How to Calculate the Shrinkage of 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.

You may say, “I can find the shrinkage rate online.” True, but I’d like to point out that there can be wide differences between a shrinkage rate posted online and the actual rate that you will get. There are so many variables-such as firing methods, equipment, clay type, etc. All of these can factor in to your actual results. And, it seems like every day a new clay type comes on the market. Why wait to learn the shrinkage rate?

In my “graphic designer life”, one task I often had was calculating proportions. Meaning, figuring out what percentage a photo or piece of art needed 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. 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. The shrinkage rate is 15%.

It’s also worth noting that there will also be some shrinkage in the drying process before your piece is fired. This can be affected by how aggressively you dry the clay, for example on a warming plate as opposed to air drying.

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.