Why does pottery clay crack when drying

8 Reasons Pottery Clay Cracks When Drying – 11 Solutions

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It’s disappointing when you work hard on a piece, only to have your pottery clay crack when it is drying.  It has happened to me a number of times.  So, I wanted to find a way of eliminating the variables that cause cracking.  After some research, this is what I learned…  

Pottery clay cracks when drying if some parts of the piece dry out more quickly than others.  When clay dries, it shrinks.  If one part of the clay is shrinking faster than the other, this puts an internal strain on the pottery. The clay cracks to cope with the strain. 

Although the main reason why pottery clay cracks is uneven drying, there are a number of factors involved in this.  Read on to find out what else is involved and how to avoid cracks in the future.

Why Does Pottery Clay Crack When Drying?

Uneven Drying is the main reason why clay cracks.  It is often said that pottery clay needs to dry slowly to avoid cracking. 

However, the key factor is not so much the speed, but how evenly the clay dries.  The reason it’s better to let clay dry slowly is that it helps you to dry your pottery evenly.

There is a lot of water in clay.  Around 20% of its body weight is water.  Therefore, as it dries out, it shrinks.  Some clay shrinks more than others.  When one part of a piece shrinks more quickly than another, it puts the pottery under a lot of strain. 

Wet clay is still quite malleable, even as it is drying out.  As a result, it can cope with this kind of strain.  However, as the clay dries it starts to harden and become more brittle.  Therefore, it is less able to manage the different rates at which it is shrinking. 

This is why you are more likely to see pottery clay crack at a later point in the drying process.  So, what causes unevenness in the drying process?

1) Uneven Thickness in Walls Can Make Pottery Clay Crack

Uneven thickness will make pottery clay crack when drying.  This is because thinner parts of your clay will dry out more quickly than thicker parts. 

As clay dries, water migrates from the deeper layers to the surface and evaporates.  The further the water has to travel to the surface the longer it takes to dry.  As such, thicker sections of your clay will dry more slowly.

This is why cups and bowls tend to crack on their base when drying.  It is partly because the base of the piece is thicker than the sides.  Lips of pots and cups tend to dry much more quickly than bases.

2) Complex Details and Designs Can Make Pottery Clay Crack

Pottery with intricate designs is more likely to have some areas that are thinner than others.  As such the details will tend to dry out more quickly than the main body of the piece.  The same is true of pottery designs with sharp angles rather than smooth curves.

This is one of the reasons why handles are more at risk of cracking than other parts of your pottery.  Handles are exposed to the air at all different angles and water evaporates quickly from them.

3) Larger Pieces of Pottery Clay Crack

Pottery clay cracks when pieces are large

The bigger your pottery the more likely it is to crack as it dries.  If your piece is thick, the surface of the ware will dry out more quickly than the deeper layers. 

Water in the deeper levels of clay has further to travel and will take longer to leave the clay.

Similarly, large slabs of clay, like big tiles or plates are prone to warp and crack too. 

This is because the edges will dry more quickly.  And because only one large surface area is exposed to air.

4) Not Wedging Your Clay (enough!)

An important reason for wedging clay is that it gives the clay an even consistency throughout.  This even consistency is called homogeneity. 

If clay is not wedged enough, then some parts of the clay body will be dense and dry.  Other parts will be soft and have a higher water content.

If you make pottery out of unwedged clay, you are immediately introducing an uneven water content to your piece.  This increases the chance of your pottery clay cracking as it dries.

5) Clay Particle Alignment

Clay is made up of tiny particles, that have a long flat shape.  In clay that has not been worked, these particles are in a random unorganized pattern. 

When a potter works on a piece of clay, the particles become more aligned.  This is particularly the case with wheel throwing.  The particles on the side of a piece of thrown pottery will align in the same direction.

By contrast, clay particles on the base of the piece tend to stay positioned more randomly.

When clay dries out, the particles shrink more across their short width than along their length. If the clay particles on the side of a piece are aligned, it will shrink more in one direction. 

By contrast, the clay in the base of a piece has not been worked as much as the sides.  As such, they are still organized more randomly.  The result of this is that the base tends to shrink less dramatically than the sides.

6) A Low Humidity Atmosphere Can Make Pottery Clay Crack

Low humidity levels in the air will cause clay to dry out more rapidly.  Humidity tends to be lower in cold climates and seasons.  Rapid drying in itself does not make your pottery clay crack.  However, it does make it harder to dry your greenware out evenly.

Positioning your pottery in a drafty area can also cause cracking.  The parts of your clay that are exposed to the draft will dry out more quickly.

7) The Type of Clay You Are Using

why pottery clay cracks as it dries

Clay with high plasticity tends to crack more on drying.  There are a couple of reasons for this.  Firstly, plastic clay has smaller clay particles. 

As such, there is more surface area in the clay for water to adhere to.  Also, there is proportionally more area in the clay for water between the particles.

Although porcelain clay is not a plastic clay body, it does have a high water content1.  As such, it is prone to shrink very quickly upon drying.  And as a result, it is at more risk of cracking.

8) Tiny Cracks in Newly Formed Pots

It is sometimes possible to see very small cracks in the edges of a piece you have just finished.  This is because, as you work with clay it dries out quite quickly.  These tiny cracks create points of vulnerability.

The reason for this is that the tiny cracks begin to dry out more quickly than the surrounding clay.  Tiny cracks can then open up into larger cracks as your ware continues to dry out.

How To Stop Your Pottery Clay Cracking:

Here are 11 simple solutions to stop your pottery clay from cracking.

1) Keep Your Clay at an Even Thickness

Pottery Clay Cracks when sides are different thicknesses

Ensure that the different parts of your pottery are a relatively even thickness.  This is particularly the case with bases of cups, bowls, and vases. 

You can check how thick the base is using a needle tool. 

A needle tool is basically a needle with a handle.  Press the needle into the base of the pottery until it touches the wheel head.  Then slide your finger down the needle until it touches the clay surface. 

Remove the needle and see how far your finger is from the tip of the needle.  This will indicate how thick the base of your pottery is.

2) Try to Keep Your Pottery Thinner Overall

It’s a good idea to try to keep the walls of your potter thinner.  However, you may want to make a large piece or something with thicker walls.  If you do, then follow some of the guidelines below about how to dry your pottery out evenly.

3) Cover Your Ware to Prevent Pottery Clay Cracking

You can control how quickly your pottery dries out by covering it with a plastic bag.  The water in the clay evaporates and increases the humidity within the bag. 

One way to do this is to make a plastic tent-like arrangement over the pottery.  If your pottery is sitting on a batt, you can trap the edges of the bag under the batt to create a seal.

Alternatively, you can leave the base of the bag loose to increase airflow.  Whether you seal the bag or position it loosely depends upon how humid your environment is.  You may need to experiment a little.

Note that water tends to condense on the inside of the plastic bag.  There is a risk that the bag will touch the surface of the clay.  If this happens it can make your clay soggy and you can cause marks on the surface of your pottery.

If this is likely to happen, it’s a good idea to cover the pottery with a soft cloth or paper towels.  These will absorb moisture from the bag, and stop it from the bag sticking to your pottery.

4) Protecting Areas That Dry Out Fast

Another way to aid even drying is to cover those parts of the clay that dry out more quickly.  Wrapping handles and lips with pieces of plastic is recommended.  This can bring the rate at which they dry in line with the rest of your pottery. 

You can also use wax resist to achieve a similar result.  Wax resist is a waxy emulsion that can be applied to handles and lips to slow moisture loss. 

If applying wax resist to a handle, remember to leave a thin strip on the handle free of wax.  This gives the moisture somewhere to escape from.

5) Using a Damp Box To Stop Your Pottery Clay Cracking

A Damp box can help prevent pottery clay from cracking

Another way to cover your pottery is by using a damp box.  You can make a damp box easily by setting a 1-2 inch layer of plaster in a plastic box.  Once hard, the plaster can be made more or less moist by adding water. 

Place your newly formed pottery in the damp box, and put the lid on it.  This creates a moist draft-free environment for the clay to dry out in slowly.  You can control the drying rate by adjusting the moisture in the plaster.  And by leaving the lid open a little.

Also, if you make a pot with a thicker base, you can place it on a dry plaster batt.  Cover the rest of the piece.  The plaster draws moisture out of the base.  This keeps the drying rate of the base in line with the thinner walls.

6) Are Drafts Making Your Pottery Clay Crack?

This is easily achieved by covering your pottery in one of the ways mentioned above.  Another simple way to protect your clay from drafts is to cover it with an upturned bucket.

Some potters claim that you can even out the drying process by using a fan.  The proviso is that the pottery is placed on an evenly rotating surface.  For example, a pot may be left rotating slowly on an electric wheel head. 

Other potters argue strongly against speeding up the drying process with blowers and heaters.

7) Turning Your Pots Over To Stop Your Pottery Clay Cracking

Once your pottery is hard enough to pick up without damaging it, you can turn it over.  This is helpful because it exposes the underside to air.

Also, if the lip of the piece sits flush against the table, it creates a seal.  The reason lips dry quickly, is that they are exposed to air from all directions.  By putting the lip flush against the table, the lip is no longer an exposed outer edge.

8) Put Your Pottery on a Shelf or Rack

This can help the airflow to circulate around your piece evenly.  It prevents certain areas, such as the underside from holding on to their moisture longer than others. 

9) How You Position Your Pottery Whilst Drying

If you make a few items with handles, position them so that the handles are pointing at one another.  When the handles are pointing inward to the center of a circle during drying, they create their own humid atmosphere.

10) Join Your Pieces of Clay Well

When joining pieces of clay, for example, a handle and a cup, the pieces must have the same moisture level. 

Also, it is not sufficient to simply press the two bits of clay together.  If you do this, they will separate from each other as the clay dries. 

Instead, you have to score the surface of both bits of clay where they are going to meet.  Scoring involves scratching grooves into the surface of the clay with a scraper, knife, or another hard tool. 

Slip is then painted or daubed onto the scored area before the two pieces are pressed together. 

why pottery clay cracks when it dries

11) Consider the Clay That You are Using

Grog in clay reduces cracking during drying.  Grog is clay that has been fired and then ground up into granules. 

Clay with grog in it shrinks less because the fired clay that makes up the grog has less water content.  As such, clay with grog contains less water and shrinks less. 

Also, the grog in the clay makes the clay particles less densely packed together.  This means that water in the clay finds it easier to migrate to the surface and evaporate.

Because grogged clay shrinks less, the pottery undergoes less stress during the drying process.  As a result, it is less likely to crack.

Grog granules come in a range of sizes.  Courser grog is tough on the hands if you are wheel throwing. However, fine grog can be added to clay and this is suitable for wheel thrown clay.

Final Thoughts

Many factors affect how clay behaves.  A few things might be making your pottery clay crack.  It is possible that you may need to experiment a little.  However, if you use the guidelines above it will help you isolate why your clay is cracking.

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References:
  1. https://www.britannica.com/science/kaolin

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