How to Determine Clay Shrinkage and Make a Clay Shrinkage Ruler

Stephani Stephenson gives a fantastic explanation of how to calculate clay shrinkage and make a clay shrinkage ruler.

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.

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This clip was excerpted from Studio Scale Architectural Ceramics with Stephani Stephenson, which is available in the the Ceramic Arts Network Shop!

To learn more about Stephani Stephenson or see images of her work, please visit

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.

**First published in 2011.
  • A sample of my work, such as this three section panel measuring 36″ across

    Now I’m wondering about the amount of expansion in the kiln of greenware during the firing process, all heated objects expand, but then we have the clay shrinking too.
    My issue today is I have a kiln that is about 17-1/4 wide brick to brick inside, but across the width at the widest points where the corners of the octagon shape meets it’s a little more.
    I have a panel in there now that is just about 17″ wide and about 2″ deep, 22″ tall, I was able to fit it in across that diagonal and gain a little more space so there is now maybe 3/8″ space on either side.
    Not the best solution but my goal is to harden up the model which I had used for a mold, so it’s just to keep it as an archival model but in hard terracotta form instead of greenware.
    I used Raku clay 06-10 and I’m taking it to cone 1 and hoping that space is enough so any expansion doesn’t have it pressed against the walls.

  • Very good, concise well described video!
    I work fairly large, so I do have to deal with shrinkage of a few inches, but for the most part I don’t need to end up with a specific size

  • Stephani S.

    Gail, if you want to account for the shrinkage with glaze firing, be sure to fire a test bar to that temperature so you can calculate your clay’s shrinkage at that ‘glaze firing’ temperature.
    Even better, put a test bar in a couple of different places in the kiln: top, middle ,bottom, front and back, to find out if there are any differences.

  • Elaine H.

    Genius! Exactly what I need just now in a commission making border tiles which have to measure 22.4 x 6.4 to 6.5cm plus some cutouts of 1.5 cm! Have been tearing my hair out but am going to go and make one of these right now- thanks very much indeed.

  • Meena, the proportion wheel (or proportion scale) is a tool I use often to determine the size I need to make a piece, in order to get the finished size I desire. You first need to determine your clay body’s percentage of shrinkage. Measure your piece when it is wet clay, and then measure it again after you have fired it. You can get the wheel at Dick Blick or any art supply store or even office supply stores. Search it on the internet to see what it looks like. It’s like a circular ruler, where you line up on the wheel, the wet size with the finished size. The wheel will show you the percentage of shrinkage. I put a small piece of tape to hold the wheel at that percentage. That way, I can determine the size I need to make the piece by looking at the corresponding desired size on the proportion wheel. It sounds complicated, but it is very simple. I use it for everything I do in the studio, from clay to paintings.

  • Subscriber T.

    Thank you stephani for your interesting and easy to understand video – I’m not making precision pieces yet, but you never know what might crop up!! Its always a pleasure to see these video’s, so thank you too to the editing team. As an aside – why do american’s still measure in inches?

  • The original clip is outstanding. Thanks Stephani so much for sharing. The comments have all been interesting too. Even though I’ve lost my math skills over the years I like to understand how a “formula” got derived so the wet/dry clarification felt very useful. But, I’m also numbers-challenged artist so if Lynn or someone could explain the concept of a “trusty proportion wheel” and how to get it or “make it,” I wager a lot of people would enjoy the information.
    Again Many thanks.

  • As a former Graphic Artist, I use my trusty proportion wheel to figure out what size the wet clay needs to be to get the correct finished size. Mark the wheel to the percentage that your clay shrinks to, and set it to that when figuring out the finished size. Much easier! My brain turned off about halfway through the video.

  • Valerie M.

    Yeah this is pretty cool. It’s a really good and clear explanation of shrinkage in clays….boy that last part though…too much math for my poor head…lol.

  • I used to think I was lucky that there were no math requirements for a degree in Fine Arts back when I went to college. I have come to rue that fact many times over the years. Thanks for the easy, clear explanations for those among us who are “math challenged”.

  • Shirley P.

    I am blown away! This has to be the best explanation of shrinkage formulas–ever. I have stumbled over this subject so often that, like Debi, it was so daunting that avoidance was how I handled it.

    Also, what Jill said is true. When you mark your original as a MASTER COPY, use yellow highlighter for writing as it won’t be picked up by the copier (unless you use a color copier). Used to work in a copy/print shop and yellow Post-Its were perfect for blanking out areas

  • Great idea with clear explanation, however readers should be aware that copiers shrink each image by a small amount. The first copy won’t matter too much unless you are multiplying to a very large clay piece but if you copy, copies, each subsequent copy will be smaller than the original. If you check your first copy for accuracy and then mark it as MASTER COPY and only use it and not subsequent copies your measurements will be more relevant.

  • Connie S.

    Wow!!! Great video. The formula calculations are easy and take the guesswork out of clay shrinkage. I appreciate this video and will be using these formulas on my projects.

  • Fantastic video! Extremely informational as I am starting to work with slabs and focusing on getting precise sizing. Shrinkage felt like a very daunting thing to me. Now I feel that I can tackle it with confidence. Thanks so much for sharing!

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