Published Feb 25, 2022
One of the most useful things for all potters and ceramic artists to learn is how to fix cracks in pottery. There's nothing more frustrating than discovering a crack in a piece you have been toiling over. The good news is, if you find cracks in the greenware stage, there is hope.
In today's post, an excerpt from her book Mastering Hand Building, Sunshine Cobb shows how to stop cracks in their tracks. She even shares her trick for repairing cracks on bone dry pieces! - Jennifer Poellot Harnetty, editor
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, such as 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 is to open it up more (1). 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 and compress it with a rib (3, 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 growth of the crack, not eliminate it as you might hope. If possible, do not add any water to a crack and use moist clay instead.
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 the 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. 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 thing 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 at a corner where one wall meets another, it is 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 does not want to stick to dry clay. Instead I recommend trying to repair small cracks with paper clay. To do this, make a small amount of very thick slip with 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. You can speed up 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 is the consistency of a sticky putty.) Then open the crack slightly by scoring it extremely well, to the scored area add a bit of water if necessary to create a bit of roughness and stickiness, then pack the crack with the slip. Check the repair before firing the piece. Repeat the process if necessary.