a sticky putty. Then, open the crack slightly by scoring it extremely well, add a bit of water to the scored area if necessary to create a bit of roughness and stickiness, then pack the crack with the paper slip. Check the repair before firing the piece. Repeat the process if needed.

Repairing Bisque Ware

If a crack has appeared after bisque firing—or if you had a small crack you rolled the dice on and it didn’t grow much during the bisque firing—you now need to repair bisque ware. If your crack is very, very small, you may be able to hide it by filling it in with glaze. But the glaze firing is hotter than the bisque firing, so there’s also a good chance your crack will expand.

For cracks that require filling in, I’ve found that using commercial repair solutions, such as Amaco Bisque Fix (www.amaco.com/products/bisque-fix-4-oz) and Bray Patch (www.archiebrayclay.com/bray-patch), are the best bets. The formulas vary, but most are a mixture of paper clay and sodium silicate. Follow the directions included with the product you choose.

Remember: don’t waste precious time on a lost cause. If you find yourself repeatedly developing cracks in your work, step back and ask yourself some questions about your building methods. See if you can identify any common issues. If you have no clue, find a teacher, mentor, or technician who has more experience and might be able to give you some advice.

Excerpted from Mastering Hand Building by Sunshine Cobb and published by Voyageur Press, an imprint of The Quarto Group. To learn more, visit www.quartoknows.com/books/9780760352731/Mastering-Hand-Building.html, or the Ceramic Arts Network Shop at https://mycan.ceramicartsnetwork.org/s/product-details?id=a1B3u000009udr1EAA. See more of Sunshine Cobb’s work online at www.sunshinecobb.com or on Instagram @shinygbird.