Shrinkage

I'm talking about clay shrinkage, not the George Costanza "I was in the pool" type of shrinkage. (One of those words that will likely live in pop culture for the generation of people who tuned in to Seinfeld every week.)

In fact, the shrinkage that occurs with clay firing happens because of extreme heat, not cold.  When clay is fired, the moisture is removed from the clay making it hard.  The higher the temperature the clay is fired to, the stronger it becomes.  Each piece is fired twice, one time for bisquing and then again after the glaze is applied.  Most of my finished pieces are fired to Cone 6 on the Pyrometric Cone Scale which is about 2232* F.  Yes, that's HOT!

Every clay body has a different shrink rate.  Most of the clay bodies I work with have about a 12% shrink rate.  While that might not sound like a lot, it sometimes is surprising how much it really is once a piece is finished. 

A before and after image of two mugs to show 12% clay shrinkage. 
The mug on the left is completed.  The mug on the right is leather hard -it has not been fired at all yet. 
Once the mug on the right is fired twice, it will be the same size as the mug on the left. 

I remember when I first started throwing and I'd make something that seemed really tall or big.  It was so disappointing to see the pieces after firing when they'd shrunk up so much.  Something that had seemed so tall and big at first, didn't seem that way in the end. 

I use a product called a "shrink rule" to help me cope with clay shrinkage.  Often customers want a specifically sized finished piece.  The shrink ruler helps eliminate a lot of the guess work that would normally occur when trying to estimate the size of a finished piece after shrinkage (assuming you know the shrink rate of the clay body you are using).

Here is a picture of a shrink ruler next to a standard 12 inch ruler
Different sides of the ruler show different shrink rates.
The shrink ruler is one of my most used tools in my studio. 

Next time you see a really large piece of pottery - think about how much bigger it was when it was created.  It'll seem even more impressive!