Clay Shrinkage – How to Calculate Shrinkage Rate and Make a Shrink Rule

How to determine clay shrinkage
Clay shrinkage. Pesky clay shrinkage. Nearly every beginner with clay has had the experience of getting their first piece out of the kiln and thinking “this was so much bigger when I made it!” Of course, clay shrinkage is a fact of life and as you build your skills you learn to accommodate for it by making your pieces a bit bigger.

In some cases, like when working on an architectural ceramic project, calculating the precise shrinkage rate is crucial. In today’s video, an excerpt from Studio Scale Architectural Ceramics, Stephani Stephenson explains how to calculate shrinkage and make a clay shrinkage ruler. I thought this would be a good clip to share because, even if you’re not working on an architectural scale, knowing how to calculate shrinkage is a good skill to have. – Jennifer Poellot Harnetty, editor

Why does clay shrink?

Clay shrinks both during the drying process and the firing process. Shrinkage in the drying process occurs due to the loss of water layers. The finer the particle size of the clay, the more water layers; hence the more shrinkage. As Vince Pitelka explains in Clay: A Studio Handbook, “When clay is fired above low-fire temperatures, glass begins forming within the platelets and seeps into the intervening spaces or voids.

As this happens, the platelets shrink, causing firing shrinkage.” It’s obvious why shrinkage is an important thing to consider in architectural projects, in which precise measurements are crucial, but clay shrinkage is also an important consideration in functional ware. Clay shrinkage can impact glaze fit and poor glaze fit can cause crazing or glaze shivering problems and reduced strength of your fired wares.

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