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. Good news! If you discover cracks in the greenware stage there is still hope.
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In today’s post, an excerpt from her book Mastering Hand Building, Sunshine Cobb shows how to stop cracks in their tracks. She also shares her secret for fixing cracks in bone dry pieces. – Jennifer Poellot Harnetty, editor.
Greenware is the best place to spot a crack and stop it from progressing. Cracks and structural problems should be repaired as you go, because cracking can often appear where there is structural weakness. Cracks are most common at the junction of two walls or where one coil attaches to another. 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. Cracks can develop in the most vulnerable areas as a piece dries.
The best times to fix cracks in clay are leather hard and soft leather tough. It may seem counterintuitive but the best way of repairing a crack or deteriorating spot in clay is to open it more (1). Clay is prone to forgetting things. 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). Depending on the dryness of the cracked clay, this will usually heal a crack.
After you have repaired the crack, if it seems that the clay is still weak, you can wrap it in plastic and let the moisture levels equalize. However, you should never add water to a crack. Water can swell cracks and cause them to grow. Use moist clay to fill cracks.
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.
Make sure you are paying attention to your first connections when making coiled pots. These connections can be complicated if the clay is not dry enough or you are connecting a fresh, wet coil with a partially built, leather-hard form. It is important to score well and ensure that the connections are accurate and properly compressed. This will minimize cracks.
The seams are where hard-slab forms crack the most. It is important to score the seams well, and use little water or slip to help set the foundation for a strong, well-attached seam.
A problem area can be the lip or edge of a form. I’ve seen many pots start to crumble or crack from the top down. You should ensure that your piece is at least a quarter inch thick. If necessary, use a coil to keep it from cracking or a chamois (a sponge or a bit of plastic) or a small strip of plastic to compress the lip. Any cracks that appear quickly should be addressed.
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. You can only fire a piece if it has a structural crack. It is possible to repair small cracks that have formed during the drying process.
It is not a good idea to try to repair a crack by adding water, wet slip or clay. This can cause a bigger crack, as well as the new clay to fall off, creating more damage. Dry clay tends to stick better to wet clay. I suggest that you repair small cracks with clay paper. You can do this by making a small amount with very thick slip made from the same clay that your piece is made. Place a towel of toilet paper in another container. Cover it with water and leave it to sit until the paper is reduced into pulp. An immersion blender will speed up this process. 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.) After opening the crack, score it with a sharp knife. Add water to make the area stickier and rougher. Then pack the slip into the crack. After the repair is complete, fire the piece. You can repeat the process if needed.