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Advice for Repairing Ceramic Greenware

repairing ceramic greenware

Note: This article is about repairing cracks in clay. It contains advice against repairing ceramic greenware. Recently, I came across an anecdote about a major American potter. 

When Adelaide Robineau’s Scarab Vase came out of the kiln, there were multiple fractures in it. Adelaide’s teacher told her to pitch the thing and start again. Stubbornly, Adelaide spent hours pounding bisque scraps into a paste. She mixed that with powdered glaze, making an approximation of magic water, and puttied the cracks. Old Addy reglazed and refired the piece. The results spawned one of the most influential pottery pieces of the past two centuries.  Adelaide is hailed as one of the first and finest independent women potters of America. So, potters, tenacity can prevail.

We all spend time repairing ceramic greenware. A potter I know painstakingly built a life-size coil pot representation of Vermeer’s Girl with a Pearl Earring. Last week, the head slumped into the shoulders when the work was put away a bit too wet.

Yesterday, after repair, the leather-hard piece developed a new fissure that went pretty much all the way round the lower face. The potter worked for three hours, repairing ceramic greenware by filling the gap and sealing the crack. Unknown if the piece will survive—it hasn’t been bisqued yet.

The moral is that we all slip up (intended) when handling clay. We end up with a mishap or a tragedy and we hope against hope for a fix. Sometimes, we get lucky and the clay gods smile down upon a repair. Mostly, we open the kiln and find what looks like ancient ruins.

My favorite ceramics/pottery instructor, Tom Vician, says your time is better spent handling clay carefully, than in repairing ceramic greenware that has failed. He adds, “Once it fails, you might spend your time more wisely building a new, usually better piece than working at repairs.”

That said, we give you by popular request our notes on repairing ceramic greenware.

Notes on Repairing Ceramic Greenware

Before you focus on being a ceramic or clay artist, you will find it helpful to get a good foundation in the physics of ceramics. Clay does what it does for a variety of predictable reasons. You can manage clay if you know what makes it behave the way it does.

For example, you have to know that parts you plan to join must be similarly wet. You can’t generally attach wet clay to leather clay. Why?

During each moment that clay is drying—even while you are working on it—the clay is shrinking. The platelets that make up the clay are losing their cushions of water and moving closer together.  When you pugged or wedged the clay, you aligned the platelets and homogenized the size of the spaces between them.

If you slap fresh moist clay onto leather hard clay, one piece has more space between the particles, and one has less. As they dry, they will shrink at different rates because the room atmosphere changes and your way of working with the clay pieces is not always consistent.

The platelets or particles move closer together as they shrink, but they move at different rates and end up with more or less space between the particles. Therefore, one piece will likely curl a bit or shift away form the other and you will see cracks. Big or small, they will be there.  Before firing, during, or after, they will usually develop.

Flyesschool.com made me laugh with this observation,

“Unknowing, forgetful or pig-headed beginners may be able to join a wet piece of clay to a near bone-dry piece (typically something they have been working on for days and have let dry out too much), but in the end, no matter what they do, the wet clay will shrink more than the drier clay and all their effort will usually go for naught.”

Keep in mind that drowning, showering, or spraying drier clay with a shock of water isn’t the answer. Neither is soaking a sponge and squeezing it our over the piece. The change is too sudden. You end up with drier clay on the interior of the piece, and sloppy wet clay on the outer surfaces. Bam! Cracks.

Learn this easy lesson and avoid many hours of sobbing frustration.

  • Gently dampen the drier piece (mist it lightly, cover it and let it steep) while you
  • set the wetter piece out on an absorbent surface to dry a bit.
  • Then score both thoroughly.
  • Slip generously.
  • Press the pieces together carefully but firmly.
  • Compress. Compress. Compress.
  • Never crash dry the finished work.

If You’re Sure the Rules Don’t Apply to You

If you insist on ignoring simple physics and your piece fractures, you might get lucky and be able to repair it.

Some potters swear by a concoction known as magic water that you can formulate yourself with readily available ingredients. Others say it doesn’t work, but a lot depends on what you’re trying to repair. Experiment.

Magic Water Recipes

2 quarts tap water
1 1/2  tablespoons liquid sodium silicate
3/4 teaspoons soda ash
Stir together. It’ll keep for a while if covered tightly in a non-transparent container.

This makes sense for joining surfaces at the same moisture content. Sodium is a repairing ceramic greenware 1powerful flux. Silica helps form glass in the firing. Dissolved soda ash penetrates wet and scored clay, thereby weaving particles together. Sodium silicate changes clay’s chemical personality and makes it dry faster (also makes it harder). This all means you create a layer that’s similar to a glaze, dries faster, and fires stronger. Joins are more secure.

OR

Soak toilet paper over night then add 15% by volume to powdered clay.
Add some vinegar. Most recipes say a “dash.” I say splash a little in. Say you’re mixing up about a quart of this recipe, use like two tablespoons of vinegar. Add two drops of sodium silicate, which you can easily purchase at a ceramic supply store. Finish just like it says above.

Magic Mud or Wonder Mud

These recipes are all over the Web. Potters from Martha Glover to Lana Wilson talk about magic water and magic mud. So try making some.

  • Chop up about a 1/3 of a cup of household paper product—napkin, toilet paper, paper towel
  • Combine that with about 2/3 cup of your clay body, dried and pounded into small pieces. If you can turn it into powder, that’s the best. If typically reconstitute powdered clay to make your clay body, you’re a step ahead.
  • Put the powdered clay into a bucket.
  • Make up some magic water and pour it over the clay powder. Pour the water until it’s an inch deeper than the clay. Stir it up to make sure all the clay and paper get wet.
  • Let the mixture soak overnight. Drain off the extra water in the morning.
  • Using an electric blender, stick blender or a mixing attachment for your drill, mix till the stuff is the consistency of thickish slip. At that point, magic mud is effective for strong joins in leather-hard clay.

repairing ceramic greenwareIf you’re hell bent on repairing ceramic greenware, you can try thickening the magic mud to a putty consistency. Then lay damp paper towels over both sides of the crack and allow them to moisten over a bit of time. Score thoroughly and putty them. Join firmly, compress the join with a rib—and you have a chance of the join holding through firing.

There’s a commercial product called APT II that some folks use. I don’t have any experience with it. It is apparently for low fire clay. Big Ceramic has 2 ounces for $5.00, if you’re interested.

Bottom Line on Repairing Ceramic Greenware

If we learn our craft—not only the art part, but the physics part—we can avoid creating cracks and fissures. If we have courage, we can recycle failed greenware, put on our big kid pants and begin again. The second try turns out better than the first. The potter learned something new. And it’s only mud, any how.

Got ideas for repairing clay work? Tell us in the comments, please!

 

 

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