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clay shrinkage ruler

Make a DIY Clay Shrinkage Ruler

You need a clay shrinkage ruler. The shrinkage rate of your clay is critical to handbuilding pieces that meet your needs an specifications. All clay bodies shrink as they are fired. They clay shrinks as it dries, while it’s bisqued and then clay shrinks more when glaze fired or finish fired.  Shrinkage is the rate at which the clay body reduces in size or mass during drying and firing.

Stoneware, for example, can shrink anywhere from 7% to 14% or so. It’s tough to make good, workable pots if you have no idea how to determine their final size. First firing is a fun moment with a class of new clay enthusiasts who have made or thrown their first cylinders. The pots come out of the kiln, obviously reduced in size, and the new artists are amazed at the change.

You can avoid shrink-shock when you’re building pottery. You can buy, make, or even download effective shrink rulers to give you a good idea of what to expect from your clay body. Here’s what our research turned up for making your own do-it-yourself shrinkage ruler.

clay shrinkage ruler
DigitalFire.com has a good discussion of shrinkage. Here they show wet, bisqued, and final fired

Make a DIY Clay Shrinkage Ruler

Read the label of your clay body package to find the shrinkage rate. Mine is 7%, others are more, few are less. We’ll make the math easy and use 10% as an example.

Subtract the shrink rate from 100% – we get 90% in our example. Meaning a finished piece will be 90% of the size of the wet clay piece. The shrinkage rate usually considers final size after glaze firing.

Figure what 90% of one inch is equivalent to. Once you know that, you can easily measure out “shrink” inches onto a ruler form. In this case, divide one inch by .9 and you get 1.11.  Now you know that one fired inch of your clay body is the same as 1.11 inches of your wet clay.

DIY clay shrinkage ruler
On Pinterest, Jan McKay shows a shrinkage ruler made of clay.

It will be challenging to find exactly where some of these conversions end up on your ruler form, sometimes you may find that using centimeters is helpful. Google will make the conversion for you. For example, Google “1.11 inches in centimeters.” You get 2.8 centimeters.

By the way, paint stirring sticks, the ones free from homecenters’ paint departments, are perfect. You can also easily cut sticks about a foot long, out of 1/4 inch thick wood. You can use heavy cardboard, but you’ll be remaking your ruler pretty often. If you root around an art supply or craft store, you’ll find flexible heavy plastic sheets that can be cut up, too. Some artists make their rulers in clay.

So now, lay your blank clay shrinkage ruler next to a standard wooden or plastic ruler. Put the centimeters on the original ruler closest to your new form.

On the blank, using an ultra fine marker, mark off your 1.1 inch intervals for “shrink inches” and then set in your quarter “shrink inches” and halves just like your standard ruler. In our example, the quarter inch will equate to 7 mm.

When you measure your wet clay against your clay shrinkage ruler, you will have a pretty fair idea of how long or wide a piece is going to be after firing.

This process takes fewer than 15 minutes. Some shrink rulers are quite expensive. The cheapest are around $15. A dollar a minute? The download is free.

Read how to choose the right clay body for your use.