Why does clay crack while it's drying, and how do I prevent it from cracking?
Hi, Marie here. In this video I'll be answering these questions and
offering you some advice and strategies to help prevent your clay from cracking while drying.
The supplies used in this video will be listed for you in the show notes below. Let's dive right in.
So why does clay crack while it's drying. The main reasons for this are uneven drying clay.
Not being compressed and the shrinkage rate of your clay. Let's take a look at uneven drying first.
As the water evaporates the tiny clay particles shrink together closing the gaps in the clay.
When the clay dries unevenly one part of the clay is moving and the other part is
not. These wet and dry clay particles push against each other during the drying process
causing tension and possibly cracking of clay. So you want the water to evaporate slowly and evenly
to prevent pressure from being put onto the clay particles from wet and dry clay. One of the best
ways to prevent this is to control the clay's drying time with even moisture. If a pottery
piece is left out in the open the rim will dry faster than the sides and the base of the pottery.
Allowing the pottery to dry slowly for the first several days helps the clay dry more evenly. There
are several ways you can do this. Tenting is one of the most popular ways to dry your pottery.
Simply cover the top of your pottery with plastic, I use plastic bags. Don't
seal the plastic around the pottery. You still need air to get in to dry the clay.
Tenting traps the moisture keeping the top of the pottery from drying faster than the bottom.
Separating your pottery pieces for drying is an important step to ensure even drying. You
want to place your pottery at least an inch apart for the air to flow in between them.
Handles and attachments should be positioned facing inward so they don't dry faster than
the piece that they're attached to. After a day remove the plastic for about an hour
or two to release some of the moisture. Rotate the pottery and cover again. Rotating and turning your
pottery over is important in the drying process. Rotating the pottery helps all the pieces to dry
at the same rate. Repeat this process the next day. Check your pottery daily until the clay feels dry
to the touch, the time it takes to dry will depend greatly on the climate, you live in. Now you can
take the plastic off and let the pottery air dry. Placing the pottery on a rack like this one or
a cookie rack will allow the air to get underneath the pottery to help the final drying process.
A damp box is another great way to dry your pottery. It holds in the moisture allowing
your clay to dry evenly as the moisture slowly escapes from the covered container.
You can make your own damp box by using a plastic container with a cover.
Make sure the top of the container is not sealed so that the moisture can escape. I just use one of
my wear boards and put plastic on. It works great. After a day remove the lid from the damp box for
about an hour or two. Rotate and turn your pottery over and place the lid back on the damp box. Repeat
this process the next day. Check your pottery daily until the clay feels dry to the touch. Then you can
leave the top off until the pottery is bone dry or take it out of the damp box to your drying area.
Another way is to use a portable greenhouse. A portable greenhouse will also keep the moisture
level even while your pottery is drying. I have a link for one in the description below.
When using a portable greenhouse after a day just unzip the door to allow some of the moisture to
escape. An hour or two is fine then rotate the pottery carefully by turning and flipping it over.
Repeat this process the next day and check your pottery daily until the clay feels dry
to the touch. Then you can leave the door open or take the pottery out to your final drying area.
Don't forget to hit the like button anytime during this video, I also have a super thanks
button where you can donate to the channel and help with supplies to make these videos. Thanks
for your support. Another reason why the clay can crack while drying is the clay particles are not
compressed. Compressing your clay helps align the particles to keep the cracks from forming.
If there isn't enough pressure placed on the clay these tiny particles remain disorganized. The clay
particles that are arranged randomly clash and place pressure against each other as the water
evaporates and this can cause the clay to crack. Like this. When the particles are more aligned they
have a better chance of bonding together as they dry, lessening the chance for the clay to crack.
Wedging your clay helps to compress and align the clay particles which can help to prevent cracking.
It's important to wedge your clay correctly so it has an even consistency throughout.
You don't want hard lumps that are drier in your soft clay this will result in an uneven drying.
I have a video and article on the different ways that you can wedge your clay. I left a link for you
in the show notes below. Hand building and thrown pottery requires different compressing techniques.
When hand building use a rubber rib to compress and smooth out the clay. Place the rib in a slight
angle, then move the rib to smooth the surface area and compress the entire piece evenly.
Flip your clay over and smooth out the other side. Besides cracking not smoothing out both sides can
also cause the clay to warp. When throwing pottery on the wheel you also need to compress the clay.
You're automatically compressing the sides and aligning the clay particles while you're throwing
your pot. However the rim and the bottom of the pot are not being compressed. It's important to
compress the rim and smooth out the surface of the clay to avoid small cracks on the rim of the pot.
Hold the sides of the rim with your anchor hand thumb and index finger, then place the index finger
of your outside hand on the top of the rim and press the clay. Be careful not to press too hard
so you don't deform the rim. Your bottom needs to be compressed to avoid s-cracks while drying.
You can do this by pressing down on the clay with your fingers
or using a sponge. I prefer to use a sponge. This will flatten and align the clay particles.
This also helps to decrease the water absorption thus keeping the clay stronger longer.
I do this after forming the base and when I finish my piece. Depending on the piece
I will even compress the clay in the middle of the process. Whenever I do this I never get cracks.
Another way to help prevent cracks while drying is to make sure that the base of the pot is not too
thick. Thicker base or uneven thickness can cause the clay to crack. If you're unsure
use a needle tool to check the thickness. Slide your index finger down the needle tool until your
finger touches the clay. Keep your finger in place as you move the needle tool away from the clay.
The thickness of the base is the distance between your finger and the tip of the needle.
Handles and different attachments can add a lot of character and function to your pottery.
There are also weak spots where cracks often form. When adding handles knobs or other features
it's important to wait until the pottery and attachments are in the same drying stage.
This is the second stage of drying. When your clay is leather hard you can no longer mold
it at this stage but the clay is strong enough to hold the attachment in place.
To ensure the attachment does not crack on your pottery it's important to score and
apply slip to your piece, then score and apply slip to your handle or attachment.
This will help to create a strong bond between the handle and your piece. Now there are potters
that'll tell you they only use water if it works for them that's fine, but I don't want to take
that chance. Wouldn't you want to do anything you could to prevent your attachments from cracking.
It's also best that your slip is the same clay body as your piece and attachment.
The type of clay you choose is also important Different clays have different shrinkage rates. The
higher the shrinkage rate the greater chance of your clay cracking because the clay is moving more.
Porcelain can shrink anywhere from 12 to 20 percent. The shrinkage rate of stoneware is
around 12 to 14 percent and earthenware has the least amount of shrinkage with about four to seven
percent. You can also consider choosing a clay with grog or adding some grog to your clay. Grog.
is made up of fired clay that's ground up. Grog can be ground up into fine medium or large bits.
Clay containing grog shrinks less and helps add strength and stability to the clay.
Which can help prevent cracking while the clay is drying. In addition, having grog in
your clay means the water won't be absorbed as much, which also speeds up the drying time.
You can also add paper fibers to your clay body or use paper clay. This also helps to
prevent the clay from cracking when drying. Can you use heat to dry your pottery? Yes.
Heat can be used to dry your pottery in several ways but you must exercise caution.
Heat guns are mainly used for wet pottery. This helps the clay stiffen to prevent it
from collapsing. The heat should be distributed as evenly as possible to prevent cracking.
A heat gun is not recommended to dry the clay completely. Candling is a term used for drying
clay in the kiln. Place your greenware in the kiln set your kiln at 180 degrees Fahrenheit or
82 degrees Celsius for about 8-12 hours. Candling will take out any moisture left in your pottery.
However candling is only recommended to do when your pottery is in the final stage of drying.
It's not recommended for wet or leather hard clay. There are several reasons for this. First it can
increase the chance of your clay cracking. It's hard on the elements in your kiln. Electric kilns
hate moisture. Plus it's not very energy efficient. You can also place your greenware next to your
kiln this is another good way to speed up the drying process if you're not sure if your pottery
is completely dry just place your pottery near the kiln when you're firing another load of pottery.
This should also be done in the final stage of your drying process. Wet or leather hard clay
will dry too fast and unevenly around the kiln. You put all that work into making your pottery
why would you want to take a chance? It's not recommended to place anything on top of your
kiln. Once your pottery is completely dry you can fire it in the kiln. Following these tips will
help to ensure that your pottery doesn't crack. Now head on over to my How-to choose clay video,
or How to throw a bowl on the pottery wheel. If you do I get to play with more clay. Let's stay dirty!