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 build a complex piece, sooner or later you will have to deal with the percentage of shrinkage that the clay will have.

You can say, “I can find the shrinkage rate online.” True, but I would like to point out that there can be big differences between the shrinkage rate posted online and the actual rate that you will get. There are so many variables, such as cooking methods, equipment, type of clay, etc. All of this can influence your actual results. And it seems like a new kind of clay comes on the market every day. Why wait to find out the shrinkage rate?

In my “graphic designer life,” one task I often had was to calculate proportions. That is, figuring out what percentage of a photo or artwork needed to be scaled up or down to fit my design.

When I started working with metallic clay, I realized that calculating shrinkage is like calculating proportions. I learned how to use a calculator and a simple formula: divide the size you want (your target size) by the size you are (the actual size of your artwork).

How is this related to calculating the shrinkage of metallic clay? Divide the size you want (it is the shrinkage rate, so it is the size of a piece after cooking) by its size (the original size of the previously cooked piece).

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

This means that my part became 85% the size it was before. But we want to know the percentage that was reduced. 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 the piece is baked. This can be affected by how aggressively the clay is dried, for example on a hot plate rather than air drying.

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