Fig.5

There are several ways to learn about clay shrinkage, but none more simple than a clay ruler. Any age or level of experience will find this to be a fun and easy way to understand how much clay shrinks at the greenware, bone dry, and bisqueware stages. A standard clay bar test will give a measurable percentage for clay body shrinkage. A clay ruler gives a simple and obvious visual example in inches.

The process is simple. Make a 12-inch clay ruler complete with inch lines using the clay body of your choice. Once bone dry, measure to see how much shrinkage occurred from the green stage and record this measurement. Include the clay ruler in your next bisque firing and measure it again after the firing to see how much shrinkage occurred from the bone dry measurement. Now you’ll have a general sense of how your work will shrink at each of these stages based on your clay ruler.


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Making a Clay Ruler


Fig.1

Fig.1 Cut perpendicular lines using a carpenter’s square, then lay a graph ruler on top to create a parallel cut at the far end of the slab.

Prepping the Slab

Roll out a slab of clay of your choice and smooth out the surface. I like to use a combination of a clear graph ruler and a carpenter’s square to measure and cut. Position the carpenter’s square to cut a straight bottom and side at a 90° angle. Next use the carpenter’s square and clear graph ruler to measure and cut the slab to 12 inches tall. The graph ruler is used to assure the parallel lines are as square as possible (figure 1).

Fig.2

Fig.2 Use the graph ruler with one end lined up with the straight edges of the slab to cut out multiple rulers.

Making the Ruler

A graph ruler is 2 inches wide so I just use that as the width for the clay rulers. Use a dull pencil to emboss vertical lines to make as many clay rulers as you can from the smoothed out slab (figure 2). When the ruler lines are done, mark the surface with horizontal lines spaced 1 inch apart to finish creating the 12-inch ruler (figures 3 and 4). Use a fettling knife to cut out the clay rulers.

During the drying and firing processes, a ruler can warp, crack, or break so having extras is always a good idea. I tend to make four or five at a time which also aides in assuring consistency. If I measure and cut correctly, all the rulers should have the same measurements once fired. This is an excellent way to assure your stats are correct. Dry the rulers sandwiched between drywall and place newspaper in-between each one. Place a little weight on top to help keep them flat while drying.

Fig.3

Fig.3 Make marks that are spaced 1 inch apart on each ruler.

Measuring the Results

The fun part is measuring the ruler at the different stages of drying and firing to actually see how your clay shrinks. I always measure the ruler when it’s bone dry and again after the bisque firing. I also fire the ruler to a higher temperature to measure the shrinkage at a variety of temperatures. This is where having multiple clay rulers around is handy. You’ll find most of the shrinkage occurs from greenware to bone dry.

For this article, I used Amaco’s Brownstone Earthenware. The results were: greenware measured 12 inches, bone dry measured 111/2 inches, and bisque fired to 1940° F measured 113/8 inches (figure 5).

Fig.4

Fig.4 Join the 1 inch marks across all of the rulers, making sure to keep the drawn lines parallel to each other (and perpendicular to the lines separating each ruler).

Some potters use a bisque ruler to measure bisque vessels to make replacement lids. The bisque measurement gives you the approximate size of what the lid should be when in the green stage. While not an exact match, it’s better than just guessing without a frame of reference for how much your clay shrinks. This is also a great exercise for students to visually see how much a clay body shrinks in a format that everyone can recognize and understand. My six-year-old son makes these all the time with great success and learns to measure and cut at the same time. I make clay rulers for all the bodies I use to get a sense of their general shrinkage. Clay rulers shouldn’t replace the clay bar shrinkage test (see Getting to Know Your Clay in the Jan/Feb 2008 issue of PMI, or available at www.ceramicartsdaily.org/education) but should be seen as an additional test to further understand your clay body and have a little fun, too.

 


Fig.5  A greenware ruler, a bone dry ruler, and a bisque fired ruler lined up to show the rate of shrinkage at each stage.

Fig.5 A greenware ruler, a bone dry ruler, and a bisque fired ruler lined up to show the rate of shrinkage at each stage.

Paul Andrew Wandless is a studio artist, workshop presenter, educator, and Vice President of the Potters Council. He authored the book Image Transfer On Clay and co-authored Alternative Kilns and Firing Techniques: Raku, Saggar, Pit & Barrel. His website is www.studio3artcompany.com and he can be emailed at paul@studio3artcompany.com.

 


This article appeared in Pottery Making Illustrated magazine’s May/June 2010 issue. To get great content like this delivered right to your door, subscribe today!


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