How To Prevent My Solid Clay Sculpture From Exploding In The Kiln

I’m looking down at the solid clay frog that had lost his leg in the kiln after firing, and I’m wondering how to prevent my solid clay sculpture from exploding in the kiln the next time. After asking my Pottery teacher and searching for hours for the answer, this is what I’ve learned. 

A Solid Clay Sculpture will take a very long time to dry completely before you can fire it in the kiln. When clay dries, it shrinks and compresses, which in turn seals the solid wet clay in the center of the structure making it harder to dry out completely. 

That is why many potters poke holes through their Clay Sculptures. They may even take the clay out of the center of the sculpture. This allows the clay to dry more evenly and faster. Some Potters will fire their kiln for days slow and low called (Candling) to get all the moisture out before firing it at a high temperature.   

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Size And Climate Does Matter

Smaller Solid Clay Sculptures do have an advantage. They dry faster and have less chance of cracking or exploding in the kiln. As your Sculpture gets bigger, the odds of explosions and cracks increases significantly.

Drying A 5-Pound Sculpture
I took several similarly shaped piece of clay and set them out to dry. After a week when I thought they were totally dry, I broke one open right down the middle. It was totally dry. That’s only because it’s been around 100 degrees in the desert. If the clay is not dry, then wait longer with the other piece of clay. This is a good way to assure your 5 Pound piece will be totally dry.

Drying A 25-Pound Solid Sculpture
It can be harder to gauge whether a 25-pound Clay Sculpture is totally dry or not. Depending on the type of pottery clay you use, and the climate you live in the Sculpture can take many months to dry.

One of my Pottery teachers took a 25-pound block of clay and let it dry out. She marked it on the calendar, June 10th and when she felt it was dry, October 28th she broke it open to see if it was totally dry. It was completely dry because she lives in the desert. Then she rehydrated the clay and used it for a sculpture.

What Type Of Clay Is Best For Solid Structures

There are Four types of Pottery Clay. Earthware is the most popular for Sculptures.

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• Earthenware is a low fire clay. With some extra grog this a good clay for big Sculptures.
• Stoneware is a high fire clay. This clay is good with smaller Sculptures. You can get stoneware with heavy grog or fire clay to help with the stability of the clay. It shapes well and can also be fired at lower temperatures.
• Porcelain is also high fire clay that is very smooth. You can use this clay for Sculptures, but it’s much harder to work with because of its smooth texture. You can also introduce grog to your porcelain to help strengthen the clay.
• Ball clay is the most plastic clay and containing very limited mineral impurities. It is not recommended for sculpting but you can also introduce grog for strength.

Benefits Of Adding Grog

Grog is fired clay with a high amount of silica and alumina. It is ground up fine or course and added to any type of unfired clay.

Sculptured Clays tend to have much more sand and grog. The grog and sand make the Clay better for a decorative item than a functional one. You can add anywhere from 10% Grog to 50% depending on the quality and look you may want.

Adding Grog to your clay is a good choice for several reasons.

• Less Cracking
  Grog makes the Clay Sculpture less likely to crack.
• Less Shrinkage
  Adding Grog lowers the shrinking rate.
• Workability
  Extra grog makes the clay easier to work with.
• Holds the Shape
  The grog makes the clay stronger to hold the clays shape better.
• Expansion and Contraction
  The more grog you use, the more the expansion and contraction rate is lowered in the kiln.
• Faster Drying
  The higher the grog content, the faster the clay will air dry.
• Faster Kiln Firing
  The higher your grog content is, the faster your kiln firing will be.

Things To Be Aware Of When Using Grog

Grog can make your clay course. The amount of grog and size of the grog particles will determine how smooth your Sculpture will be.

Glazes can cause pitting and Pinholes to highly Grogged Pottery.

Few Things you can try to prevent the glaze from pitting or pinholes 

• Give the glaze time to soak in or slow down the cooling.
• Select glazes that decompose and produce fewer gases.
• Be Careful not to apply glazes too think.
• Use a finer slip in between the clay body and glaze.

What Causes Solid Clay To Explode When Heated Up

The primary cause is moisture which in turn puts pressure on the clay piece. Because the clay shrinks when it dries it puts pressure on the pottery.
Another big problem is when you don’t wedge your clay the correct way thus leaving air pockets in your piece. Although it’s geared toward regular pottery making check out our article on What will happen if there is an air pocket that goes into the kiln to find out how important removing air pockets and moisture actually is.

Firing Solid Clay Sculpture

Firing a solid piece of clay will not explode if done right. If you choose the right clay, the odds of cracking your piece go way down. Also firing a solid Sculpture requires a very long firing time.
A Sculpture on average can take around 12 hours to fire. Whereas a solid Sculpture can take up to 72 hours (depending on the size) to fire completely.

There really is no limit to how thick your Clay should be. It all comes down to how long you want to wait for the center of your piece to dry and knowing when it is dry.
Slow and low is the only way to go with a solid Sculpture. Most Potters I have talked to said the same thing. If you don’t fire slow and low, she will crack or even explode.

When To Hollow Out Your Clay Sculpture

Molding Stage
If your clay is too soft, it will be harder to keep it in the shape you sculpted and make reattaching more difficult.

Leather Hard
It’s best to hollow out your piece when it is leather hard. When your clay is leather hard, it makes it easier to work with. You can cut the top or middle off without compromising the shape of your sculpture making it easier to put the Sculpture back together and smoothing it back into place.

Bone Dry
If the outer layer of your Sculpture is bone dry, you will no longer be able to cut it open and dig out the moist clay in the center without compromising the piece.

Why Hollow Out Your Clay Sculpture

You may want to carefully cut their sculptures open and remove the clay in the center for several reasons.

• Weight
  If you don’t want your Sculpture to be that heavy.
• Wait
  You don’t want to wait that long for your piece to air dry.
• Save Energy
  You may want to be economical and not have to fire the kiln as long. A solid piece does take much longer to fire.
• Explosion or Cracks
  You may decide not to take the chance of having your piece crack or explode.
• Unsure If The Center Is Dry
  If your not confident that you will know when your solid sculpture is totally dry in the center and you don’t want to take the chance.

Reminder – If you decide to hollow out your piece it must be done in the leather hard stage.

Do You Have To Fire Clay Sculptures

If you don’t fire your Clay Sculpture in the kiln, your piece will be very fragile and has a higher chance of decomposing over time. You will want to fire your Sculpture to make it more durable and have a nice glaze on it.

POTTERY CRAFTERS THOUGHTS…  It’s fun to take a solid piece of clay and sculpt it into a beautiful piece of art. The possibilities are endless. And so is your imagination.