I had someone write to me asking about metal clay shrinkage. It's been discussed in this blog a little bit. Here is what Linda Kline wrote about in November 2013.
What is the formula for adjusting your clay size for shrinkage? First, it depends on the clay you are using. Each clay type has a different shrinkage value. Draw your design at the desired finished size. Scan your drawing into a printer and adjust for shrinkage by enlarging your design according to the clay type you are using. PMC3 and PMC Flex have a maximum shrinkage of roughly 15% (.15) when fired at 1650F for 2 hours. It can be less if fired at a lower temperature and for a shorter time. The enlargement factor for 15% shrinkage is 118%.
Here's how to calculate the enlargement factor.
Subtract the expected clay shrinkage percentage from 1. In this case the clay has a 15% shrinkage rate.
1 - .15 = .85 (Another way to think of it is 100% - 15% = 85% or .85.)
Divide 1 by the resulting number: 1/.85 = 1.18 or 118%
If firing the clay at a shorter or cooler firing schedule then base the calculation on the lower end of the shrinkage scale (10% - 12% in the case of PMC3, PMC Flex, or PMC+).
Here is a simple chart for other clay shrinkage so you won’t have to do the math.
Until next time, have fun claying around!
by Janet Alexander