Calculating Shrinkage of Metal Clay

First of a Four Part Series: Designing for Metal Clay

Knowing how much clay shrinks, and how to work with that shrinkage is one of the fundamental skills necessary for working with metal clay. If you ever want to construct a complex piece—sooner or later you are going to have to deal with the percentage of shrinkage the clay will have.

In my “graphic designer life”, one task I often have is calculating proportions. Meaning, figuring out what percentage a photo or piece of art needs to be enlarged or reduced to fit my layout.

When I began to work with metal clay, I realized that calculating shrinkage is just like calculating proportions.

a proportion wheel

A very long time ago, in the graphics trade, we used a tool called a proportion wheel to figure these things out. This tool is basically an antique artifact now. Even I stopped using it long before the digital revolution changed how everything was done. I learned to use a calculator and a simple formula: divide the size you want (your target size) by the size you have (your artwork’s actual size).

How does this relate to calculating the shrinkage of metal clay? Divide the size you want (it’s the shrinkage rate—so it’s the post-fired size of a piece) by the size you have (your piece’s original pre-fired size).

I carefully measure every piece before I fire it, take notes, and then measure it again after firing. Let’s say I made a sterling silver test strip, 13 x 30 mm. After firing it was 11 x 25.5 mm. Here’s how I calculate the shrinkage rate: the smaller size is my target size (remember, shrinkage size is what we’re after—so the smaller number is our target number). 11÷13=.846 (85). 25.5÷30= .85.

This means that my piece became 85% of the size it once was. But we want to know the percentage that it shrank. Subtract 85 from 100, which equals 15. My piece shrank 15%.

There will be variations in your shrinkage after firing, but keep measuring your pieces and checking the shrinkage. After a while a pattern emerges and you will learn what the average shrinkage rate is for a particular type of clay.

Next time I will talk about how to use this information to size your designs.

Evelyn

About Evelyn

I'm a metal clay artist who is learning something new every day. My goal in my work is to make things that are well crafted and distinctively from my own hands, like the work of a good tailor. My jewelry can be found at http://www.evelynpelati.com.


Comments

Calculating Shrinkage of Metal Clay — 3 Comments

  1. Great post and this topic is key when setting gems in a fired metal clay piece. The clay shrinks during drying, which is another factor. Then shrinks during firing. The way an artist works the clay (roll, press, etc) will give them results that may be different than other artists. You are so right that tracking your own shrinkage percents will show your pattern.

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